This is one of the webpages of LibaridA. Maljian at the Department of Physics at CSLA at NJIT.
New Jersey Institute of Technology
College of Science and Liberal Arts
Department of Physics
The Earth in Space
Spring 2024
Fourth Examination lecture notes
Introduction to the Atmosphere
An atmosphere is a thin layerof gas gravitationally held to a moon, a planet, or a star. The Earth’s atmosphere is roughly eightypercent nitrogen, roughly twenty percent oxygen, and tiny amounts of othergases. The tiny amounts of other gasesare quite important, as we will discuss shortly. Every second of every day of our lives, weare breathing mostly nitrogen (roughly eighty percent) and a fair amount ofoxygen (roughly twenty percent). Thisroughly twenty-percent abundance of oxygen is an enormous fraction; otherplanets have nowhere nearly this much oxygen in their atmospheres. Other planetary atmospheres have only tinyamounts of oxygen with a large abundance of carbon dioxide, as is the case withthe atmospheres of planets Venus and Mars for example. The Earth’s atmosphere has a large fractionof oxygen but only a tiny amount of carbon dioxide. To understand why the Earth’s atmosphere isso different from other planetary atmospheres, we must discuss the history ofthe Earth’s atmosphere. When the Earthformed roughly 4.6 billion years ago, its atmosphere was almost entirelyhydrogen gas and helium gas; this is called theEarth’s primary atmosphere. The primaryatmosphere was almost entirely hydrogen and helium because the Earth togetherwith the entire Solar System was born from a nebula, an enormous cloud of gascomposed of mostly hydrogen and helium.Indeed, most of the universe is composed of hydrogen and helium. The more massive (or heavier) an atom ormolecule, the slower it moves; the less massive (or lighter) an atom ormolecule, the faster it moves. This israther remarkable. Suppose all the airin a room is at the same temperature, which means that all the air molecules inthe room have the same average energy.Nevertheless, the oxygen molecules are moving slower on average sincethey are more massive (or heavier), while the nitrogen molecules are movingfaster on average since they are less massive (or lighter), even though all ofthe air is at the same temperature, which means both the nitrogen molecules andthe oxygen molecules have the same average energy! Hydrogen is the least massive (lightest) atomin the entire universe, and helium is the second least massive (secondlightest) atom in the entire universe.Hydrogen and helium are so light that they move so fast that they canescape from the Earth’s gravitational attraction. Thus, the Earth lost its primary atmospherebecause its own gravity was too weak to hold onto hydrogen and helium. This occurred with all four of the innerplanets orbiting the Sun (Mercury, Venus, Earth, and Mars). These four inner planets have weaker gravitysince they are smaller with less mass as compared with the four outer planetsorbiting the Sun (Jupiter, Saturn, Uranus, and Neptune). These four outer planets have strongergravity since they are larger with more mass; thus, they have retained theirprimary (hydrogen and helium) atmospheres to the present day. Even the gravity of the outer planets is weakcompared with the gravity of the Sun; therefore, the Sun has certainly retainedits primary (hydrogen and helium) atmosphere to the present day. To summarize, the Sun and the four outer planetshave retained their primary (hydrogen and helium) atmospheres, but the fourinner planets have lost their primary (hydrogen and helium) atmospheres. After the Earth lost its primary atmosphere,this left behind a secondary atmosphere composed of an abundance of watervapor, nitrogen, carbon dioxide, methane, and other gases. These gases came from volcanicoutgassing. Since the Earth was bornalmost entirely molten, volcanic eruptions everywhereon its surface ejected not just lava but gases as well, as we discussed earlierin the course. These gases aresignificantly more massive (or heavier) than hydrogen and helium. Therefore, they did not move fast enough toescape from the Earth’s gravitational attraction. As the Earth cooled, the water vaporcondensed into liquid water which precipitated back down onto the planet forsuch a long period of time that most of the surface ofthe Earth became flooded, thus forming the oceans. At this point, the Earth maybe regarded as having a normal atmosphere with an abundance of carbondioxide similar to other planets such as Venus and Mars. However, roughly onebillion years after the Earth formed (roughly 3.6 billion years ago), somethingextraordinary occurred in the oceans that to our knowledge did not occuranywhere else in the entire universe: life appeared. The first lifeforms were primitiveunicellular microorganisms, such as bacteria and blue-green algae. Some of the carbon dioxide in the atmospheredissolved into the oceans, which these primitive lifeforms converted tooxygen. Over the next roughly onebillion years, the rock at the ocean floor was oxidized bythe oxygen that these primitive microorganisms continually synthesized. When nearly all the rock at the ocean floor was oxidized, the oxygen that these primitive lifeformscontinued to synthesize then began to accumulate within the oceans. Some of this accumulating oxygen thendissolved back into the atmosphere.After an additional roughly two billion years, these microorganismsactually succeeded in extracting almost all of the carbon dioxide from theatmosphere, replacing it with oxygen.Thus, as of roughly 600 million years ago, planet Earth attained itstertiary atmosphere that we enjoy to the present day: roughly eighty percentnitrogen, roughly twenty percent oxygen, and tiny amounts of other gases. Again, the tiny amounts of other gases arequite important, and we will discuss these trace gases shortly.
The pressure of the Earth’satmosphere is typically a maximum at mean sea level and decreases exponentiallywith increasing elevation. The equationthat describes this decreasing pressure with increasing elevation is called thelaw of atmospheres, but we do not need this equation to understand why this isthe case. The Earth’s gravity pulls airdownward; therefore, the air becomes thinner as we climb the atmosphere, makingthe air pressure less at higher elevations.The average air pressure at mean sea level is called one atmosphere ofpressure, equal to 101325 pascalsof pressure. One pascalof pressure is one newton of force per square meter of area. This average air pressure of 101325 pascals is close enough to onehundred thousand pascals that meteorologists havedefined another unit of air pressure: the bar.One bar of air pressure is exactly one hundred thousand pascals of air pressure. Thus, the average air pressure at mean sealevel is equal to 1.01325 bars of pressure; this is also 1013.25 millibars of pressure.When meteorologists report the air pressure on any given day, they mayreport that the air pressure is several millibarsabove average or several millibars belowaverage. A device to measure airpressure is called a barometer. To build a primitive barometer, we insert along, narrow container inverted into any liquid. The air pressure will push downward onto theliquid, thus forcing the liquid upward into the long, narrow invertedcolumn. If the air pressure is greaterthan average, it will push downward more strongly onto the liquid, thus forcingthe liquid further upward the inverted column, making the column of liquidtaller. If the air pressure is less thanaverage, it will push downward less strongly onto the liquid, thus forcing theliquid not as far upward the inverted column, making the column of liquidshorter. Thus, by measuring the heightof the column of liquid and performing a calculation, we can determine the airpressure that has pushed downward onto the liquid. Most barometers use the element mercury asthe liquid; at average air pressure at mean sea level, liquid mercury will be pushed 760 millimeters (or 29.9 inches) up thenarrow column. Thus, the average airpressure at mean sea level is also equal to 760 millimeters of mercury (or 29.9inches of mercury). When meteorologistsreport the air pressure on any given day, they may report that the air pressureis several millimeters of mercury higher than average or several millimeters ofmercury lower than average. Barometers almost always use liquid mercury because mercury is betweenthirteen and fourteen times more dense than water. In other words, mercury is between thirteenand fourteen times heavier than water; thus, gravity pulls mercury thirteen orfourteen times more strongly than water, making the column of mercury only 760millimeters (or 29.9 inches) tall. If abarometer used water instead of mercury, the column of water would be betweenthirteen or fourteen times taller; this would make barometers more than tenmeters (almost thirty-four feet) tall!It is not convenient to carry such a tall device; it is much more convenientto carry a barometer that is only 760 millimeters (or 29.9 inches) tall. This is why almost all barometers use mercuryinstead of water.
Meteorologists have definedlayers of the Earth’s atmosphere based on the variation of the temperature ofthe Earth’s atmosphere with elevation.The lowest layer of the atmosphere is the troposphere. With increasing elevation, the troposphere is followed by the stratosphere, then the mesosphere, andfinally the thermosphere. Beyond thethermosphere is the exosphere, where the air smoothly transitions from theEarth’s atmosphere to the surrounding outer space, as we will discussshortly. The temperature typically coolswith increasing elevation within the troposphere, the lowest layer of theatmosphere. However, we reach a certainelevation at which the temperature stops becoming cooler and begins instead tobecome warmer with increasing elevation.This elevation defines the end of the troposphere and the beginning ofthe stratosphere. This precise elevationis called the tropopause. We may regard the tropopause as the boundarybetween the troposphere and the stratosphere, but the tropopause is more correctly defined as the end of thetroposphere. The temperature thentypically becomes warmer with increasing elevation within thestratosphere. The reason for thiswarming is a heat source within the stratosphere that we will discussshortly. However, we reach a certainelevation at which the temperature stops becoming warmer and begins instead tobecome cooler with increasing elevation.This elevation defines the end of the stratosphere and the beginning ofthe mesosphere. This precise elevation is called the stratopause. We may regard the stratopauseas the boundary between the stratosphere and the mesosphere, but the stratopause is more correctly definedas the end of the stratosphere. Thetemperature then typically becomes cooler with increasing elevation within themesosphere. However, we reach a certainelevation at which the temperature stops becoming cooler and begins instead tobecome warmer with increasing elevation.This elevation defines the end of the mesosphere and the beginning ofthe thermosphere. This precise elevationis called the mesopause. We may regard the mesopauseas the boundary between the mesosphere and the thermosphere, but the mesopause is more correctly definedas the end of the mesosphere. Thetemperature then typically becomes warmer with increasing elevation within thethermosphere. The reason for thiswarming is a heat source within the thermosphere that we will discussshortly. However, we reach a certainelevation at which the temperature stops becoming warmer and begins instead tobecome cooler with increasing elevation.This elevation defines the end of the thermosphere and the beginning ofthe exosphere. This precise elevation is called the thermopause. We may regard the thermopauseas the boundary between the thermosphere and the exosphere, but the thermopause is more correctly definedas the end of the thermosphere. Thetemperature then typically becomes cooler with increasing elevation within theexosphere, smoothly transitioning into the very cold temperatures of thesurrounding outer space. To summarize,the temperature typically becomes cooler with increasing elevation within thetroposphere, the mesosphere, and the exosphere, while the temperature typicallybecomes warmer with increasing elevation within the stratosphere and thethermosphere.
It may seem reasonable to askfor the precise elevation at which the Earth’s atmosphere ends and outer spacebegins, but this is in fact an ill-defined question. The concentration of gases in the Earth’satmosphere becomes thinner and thinner with increasing elevation until we reacha certain elevation at which the concentration of gases matches theconcentration of gases of the surrounding outer space. It is a common misconception that outer spaceis a perfect vacuum, but this is false.There is no such thing as a perfect vacuum; in fact, a perfect vacuumwould violate the laws of physics. Inactuality, the entire universe is filled withextremely diffuse gas. Therefore, theEarth’s atmosphere smoothly transitions into the gases of the surrounding outerspace. We may actually interpret theEarth’s atmosphere as extending forever, filling the entire universe. The same interpretation canbe applied to all other planetary atmospheres. In other words, there is no well-definedexopause. The tropopause is the end ofthe troposphere, the stratopause is the end of thestratosphere, the mesopause is the end of themesosphere, and the thermopause is the end of thethermosphere. If there were an end ofthe exosphere (which would also be the end of the entire atmosphere), that endwould be called the exopause, but there is no well-defined exopause. Nevertheless, if we insist upon a boundarybetween the Earth’s atmosphere and outer space, we may arbitrarily use theelevation of the tropopause, since the Earth’s gravity pulls most of the air inthe entire atmosphere down into the troposphere. Indeed, the vast majority of allmeteorological phenomena (commonly known as weather) occurs within thetroposphere, the lowest layer of the atmosphere. The exceptions to this are rare. For example, the jet stream is a fast-movingcurrent of air around the tropopause, much higher in elevation than mostmeteorological phenomena (weather). So, we may arbitrarily regard the thickness of the Earth’satmosphere as the elevation of the tropopause as a rough estimate. The elevation of the tropopause is roughlyten kilometers above mean sea level.Using this as a rough estimate for the thickness of the Earth’satmosphere, we conclude that the atmosphere is extremely thin as compared withthe average radius of the Earth, roughly 6400 kilometers. To summarize, the Earth’s atmosphere extendsindefinitely far according to strict interpretations, but the Earth’satmosphere is only a few kilometers thick for all practical purposes. The Earth’s atmosphere keeps us alive in avariety of different ways. After wediscuss all these ways the atmosphere keeps us alive, we will be humbled. In this vast universe, we are only able tosurvive within a very thin layer of air surrounding a single planet: theatmosphere of planet Earth.
The most obvious way theEarth’s atmosphere keeps us alive is with its abundance of oxygen. Humans and all animals must inhale oxygen tosurvive. This is because humans andanimals must chemically react oxygen with glucose (a simple sugar) to extractthe energy they need for their survival.Humans ingest various sugars as well as complex carbohydrates such asbread, rice, cereal, and pasta. Ourbodies digest complex carbohydrates as well as sugars, breaking them down intoglucose (a simple sugar). There is atremendous amount of energy stored within the chemical bonds of the glucosemolecule, which our bodies access by reacting it with oxygen. The human body is composed of roughly onehundred trillion cells. Within thesecells, the following chemical reaction occurs: glucose plus oxygen yieldsenergy plus carbon dioxide and water as waste products. This chemical reaction is called cellularrespiration and is more properly written C6H12O6+ 6 O2 → energy + 6CO2 + 6 H2O. When we inhale, the oxygen that enters ourlungs is transferred to our blood; our blood thencarries the oxygen to the trillions of cells of our bodies. The oxygen enters our cells and chemicallyreacts with glucose to yield energy. Thecarbon dioxide that is produced as a waste productfrom the reaction is transferred back into our blood; our blood then carriesthe carbon dioxide back to our lungs, and we then exhale. Cellular respiration not only explains whyhumans and animals must inhale oxygen; cellular respiration also explains whyhumans and animals must exhale carbon dioxide.Plants inhale carbon dioxide to use together with water as the rawmaterials to synthesize glucose. Plantsuse the energy of sunlight to initiate this reaction, which is why thischemical reaction is called photosynthesis. The photosynthesis reaction is more properlywritten 6 CO2 + 6H2O + energy → C6H12O6 + 6 O2. Notice thatthe photosynthesis reaction is precisely the reverse of the cellularrespiration reaction. Note also thatoxygen is a waste product of this photosynthesis reaction. Plants inhale carbon dioxide and exhaleoxygen, the precise reverse of humans and animals. Therefore, the relationship between animals(including humans) and plants is a symbiotic relationship. Animals (including humans) exhale carbon dioxide,which plants then inhale. Plants thenexhale oxygen, which animals (including humans) then inhale. Animals (including humans) then exhale carbondioxide, which plants then inhale, and so on and so forth.
Another way the Earth’satmosphere keeps us alive is by maintaining a habitable temperature forlife. Based on the Earth’s distance fromthe Sun, our planet should be too cold for life to exist. The temperature of our planet should be muchcolder than the freezing temperature of water; not only should all the oceansbe frozen, but the continents should be frozen over as well. However, there are tiny amounts of gaseswithin the Earth’s atmosphere that absorb and then emit some of the heat thatour planet radiates. These gases are called greenhouse gases.The most important greenhouse gas is water vapor. Carbon dioxide, methane, and other gases aresecondary greenhouse gases. These gasesabsorb some of the heat that the Earth radiates, and these gases then reradiatethis heat themselves. Although thesegases reradiate some of this heat into outer space, these gases also reradiatesome of this heat back to the Earth.This causes the temperature of planet Earth to be significantly warmerthan it would have been otherwise, based on its distance from the Sun. In fact, the Earth issufficiently warmed that its average surface temperature is habitablefor life. This warming is called the greenhouse effect, since it is rather like agreenhouse that is warm even in the wintertime.The Earth’s atmosphere is mostly nitrogen and oxygen, but neither ofthese gases can absorb or radiate heat efficiently. In other words, neither nitrogen nor oxygenare greenhouse gases. Primarily watervapor and secondarily carbon dioxide, methane, and other gases are able toabsorb and radiate heat efficiently. Thetiny amounts of water vapor, carbon dioxide, methane, and other gases in theEarth’s atmosphere warm the planet to a habitable temperature. This is a second way the Earth’s atmospherekeeps us alive.
The Sun not only radiatesvisible light; the Sun radiates all forms of electromagnetic radiation. The Electromagnetic Spectrum is a list of allthe different types of electromagnetic waves in order as determined by either thefrequency or the wavelength. Startingwith the lowest frequencies (which are also the longest wavelengths), we haveradio waves, microwaves, infrared, visible light (the only type ofelectromagnetic wave our eyes can see), ultraviolet, X-rays, and gamma rays atthe highest frequencies (which are also the shortest wavelengths). All of these are electromagnetic waves. Therefore, all of them maybe regarded as different forms of light.They all propagate at the same speed of light through the (near-perfect)vacuum of outer space for example. Wenow realize that whenever we use the word light in colloquial English, weprobably mean to use the term visible light, since this is the only type oflight that our eyes can actually see.The visible light band of the Electromagnetic Spectrum is actually quitenarrow. Nevertheless, the visible lightband of the Electromagnetic Spectrum can be subdivided. In order, the subcategories of the visiblelight band of the Electromagnetic Spectrum starting at the lowest frequency(which is also the longest wavelength) are red, orange, yellow, green, blue,indigo, and violet at the highest frequency (which is also the shortestwavelength). We now realize whyelectromagnetic waves with slightly lower frequencies (or with slightly longerwavelengths) than visible light are called infrared,since their frequencies (or wavelengths) are just beyond red visiblelight. In other words, infrared light ismore red than red! We also realize whyelectromagnetic waves with slightly higher frequencies (or with slightly shorterwavelengths) than visible light are calledultraviolet, since their frequencies (or wavelengths) are just beyond violetvisible light. In other words,ultraviolet light is more purple than purple!The Sun radiates all of these electromagnetic waves. For example, the near ultraviolet from theSun causes suntans, and too much near ultraviolet from the Sun causessunburns. The far ultraviolet has evenmore energy, and the Sun radiates sufficient far ultraviolet that we should bekilled from its far ultraviolet radiation. X-rays have even greater energy; thus, theX-rays from the Sun should kill us in a fairly shortamount of time. Something must beshielding us from the Sun’s far ultraviolet and from the Sun’s X-rays. Our atmosphere provides these shields. The symbol for the oxygen atom is O. Under normal temperatures and pressures, theoxygen atom will never remain by itself; it will always chemically bond toanother atom. If there are no otheratoms nearby, the oxygen atom will chemically bond to another oxygen atom. Two oxygen atoms chemically bonded to eachother is called the oxygen molecule, which is written O2.Molecular oxygen is also known as normal oxygen, since oxygen is almost always in this state under normal temperatures andpressures. Roughly twenty percent of theEarth’s atmosphere is molecular (normal) oxygen for example, and this is the form of oxygen that plants exhale as well as the formhumans and all animals must inhale.Notice this is the form of oxygen appearing in both the cellularrespiration reaction and the photosynthesis reaction written above. Whenever we use the simple word oxygen, weare not being clear. Dowe mean atomic oxygen O or do we mean molecular oxygen O2? We probably mean molecular oxygen, since thisis normal oxygen. If molecular oxygenabsorbs far ultraviolet, a chemical reaction willsynthesize a strange form of oxygen: three oxygen atoms chemically bonded toeach other. This strange form of oxygen is written O3 and iscalled ozone. The synthesis of ozone ismore properly written 3 O2 + energy → 2O3. Ozone istoxic, since inhaling O3 causes severerespiratory problems. This is ironic,since ozone also keeps us alive. Themolecular oxygen in the Earth’s atmosphere absorbs the far ultraviolet from theSun, synthesizing ozone. Therefore, thefar ultraviolet from the Sun never reaches the surface of the Earth, since it is absorbed by molecular oxygen to synthesize ozone. In fact, there is a layer of ozone in thestratosphere below which far ultraviolet does not penetrate. This layer is commonlyknown as the ozone layer, but it is more correctly called theozonosphere. The ozonosphere is the heatsource within the stratosphere that is responsible for warming temperatureswith increasing elevation within that atmospheric layer. Much higher in the atmosphere within thethermosphere, various atoms and molecules absorb X-rays from the Sun. X-rays have so much energy that absorbingthem strips electrons completely free from an atom or molecule. In other words, atoms ormolecules are ionized by X-rays.Therefore, the X-rays from the Sun never reach the surface of the Earth,since they are absorbed by atoms and molecules to synthesizeionized atoms and molecules. In fact, there is a layer of ionized atoms and molecules in thethermosphere below which X-rays do not penetrate. This layer is calledthe ionosphere, and it is the heat source within the thermosphere that isresponsible for warming temperatures with increasing elevation within thatatmospheric layer.
Let us summarize all the waysthe Earth’s atmosphere keeps us alive.Firstly, humans and animals would not have oxygen to react with glucoseto extract energy for their survival if unicellular microorganisms did notremove most of the carbon dioxide from the atmosphere, replacing it withmolecular oxygen. Secondly, planet Earthwould be too cold for life to exist without the presence of greenhouse gasesthat make the planet warm enough to be habitable for life. Thirdly, life on Earth wouldbe killed from the far ultraviolet from the Sun if it were not for theozonosphere. Fourthly, life on Earth would be killed from the X-rays from the Sun if it were notfor the ionosphere. As we discussedearlier in the course, the atmosphere would be substantiallyionized by the Sun’s solar wind without the Earth’s magnetic fielddeflecting most of these charged particles from the Sun that continuallybombard our planet. Thisis a fifth way our planet keeps us alive. If only one of these were the case, we wouldnot be here from the lack of the other four.If two were the case, we would not be here from the lack of the otherthree. If three were the case, we wouldnot be here from the lack of the other two.If four were the case, we would not be here from the lack of theremaining one. The fact that all five ofthese are the case on the same planet is truly miraculous. Again, we are humbled. In this vast universe, we are only able tosurvive within a very thin layer of air surrounding a single planet: theatmosphere of planet Earth.
The surface temperature ofthe Earth causes the Earth to radiate heat from its surface. This explains why the troposphere becomescooler with increasing elevation; as we climb the troposphere, we are furtherand further from the surface of the Earth and thus furtherand further from this source of heat.Air in the lower troposphere (near mean sea level) isoften heated by the Earth’s surface.Since hot fluids rise, this hot air may rise to the upper troposphere(near the tropopause). This rising airmay cool, as we will discuss shortly.Since cool air sinks, air in the upper troposphere (near the tropopause)may sink to the lower troposphere (near mean sea level), where it may be warmedagain thus causing it to rise again. Insummary, there is significant convection within the troposphere caused bycontinually circulating air within the troposphere. This convection (circulation) of air withinthe troposphere is ultimately responsible for meteorological phenomena(commonly known as weather), as we will discuss. This explains why the vast majority of allmeteorological phenomena (weather) occurs within the troposphere, the lowestlayer of the atmosphere. This alsoexplains why the lowest layer of the Earth’s atmosphere iscalled the troposphere, since the Greek root tropo- means turning. The ozonosphere is in the upper stratosphere,near the stratopause.This explains why the temperature warms as we climb thestratosphere. After passing the tropopause,we approach the ozonosphere, which serves as a heat source, thus causingwarming temperatures. After passing the stratopause, we are further and further from theozonosphere. This explains why thetemperature cools as we climb the mesosphere.Note that cooler air resides in the lower stratosphere while warmer airresides in the upper stratosphere. Sincecool fluids do not rise, the cool air in the lower stratosphere does not riseto the upper stratosphere. Conversely,since warm fluids do not sink, the warm air in the upper stratosphere does notsink to the lower stratosphere.Therefore, there is no convection (circulation) of air within thestratosphere. The air in the stratosphereremains layered based on temperature.This explains why this layer of the atmosphere iscalled the stratosphere, since the air is stratified or layered. The word stratify is derived from a Latinword meaning layer. As we discussedearlier in the course, the word stratum (a layer of sedimentary rock) derivesfrom the same Latin word. The air in themesosphere cools as we climb the mesosphere, just as air in the tropospherecools as we climb the troposphere. Thismay lead us to conclude that there is significant convection (circulation) ofair within the mesosphere just as in the troposphere, thus causing an abundanceof meteorological phenomena (weather) within the mesosphere. However, the Earth’s gravity pulls most ofthe air in the entire atmosphere down into the troposphere. Although there is convection (circulation) ofair within the mesosphere, the air within this layer is too thin for thisconvection (circulation) to result in an abundance of meteorological phenomena(weather) within the mesosphere. TheGreek root meso- means middle. For example, Central America is sometimes called Mesoamerica, as in Middle America. Therefore, the word mesosphere simply meansmiddle sphere or middle layer. Theionosphere is in the upper thermosphere, near the thermopause. This explains why the temperature warms as weclimb the thermosphere. After passingthe mesopause, we approach the ionosphere, whichserves as a heat source, thus causing warming temperatures. After passing the thermopause,we are further and further from the ionosphere.This explains why the temperature cools as we climb the exosphere. Note that cooler air resides in the lowerthermosphere while warmer air resides in the upper thermosphere. Since cool fluids do not rise, the cool airin the lower thermosphere does not rise to the upper thermosphere. Conversely, since warm fluids do not sink,the warm air in the upper thermosphere does not sink to the lowerthermosphere. Therefore, there is noconvection (circulation) of air within the thermosphere. The air in the thermosphere remains layeredbased on temperature. This is similar tothe air in the stratosphere, but note that the air in the thermosphere is muchthinner than the air in the stratosphere, since the Earth’s gravity pulls airdownward. As we climb the exosphere, theair becomes cooler and cooler, smoothly transitioning into the very coldtemperatures of the surrounding outer space.The Greek root exo- means outside orexternal. For example, an exoskeleton isa skeleton that is outside (surrounding) an organism. Therefore, the word exosphere simply meansexternal sphere or external layer. Theexosphere is a layer of gas that is not strictly part of the Earth’s atmospherebut is outside (surrounding) the Earth’s atmosphere that smoothly transitions intothe gas of the surrounding outer space.
All of us have a basicunderstanding of the seasons: it is warmer in summertime and colder inwintertime. It is acommon misconception that the seasons occur because of the varyingdistance of planet Earth from the Sun. Supposedly when our planet Earth is closer to the Sun, it iswarmer causing summertime, and supposedly when our planet Earth is further fromthe Sun, it is colder causing wintertime.This argument seems reasonable, but it is completely wrong. The orbit of the Earth around the Sun isalmost a perfect circle, meaning that the Earth is roughly the same distancefrom the Sun throughout the entire year.Of course, the true shape of the Earth’s orbit around the Sun is anellipse; sometimes the Earth is closer to the Sun than average, and other timesthe Earth is further from the Sun than average.However, the eccentricity of the Earth’s elliptical orbit is so close tozero that the orbit is almost a perfect circle.The eccentricity of an ellipse quantifies the elongation of the ellipse.When the eccentricity is zero, theellipse is a perfect circle. When theeccentricity is close to zero, the ellipse is almost a perfect circle. The eccentricity of the Earth’s ellipticalorbit around the Sun is so close to zero that its orbit is almost a perfectcircle. When the Earth is at perihelion(closest to the Sun), it is roughly 2.5 million kilometers (roughly 1.5 millionmiles) closer to the Sun than average.When the Earth is at aphelion (furthest from the Sun), it is roughly 2.5million kilometers (roughly 1.5 million miles) further from the Sun thanaverage. These closer or furtherdistances may seem large, but the Earth is on average roughly 150 millionkilometers (roughly 93 million miles) from the Sun. Therefore, these closer or further distancesare less than two-percent variations from the average distance between theEarth and the Sun, and this is not enough of a difference to cause theseasons. There is a spectacular piece ofevidence that will forever bury the misconception that the varying distance ofthe Earth from the Sun causes the seasons: the Earth is closest to the Sun inwintertime and furthest from the Sun in summertime! The Earth is at its perihelion on roughlyJanuary 03rd every year, but early January is inwintertime! The Earth is at its aphelionon roughly July 03rd every year, but early July is insummertime! Therefore, it is absolutely not the Earth’s varying distance from the Sunthat causes the seasons. Caution: we donot argue that distance from the Sun is completely irrelevant. Obviously, if we were to move the Earth fiftymillion kilometers closer to the Sun, of course the planet would become so hotthat it would no longer be habitable for life (all life, including all of us,would die). Obviously, if we were tomove the Earth fifty million kilometers further from the Sun, of course theplanet would become so cold that it would no longer be habitable for life (alllife, including all of us, would die).However, if we were to move the Earth only a couple million kilometerscloser to or further from the Sun, this would not be enough to affect theEarth’s average temperature. The proofof this assertion is that this already occurs; every year as the Earth orbitsthe Sun on its elliptical orbit, the Earth moves roughly 2.5 million kilometerscloser to the Sun at perihelion and roughly 2.5 million kilometers further fromthe Sun at aphelion, and these variations do not affect the average temperatureof the planet. In fact, planet Earth isclosest to the Sun in wintertime and furthest from the Sun in summertime!
After discussing intremendous detail what does not cause the seasons, we must finally discuss whatdoes cause the seasons. The Earth’srotational axis is tilted from the vertical, the vertical being defined asperpendicular to the plane of the Earth’s orbit around the Sun. The tilt of any planet’s rotational axis is called the obliquity of the planet. The seasons are causedby the Earth’s obliquity, the tilt of its rotational axis. The obliquity of planet Earth is roughly 23½degrees. As the Earth orbits the Sun,this 23½ degrees of obliquity remains fixed to an excellent approximation. Thus, as the Earth orbits the Sun, sometimesthe Earth’s northern hemisphere will be tilted towardthe Sun, causing that hemisphere to receive more direct sunlight thus causingwarmer summertime. Thewarmer summertime is also caused by daytime being longer than nighttime, as wewill discuss shortly. At the sametime the Earth’s northern hemisphere is tilted towardthe Sun, the Earth’s southern hemisphere is tilted away from the Sun, causingthat hemisphere to receive less direct sunlight thus causing colderwintertime. The colderwintertime is also caused by nighttime being longer than daytime, as we willdiscuss shortly. Six months laterwhen the Earth is on the opposite side of its orbit, the Earth’s northernhemisphere will be tilted away from the Sun, causing that hemisphere to receiveless direct sunlight and causing that hemisphere to have longer nighttime than daytime,thus causing colder wintertime. At thesame time the Earth’s northern hemisphere is tilted away from the Sun, theEarth’s southern hemisphere is tilted toward the Sun, causing that hemisphereto receive more direct sunlight and causing that hemisphere to have longerdaytime than nighttime, thus causing warmer summertime. This is remarkable; the seasons are reversed in the two hemispheres at the same time! As another example, when it is springtime inthe northern hemisphere, it is autumntime in thesouthern hemisphere at the same time.This means that the Earth is at perihelion (closest to the Sun) duringthe southern hemisphere’s summertime, and the Earth is at aphelion (furthestfrom the Sun) during the southern hemisphere’s wintertime. We may be tempted to conclude that thesouthern hemisphere’s summertime is especially hot, and the southernhemisphere’s wintertime is especially cold.This is false; the opposite is true!Summers are typically hotter in the northern hemisphere as compared withsummers in the southern hemisphere, and winters are typically colder in thenorthern hemisphere as compared with winters in the southern hemisphere! In other words, both summers and winters are more mild in the southern hemisphere as compared with the northernhemisphere, where both summers and winters are more severe. As we discussed earlier in the course, thisis because the continents are presently somewhat crowded together in thenorthern hemisphere, making the southern hemisphere mostly covered with ocean(water). Water has a large heatcapacity, meaning that it is difficult to change the temperature of water. Therefore, the abundance of water in thesouthern hemisphere stabilizes temperatures, causing smaller temperaturevariations in the southern hemisphere as compared with larger temperaturevariations in the northern hemisphere.This spectacularly emphasizes that variations in the distance from theSun do not determine seasonal temperatures.Again, summers are more mild (less hot) in the southern hemisphere, eventhough the Earth is closest to the Sun during summertime in the southernhemisphere, while summers are more severe (more hot) in the northernhemisphere, even though the Earth is furthest from the Sun during summertime inthe northern hemisphere! Similarly,winters are more mild (less cold) in the southern hemisphere, even though theEarth is furthest from the Sun during wintertime in the southern hemisphere,while winters are more severe (more cold) in the northern hemisphere, even thoughthe Earth is closest to the Sun during wintertime in the northernhemisphere! The large heat capacity ofliquid water is also responsible for moderating the temperature differencebetween daytime and nighttime on our planet Earth. The daytime side of any planet faces towardthe Sun, while the nighttime side of any planet faces away from the Sun. For most planets, nighttime is much colderthan daytime, but the nighttime side of planet Earth is only slightly coolerthan its daytime side, thanks to the stabilizing effect of all the water thatcovers most of the planet.Extraordinarily, the nighttime side of planet Earth may at times becomewarmer than the daytime side depending upon weather patterns, as we will discuss. As another example of how water stabilizestemperatures on our planet Earth, other planets have north poles and southpoles that are much colder than their equators.Although the Earth’s poles are cold and the Earth’s equator is hot byhuman standards, the difference in temperature is nevertheless moderate ascompared with other planets. Without theabundance of water that covers our planet Earth, our poles would be too coldand our equator would be too hot to be habitable for life.
The moment when the Earth’snorthern hemisphere is tilted the most toward the Sunis called the summer solstice. Thisoccurs on average June 21st every year; some years it could occur one or twodays earlier, while other years it could occur one or two days later. Since the northern hemisphere is tilted the most toward the Sun on the summer solstice,the Sun appears to be highest in the sky, since the northern hemispherereceives the most direct sunrays. Thisis also the longest daytime and the shortest nighttime of the year in thenorthern hemisphere. The preciseduration of daytime and nighttime depends upon our latitude. At the midlatitudes,there are roughly fifteen hours of daytime and only roughly nine hours ofnighttime on the summer solstice. Notethat the sum of fifteen hours and nine hours is twenty-four hours. Six months later when the Earth is on theother side of its orbit around the Sun, there is a moment when the Earth’snorthern hemisphere is tilted the most away from theSun. This moment is called the wintersolstice, occurring on average December 21st every year; some years it couldoccur one or two days earlier, while other years it could occur one or two dayslater. Since the northern hemisphere is tilted the most away from the Sun on the winter solstice,the Sun appears to be lowest in the sky, since the northern hemisphere receivesthe least direct sunrays. This is alsothe longest nighttime and the shortest daytime of the year in the northernhemisphere. The precise duration ofnighttime and daytime depends upon our latitude, but it will always be thereverse of the summer solstice. At the midlatitudes for example, there are roughly fifteen hoursof nighttime and only roughly nine hours of daytime on the wintersolstice. Note again that the sum offifteen hours and nine hours is twenty-four hours. Halfway in between the solstices are twoother moments called the equinoxes when the Earth’s axis is not tilted towardor away from the Sun, resulting in equal amounts of daytime and nighttime (twelvehours each). This is why they are called equinoxes, since there are equal amounts ofdaytime and nighttime. Roughly threemonths after the summer solstice (roughly three months before the wintersolstice) is the autumn equinox, occurring on average September 21st everyyear; some years it could occur one or two days earlier, while other years itcould occur one or two days later. Roughly three months after the winter solstice (roughlythree months before the summer solstice) is the vernal equinox (or the springequinox). The vernal equinox (springequinox) occurs on average March 21st every year; some years it could occur oneor two days earlier, while other years it could occur one or two dayslater. It is a common misconception thatsince every day is twenty-four hours, supposedly every day has twelve hours ofdaytime and twelve hours of nighttime.This is false for almost every day the entire year. In fact, there are only two days of theentire year when this is the case: the equinoxes. Once we pass the vernal equinox (spring equinox),every day for the next six months there is more daytime than nighttime, withmaximum daytime on the summer solstice.Once we pass the autumn equinox, every day for the next six months thereis more nighttime than daytime, with maximum nighttime on the winter solstice.
The term summer solstice hasat least two different yet interrelated meanings. We may interpret the summer solstice as thelocation on the Earth’s orbit where its northern hemisphere istilted the most toward the Sun.We may also interpret the summer solstice as the moment in time when theEarth’s northern hemisphere is tilted the most toward the Sun, occurring onaverage June 21st every year. Obviously, the Earth is located at itsorbital summer solstice at the moment of the temporalsummer solstice. The terms wintersolstice, vernal equinox, and autumn equinox have similar interpretations. The term winter solstice has at least twodifferent yet interrelated meanings. Wemay interpret the winter solstice as the location on the Earth’s orbit whereits northern hemisphere is tilted the most away fromthe Sun. We may also interpret thewinter solstice as the moment in time when the Earth’s northern hemisphere istilted the most away from the Sun, occurring on average December 21st every year.Obviously, the Earth is located at its orbital winter solstice at the moment of the temporal winter solstice. The term vernal equinox (spring equinox) hasat least two different yet interrelated meanings. We may interpret the vernal equinox (springequinox) as the location on the Earth’s orbit where its axis isnot tilted toward or away from the Sun as it journeys from the orbitalwinter solstice toward the orbital summer solstice. We may also interpret the vernal equinox(spring equinox) as the moment in time occurring on average March 21st every year when the Earth’s axis is not tilted towardor away from the Sun after the temporal winter solstice but before the temporalsummer solstice. Obviously, the Earth islocated at its orbital vernal equinox at the moment ofthe temporal vernal equinox. Finally,the term autumn equinox has at least two different yet interrelatedmeanings. We may interpret the autumnequinox as the location on the Earth’s orbit where its axis isnot tilted toward or away from the Sun as it journeys from the orbitalsummer solstice toward the orbital winter solstice. We may also interpret the autumn equinox asthe moment in time occurring on average September 21stevery year when the Earth’s axis is not tilted toward or away from the Sunafter the temporal summer solstice but before the temporal wintersolstice. Obviously, the Earth islocated at its orbital autumn equinox at the moment ofthe temporal autumn equinox.
As we discussed earlier inthe course, the latitude of any location on planet Earth isdefined as its angle north or south from the equator. The colatitude of any location on planetEarth is defined as its angle from the north pole. Since there are ninety degrees of latitudefrom the equator to the north pole, this makes thecolatitude equal to ninety degrees minus the latitude. For example, if our latitude is ten degreesnorth, this means we are ten degrees of latitude north of the equator, makingus eighty degrees from the north pole; therefore, our colatitudeis eighty degrees. Indeed, ninety minusten equals eighty. As another example,if our latitude is seventy degrees north, this means we are seventy degrees oflatitude north of the equator, making us twenty degrees from the north pole; therefore, our colatitude is twentydegrees. Indeed, ninety minus seventyequals twenty. The only location onplanet Earth where our latitude and our colatitude equal the same number is atforty-five degrees north latitude, since that would place us halfway between(equidistant from) the equator and the north pole. Indeed, ninety minus forty-five equalsforty-five. The altitude of the Sun atnoon on the summer solstice equals our colatitude plus the obliquity. The altitude of the Sun at noon on the wintersolstice equals our colatitude minus the obliquity. The altitude of the Sun at noon on eitherequinox equals our colatitude.Everything we have discussed applies not just to planet Earth but alsoto any other planet orbiting the Sun.The obliquity of any planet is the angular tilt of its rotational axisfrom the vertical direction that is perpendicular to its own orbital planearound the Sun. The poles of any planetare where its own rotational axis intersects the planet. The equator of any planet is halfway betweenthe two poles of the planet. Ourlatitude on that planet would be our angle north or south from that planet’sequator. The planet’s equator would be0° latitude on that planet. The planet’snorth pole would be 90°Nlatitude on that planet, and the planet’s south pole would be 90°S latitude on that planet. Our colatitude on that planet would be ourangle from that planet’s north pole, which would againbe ninety degrees minus our latitude.The summer solstice of any planet is the moment when its northernhemisphere is tilted the most towards the Sun. The winter solstice for any planet is themoment when its northern hemisphere is tilted the mostaway from the Sun. The equinoxes of anyplanet is halfway between the solstices when its rotational axis is not tilted toward or away from the Sun. The altitude of the Sun at noon on each ofthese dates would be the same equations: colatitude plus obliquity on thesummer solstice, colatitude minus obliquity on the winter solstice, andcolatitude on the equinoxes. The onlydifference in this analysis for other planets are the actual dates of thesolstices and the equinoxes. For anyplanet orbiting the Sun, the time from one solstice to the next solstice (whichis also the time from one equinox to the next equinox) is one-half of theplanet’s orbital period around the Sun.The time from one solstice to the next equinox (which is also the timefrom one equinox to the next solstice) is one-quarter of the planet’s orbitalperiod around the Sun. As an example,consider a hypothetical planet with an obliquity of thirty degrees, and supposewe live at fifty degrees north latitude on this hypothetical planet. Since our latitude is fifty degrees north,our colatitude is forty degrees, since ninety minus fifty equals forty. Hence, the altitude of the Sun at noon on thesummer solstice would be seventy degrees, since the colatitude plus theobliquity is forty plus thirty, which equals seventy. The altitude of the Sun at noon on the wintersolstice would be ten degrees, since the colatitude minus the obliquity isforty minus thirty, which equals ten.The altitude of the Sun at noon on either equinox would be fortydegrees, since that is our colatitude.
It is a common misconceptionthat the Sun is directly overhead at noon.This misconception comes from the phrase high noon. Of course, the Sun is highest in the sky atnoon, giving this phrase some validity.Nevertheless, the Sun is never ever directly overhead at most locationson Earth. For example, suppose we liveat forty degrees north latitude. Ourcolatitude would be fifty degrees, since ninety minus forty equals fifty. The highest the Sun would ever be at thislocation is on the summer solstice, when its altitude at noon is 73½ degrees,since our colatitude plus obliquity is fifty plus 23½, which is indeed 73½degrees. Although 73½ degrees is a highaltitude, it is not directly overhead.Directly overhead would be ninety degrees of altitude. If the Sun is notdirectly overhead at noon on the summer solstice, it would only be lower in thesky every other day of the year. Thisshows that the Sun is never ever directly overhead at most locations onEarth. Is there anywhere on planet Earthwhere the Sun is directly overhead at noon on the summer solstice? Yes, at a latitude equal to the same numberof degrees north of the equator as the obliquity of planet Earth. To prove this, suppose we live at 23½ degreesnorth latitude, then our colatitude would be 66½degrees, since ninety minus 23½ equals 66½. Thus, the altitude of the Sun at noon would beour colatitude 66½ degrees plus the obliquity ofplanet Earth 23½ degrees, but 66½ plus 23½ equals ninety degrees of altitude,directly overhead! This location of 23½degrees north latitude is so important that it deserves a special name: theTropic of Cancer, as we discussed earlier in the course. There is only one location on planet Earthwhere the Sun is directly overhead at noon on the winter solstice: 23½ degrees southlatitude. This location is so importantthat it deserves a special name: the Tropic of Capricorn, as we discussedearlier in the course. The words Cancerand Capricorn refer to astronomical constellations of the zodiac; the reasonthese lines of latitude are named for astronomicalconstellations of the zodiac is beyond the scope of this course. There is only one locationon planet Earth where the Sun is directly overhead at noon on the equinoxes:the equator at zero degrees latitude.Thousands of years ago, primitive humans did not understand that theEarth is a planet with a tilted rotational axis orbiting the Sun. For thousands of years, humans believed thatthe motion of the Sun was responsible for the seasons. Although today we understand that it is actually the Earth that is orbiting the Sun, wemust also confess that when we look up into the sky, it does appear as if theSun is moving. Therefore, we shouldunderstand the seasons from the frame of reference of the Earth, which was theonly understanding of humans for thousands of years. From the frame of reference of the Earth, theSun appears to be directly on top of the Tropic of Cancer on the summersolstice. For the next six months, theSun appears to move south, arriving on top of the equator three months later onthe autumn equinox and arriving on top of the Tropic of Capricorn three monthsafter that on the winter solstice. Forthe following six months, the Sun appears to move north, arriving on top of theequator three months later on the vernal equinox (spring equinox) and arrivingon top of the Tropic of Cancer three months after that on the summersolstice. Again, it isnot the Sun that is actually moving north and south; in actuality, theEarth is orbiting the Sun. Nevertheless,we live on planet Earth, and so we must understand the seasons from the frameof reference of the Earth. To summarize,it is only possible for the Sun to appear directly overhead at noon if we livesomewhere between the Tropic of Cancer and the Tropic of Capricorn. If we live north of the Tropic of Cancer orsouth of the Tropic of Capricorn, the Sun never ever appears to be directlyoverhead.
The Arctic Circle is 66½degrees north latitude, making its colatitude 23½ degrees, since ninety minus66½ equals 23½. The altitude of the Sunat noon at the Arctic Circle on the winter solstice would be zero degrees,since our colatitude minus the obliquity would be 23½ minus 23½, which isobviously zero. An altitude of zerodegrees means the Sun is on the horizon, such as during sunrise. At even more northern latitudes, the altitudeof the Sun on the winter solstice will be a negative number, which means it isbelow the horizon. In other words, wecannot see the Sun, making it nighttime even though it is noon! Before noon or after noon, the Sun will beeven further below the horizon. Thus,the entire day is in perpetual nighttime!The same occurs on the Antarctic Circle at 66½ degrees south latitude:the altitude of the Sun at noon on the summer solstice is zero degrees, meaningthat it is on the horizon. At even moresouthern latitudes, the altitude of the Sun on the summer solstice will be anegative number, which means it is below the horizon. Again, we cannot see the Sun, making itnighttime even though it is noon! Beforenoon or after noon, the Sun will be even further below the horizon. Thus, the entire day is in perpetualnighttime! These extreme northernlatitudes and extreme southern latitudes are the only places on Earth where theSun may never rise on some days of the year and may never set on other days ofthe year. At the north pole, six monthsof continuous nighttime occurs from the autumn equinox all the way to thevernal equinox (spring equinox), and then six months of continuous daytime occursfrom the vernal equinox (spring equinox) all the way to the autumnequinox. The reverse occurs at the south pole: six months of continuous nighttime occurs fromthe vernal equinox (spring equinox) all the way to the autumn equinox, whilesix months of continuous daytime occurs from the autumn equinox all the way tothe vernal equinox (spring equinox).Even when the clock time is midnight, the Sun may still be in the skycausing daytime at these extreme latitudes.This is the origin of the phrase midnight Sun.
There areseveral religious holidays thathave their origins in the solstices and the equinoxes. For example, Christmas Day is observed onDecember 25th every year. Notice that this is shortly after the wintersolstice. Before Christmas Day becamethe celebration of the birth of Jesus Christ, this was a pagan holidaycelebrating the winter solstice. Whywould pagans celebrate the day when the Sun appeared to be lowest in the sky atnoon with the most number of nighttime hours?Primitive humans observed the Sun appear lower and lower in the skyafter the summer solstice; many primitive humans were probably terrified thatthe Sun would continue to move downward until it disappeared below thehorizon. However, by simply payingattention every year, we observe that the Sun stops moving downward on thewinter solstice, and then begins to move upward. This was a reason to celebrate for manyancient pagans. This pagan celebrationbecame the celebration of the birth of Jesus Christ, since early Christians sawthe birth of Jesus Christ as bringing more and more light into a spirituallydark world. In actuality, Jesus was notborn on December 25th.We will never be certain of the exact day of the birth of Jesus. Most of the details of the vast majority ofpersons throughout human history, including Jesus, were neverrecorded. We will also never becertain of the exact year of the birth of Jesus. Although it is commonly believed that theyear of the birth of Jesus was anno Domini 1, inactuality no one recorded the year that Jesus was born. Again, most of the details of the vastmajority of persons throughout human history, including Jesus, were never recorded.The people of the world did not declare the year to be anno Domini 1when Jesus was born. The ancient Romansdesignated years using a few different numbering schemes, among which was thenumber of years since the founding of the city of Rome, which was more thanseven centuries before the birth of Jesus.Using this particular convention, the ancient Romans designated years using either of the Latin phrases “ab urbe condita” or “anno urbisconditae,” meaning “in the year since the founding of the city.” Either Latin phrase was abbreviated AUC. This numberingscheme continued to be used by Europeans even afterthe fall of the Western Roman Empire.More than five centuries after the birth of Jesus and roughly fiftyyears after the Western Roman Empire fell, the Christian monk Dionysius Exiguus tried to determine the year that Jesus wasborn. This monk declared that the year AUC 1278 should be redesignatedA.D. 525, where A.D. is the abbreviation for the new Latin phrase “anno Domini”meaning “in the year of our Lord,” effectively meaning the number of yearssince the birth of Jesus. Over the nextfew centuries, Europeans retroactively changed the years of historical datesfrom AUC to A.D. based on this declaration byDionysius Exiguus.Europeans even changed the years of historical dates before the birth ofJesus from AUC to B.C., which is simply anabbreviation for “before Christ.” Modernscholarship has revealed that the year that Jesus was born as determined byDionysius Exiguus is probably a few years inerror. Modern scholarship estimates that6 B.C. is a more accurate estimate for the year of the birth of Jesus. Many students are offendedby this assertion. Students claim thatit would be contradictory for Jesus to have been born in the year 6 B.C., sinceB.C. is the abbreviation for before Christ and no one can be born before theyear of their own birth! Again, we mustremember that no one recorded the date that Jesus was born. We must remember that thebelief that the year of the birth of Jesus was A.D. 1 is based upon an estimateby a monk who lived roughly five centuries after Jesus was born. In fact, we should marvel that the estimatemade by Dionysius Exiguus for an unrecorded eventthat occurred roughly five centuries earlier was only a few years inerror! As another example of a religiousholiday that has its origin in the solstices and the equinoxes, Easter isalways the first Sunday after the first Full Moon after the vernal equinox(spring equinox) every year. Since Jesuswas Jewish, what is traditionally called the LastSupper was in actuality a Passover celebration.The Jewish calendar is a lunar calendar, and hence Jewish holy days aredetermined by the cycles of the Moon.Placing Easter on the first Sunday after the first Full Moon after thevernal equinox (spring equinox) is an attempt to keep the date of Easter asclose as possible to the date of Passover.
A thermometer is a device tomeasure temperature. The operation of athermometer is based on the principle of thermalexpansion and thermal contraction. Mostsubstances expand when they become warmer, and most substances contract whenthey become colder. To build a primitivethermometer, we take any object and measure its length at one temperature, andwe measure its different length at a different temperature. We draw marks at each of these lengths, andwe draw other marks between these two marks.To determine the temperature, we simply read off whichever mark the endof the object meets based on its length at that temperature. Unfortunately, most substances expand andcontract by only tiny amounts when their temperature changes. Hence, the marks are oftentoo close together, making differences in length difficult tomeasure. However, the element mercuryexpands by quite a noticeable amount as it becomes warmer and contracts byquite a noticeable amount as it becomes colder.Therefore, most thermometers use liquid mercury, since the marks arethen well separated and easy to read. Anactinometer is a device that measures solarradiation. To build a primitive actinometer, we take any object and use a thermometer tomeasure its initial temperature. Then,we place the object in sunlight for a certain amount of time, perhaps onehour. As the object absorbs solarradiation, it becomes hotter. We use athermometer to measure its hotter temperature afterwards, and from thedifference in temperature between its hotter final temperature and its colderinitial temperature, we can calculate the amount of solar radiation the objectabsorbed. Note that we must wrap theobject in black cloth to ensure that it absorbs all of the solarradiation. Otherwise, the object willonly become hotter by some of the solar radiation that it absorbed, since theobject will reflect the rest of the solar radiation. It is convenient to use a bucket of water asthe object, since we know the heat capacity of water. An actinometerwould measure the greatest amount of solar radiation on the summer solstice,and an actinometer would measure the least amount ofsolar radiation on the winter solstice.However, the summer solstice is almost never the hottest day of theyear; a thermometer measures the hottest air temperature roughly a month later,in late July in the northern hemisphere.Similarly, the winter solstice is almost never the coldest day of theyear; a thermometer measures the coldest air temperature roughly a month later,in late January in the northern hemisphere.These delays occur because the Earth is mostly coveredwith water, which has a large heat capacity.In other words, it is difficult to change the temperature of water. The northern hemisphere receives the mostdirect sunrays on the summer solstice around June 21st, but it still takesanother month for the air to warm to maximum temperature sometime in lateJuly. In fact, it takes yet anothermonth for the ocean waters to warm to maximum temperature, in late August inthe northern hemisphere. This is whymost people in the northern hemisphere take their summer vacations in August,so that they may enjoy swimming in the ocean when it is warmest. The northern hemisphere receives the leastdirect sunrays on the winter solstice around December 21st, but the oceans haveretained so much heat from the summertime that it still takes another month forthe air to cool to minimum temperature sometime in late January. In fact, it takes yet another month for theocean waters to cool to minimum temperature, in late February in the northernhemisphere.
Local (Small-Scale) Meteorological Dynamics
Aside from the seasonaltemperature variations we have discussed, many other variables affect the airtemperature on a daily basis, on an hourly basis, and even shortertimescales. These variations in airtemperature cause variations in air pressure.These variations in air pressure cause the meteorological phenomena(commonly known as weather) that we will discuss. Air pressure is the force that the air exertsper unit area. This pressure (force perunit area) is ultimately caused by molecularcollisions. Therefore, we may interpretair pressure as a measure of how frequently air molecules collide with eachother. Warm air is at a lower pressure,while cold air is at a higher pressure.This is because the molecules of warm air are moving faster, enablingthem to move further from one another; hence, they collide with each other lessfrequently since they are further apart from one another. Conversely, the molecules of cold air aremoving slower; they cannot move far from one another and hence they collidewith each other more frequently. Supposeall the air in a certain room is at the same pressure, and consider a parcel ofair in the middle of the room. Since theair pressure on either side of the parcel of air is the same, it will sufferequal molecular collisions from either side of itself. These equal molecular collisions will balanceeach other, and the parcel of air will not move. Now suppose instead that the air on one sideof the room is at a higher air pressure for whatever reason, and suppose theair on the other side of the room is at a lower air pressure for whateverreason. Again, consider a parcel of airin the middle of the room. This parcelof air will now suffer greater molecular collisions from the higher-pressureside of the room, and the parcel of air will suffer fewer molecular collisionsfrom the lower-pressure side of the room.The net result is that the parcel of air will bepushed from the higher pressure toward the lower pressure. The parcel of air will move, since it is pushed by a pressure imbalance. This pressure imbalance is so important thatit deserves a name; it is called the pressure gradientforce. If the air pressure throughoutthe room were the same, there would be no pressure gradient and hence no force;the air would not move. If there arevariations in pressure, the pressure gradient force pushes air from higherpressure toward lower pressure. Movingair is called wind.Hence, the pressure gradient force causes wind to blow.
A geometrical curveconnecting locations of equal air pressure is calledan isobar. The pressure gradient forceis always zero along any isobar, since every point on an isobar is at the samepressure. Since the pressure gradientforce is always zero along any isobar, the pressure gradient force must pointdirectly perpendicular to isobars. Ifthe pressure gradient force did not point directly perpendicular to isobars,then we would be able to break the force into two components: one componentdirectly perpendicular to the isobars and the other component along theisobars. However, the component alongthe isobars must be zero, as we just argued.Therefore, the pressure gradient force can only have one component: thecomponent directly perpendicular to the isobars. The pressure gradient force does not have twocomponents; it only has one component that is directly perpendicular to theisobars. Every point on an isobar is atthe same air pressure, but two different isobars will of course be at twodifferent pressures. Suppose as we movefrom one isobar to a neighboring isobar, the pressure always drops by a definiteamount, perhaps ten millibars. If isobars are closely spaced to each other,this means that the pressure drops by ten millibarsover a narrow distance. In other words,the pressure gradient will be steep, thus causing strong winds. If the isobars are widely spaced from eachother, this means that the pressure drops by ten millibarsover a wide distance. In other words,the pressure gradient will be shallow, thus causing light winds. This is remarkable, since the pressure dropfrom one isobar to the neighboring isobar is always a fixed amount: ten millibars in these examples. Nevertheless, the ten-millibarpressure drop is steep if the isobars are closely spaced to each other, whilethe ten-millibar pressure drop is shallow if theisobars are widely spaced from each other.Again, this is remarkable: the same ten millibar pressure drop is steep causing strong windsto blow in one case, while the same ten millibarpressure drop is shallow causing light winds to blow in another case.
An anemometer is a devicethat measures the velocity of wind, meaning that an anemometer measures boththe speed and the direction of wind. Ananemometer is essentially a wind vane together with a flag. As the wind blows, the wind vane turns with acertain angular speed. From that angularspeed, we can calculate the speed with which the wind blows. A wind vane shaped like a rooster is called a weatherco*ck.The flag reveals the direction with which the wind blows; whichever waythe flag flutters is the direction the wind is blowing. The Beaufort scale is a wind scale, named forthe Irish oceanologist/oceanographer FrancisBeaufort. The Beaufort scale usesnumbers from zero (for no winds) to twelve (for hurricane-speed winds). A low number on the Beaufort would be a lightwind, which is called a breeze. A middle number on the Beaufort scale would simply be called a wind. A high number on the Beaufort scale would bea strong wind, which is called a gale.
We have already discussedenough basic meteorology to analyze some simple weather patterns. Suppose we are at the beach in the daytimewhen the Sun warms the Earth. Sincewater has a large heat capacity, the ocean does not become as warm as thecontinent. All of us have experiencedthis while at the beach in the daytime; no matter how hot the daytimetemperature, the ocean water is not as warm as the sand. Therefore, the air above the continent iswarmer than the air above the ocean.Thus, the air above the continent is at a lower pressure as comparedwith the air above the ocean, which is at a relatively higher pressure. Since the pressure gradient force pushes airfrom high pressure toward low pressure, wind will blow from the ocean towardthe continent. This iscalled the sea breeze. Inmeteorology, we always name wind based on the direction it is blowing from,which is the opposite of the direction the wind is blowing toward. For example, a wind blowing from the north(which means it is blowing toward the south) is calleda north wind. As another example, a windblowing from the southwest (which means it is blowing toward the northeast) is called a southwest wind.The sea breeze is blowing from the ocean toward the continent; hence, itis called the sea breeze. We often feel this sea breeze while at thebeach in the daytime. The sea breeze isa steady wind blowing from the ocean toward the continent during thedaytime. In the nighttime, the oppositeoccurs. Suppose we are at the beach inthe nighttime when the Earth cools.Since water has a large heat capacity, the ocean does not become as coldas the continent. Perhaps some of ushave experienced this while at the beach in the nighttime; no matter how coolthe nighttime temperature, the ocean water is not as cold as the sand. Therefore, the air above the continent iscolder than the air above the ocean.Thus, the air above the continent is at a higher pressure as comparedwith the air above the ocean, which is at a relatively lower pressure. Since the pressure gradient force pushes airfrom high pressure toward low pressure, wind will blow from the continenttoward the ocean. This is called the land breeze.Again, we always name wind based on the direction it is blowing from,which is the opposite of the direction the wind is blowing toward. The land breeze is blowing from the continenttoward the ocean; hence, it is called the landbreeze. Perhaps some of us have feltthis land breeze while at the beach in the nighttime. The land breeze is a steady wind blowing fromthe continent toward the ocean during the nighttime. If we are facing the ocean, we feel the landbreeze upon our backs; if we turn our backs to the ocean, we feel the landbreeze upon our fronts. Similar to thesea breeze and the land breeze is the valley breeze and the mountainbreeze. In the daytime, the Sun warmsthe air. Hot air is less dense, and so hot air will be buoyed upward by the surrounding air. This is why hot air rises. Therefore, daytime winds will blow from avalley up toward a mountain. This is called the valley breeze.Again, we always name wind based on the direction it is blowing from,which is the opposite of the direction the wind is blowing toward. The valley breeze is blowing from the valleyup toward the mountain; hence, it is called the valleybreeze. Perhaps some of us have feltthis valley breeze while on a mountain in the daytime. The valley breeze is a steady wind blowingfrom the valley up toward the mountain during the daytime. In the nighttime, the opposite occurs. The air cools in thenighttime. Cold air is more dense, and so cold air will descend into thesurrounding air. This is why cold airsinks. Therefore, nighttime winds willblow from a mountain down into a valley.This is called the mountain breeze. Again, we always name wind based on the directionit is blowing from, which is the opposite of the direction the wind is blowing toward. The mountainbreeze is blowing from the mountain down into the valley; hence, it is called the mountain breeze. Perhaps some of us have felt this mountainbreeze while in a valley in the nighttime.The mountain breeze is a steady wind blowing from the mountain downtoward the valley during the nighttime.To summarize, during the daytime the sea breeze blows from the oceantoward the continent, while during the nighttime the land breeze blows from thecontinent toward the ocean. During the daytime the valley breeze blows from the valley up towardthe mountain, while during the nighttime the mountain breeze blows from themountain down toward the valley.
Fictitious forces or pseudoforces are forces that do not actually exist; theyonly seem to exist in certain frames of reference. For example, suppose we are in a stationarycar waiting at a red traffic light. Whenthe red traffic light turns green, we place our foot upon the car’s acceleratorpedal. As the car accelerates forward,everyone and everything in the car feels a backward force. We actually feel ourselves pulled backwardinto the backrest of our chair. Anythinghanging from the rearview mirror also swings backward. This backward force is a fictitious force ora pseudoforce.It does not exist; it only seems to exist within the car as the caraccelerates forward. Although everyoneand everything within the car feels this backward force, it nevertheless doesnot actually exist. In actuality,everyone and everything within the car remains stationary for a moment as thecar and its chairs accelerate forward, and hence the backrests of the chairsaccelerate forward and collide with our own backs. This is amusing: within the car we feel pulled backward into the backrests of thechairs, but in actuality we remain stationary while the backrests of the chairsaccelerate forward into our backs!Although we feel a backward force within the car, we neverthelessconclude that this backward force is a fictitious force or a pseudoforce. It doesnot actually exist; it only seems to exist within the car as the caraccelerates forward. As another example,suppose we are in a moving car when we see a green traffic light turn yellow,and so we place our foot upon the car’s brake pedal. As the car slows down, everyone andeverything in the car feels a forward force.We actually feel ourselves pulled forward off ofthe backrest of our chair. Anythinghanging from the rearview mirror also swings forward. In extreme cases, we may feel pulled forwardso strongly that our heads may collide with the windshield. This forward force is a fictitious force or apseudoforce.It does not exist; it only seems to exist within the car as the carslows down. Although everyone andeverything within the car feels this forward force, it nevertheless does notactually exist. In actuality, everyoneand everything within the car remains in motion for a moment as the car and itschairs and its windshield slow down, and hence thebackrests of the chairs move away from our own backs while the windshield movestoward our heads. This is amusing:within the car we feel pulled forward off of thebackrests of the chairs and toward the windshield, but in actuality thebackrests of the chairs move away from our backs and the windshield movestoward our heads! Although we feel aforward force within the car, we nevertheless conclude that this forward forceis a fictitious force or a pseudoforce. It does not actually exist; it only seems toexist within the car as the car slows down.As yet another example, suppose we are in a moving car when we see thatthe highway ramp ahead curves to the left, and so we turn the steering wheel tothe left so that the car will remain on the highway ramp. As the car turns left, everyone andeverything in the car feels a rightward force.We actually feel ourselves pulled rightward away from the driver’s sideof the car and toward the passenger’s side of the car. Anything hanging from the rearview mirror alsoswings rightward and continues to remain suspended rightward in apparentdefiance of the Earth’s downward gravity as the car turns left! This rightward force is a fictitious force ora pseudoforce.It does not exist; it only seems to exist within the caras the car turns left. Although everyoneand everything within the car feels this rightward force, it nevertheless doesnot actually exist. In actuality,everyone and everything within the car remains in forward motion as the carturns left, and hence the driver’s side of the car turns away from us while thepassenger’s side of the car turns toward us.This is amusing: within the car we feel pulledrightward toward the passenger’s side of the car, but in actuality we remain inforward motion while the passenger’s side of the car turns leftward towardus! Although we feel a rightward forcewithin the car, we nevertheless conclude that this rightward force is a fictitiousforce or a pseudoforce. It does not actually exist; it only seems toexist within the car as the car turns left. As a fourth example, projectiles will appearto suffer from deflections within a rotating frame of reference. This deflecting force is a fictitious forceor a pseudoforce.It does not exist; it only seems to exist within the rotating frame ofreference. In actuality, the projectilesare not deflected; the projectiles in fact continuemoving along straight paths. The frameof reference is rotating, and the rotation of the entire frame of referenceseems to cause projectiles to deviate from straight trajectories. This particular fictitious force or pseudoforce is called the Coriolisforce, named for the French physicist Gaspard-Gustave de Coriolis who firstderived the mathematical equations describing this particular fictitious forceor pseudoforce.The Coriolis force appears to cause rightward deflections in frames ofreference rotating counterclockwise, and the Coriolis force appears to causeleftward deflections in frames of reference rotating clockwise. The Coriolis force appears to cause strongerdeflections if the frame of reference is rotating faster and appears to causeweaker deflections if the frame of reference is rotating slower. The Coriolis force appears to vanish if theframe of reference stops rotating. TheCoriolis force only appears to cause deflections; it does not cause projectilesto speed up or slow down.
If the Earth were notrotating, the study of the Earth’s atmosphere would be relatively simple. The pressure gradient force would simply pushwind perpendicular to isobars from high pressure toward low pressure. However, the Earth is rotating, and therotation of the Earth causes gross complications in the Earth’satmosphere. The Earth rotates from westto east. Therefore, the northernhemisphere rotates counterclockwise when viewed from above the north pole, while the southern hemisphere rotates clockwisewhen viewed from above the south pole.We conclude that there is a Coriolis force on planetEarth that appears to cause rightward deflections in the northern hemisphereand appears to cause leftward deflections in the southern hemisphere. The Coriolis force would appear to bestronger if the Earth were rotating faster, while the Coriolis force wouldappear to be weaker if the Earth were rotating slower. The Coriolis force would appear to vanish ifthe Earth were to stop rotating altogether.The Coriolis force only appears to cause deflections; it does not causeprojectiles to speed up or slow down.Finally, the Coriolis force is weak near the equator (the Coriolis forceis in fact zero at the equator), and the Coriolis force becomes stronger andstronger as we move away from the equator toward the poles (the Coriolis forceis in fact strongest at the poles). Thepressure gradient force still pushes air perpendicular to isobars from highpressure toward low pressure, but in addition the Coriolis force causesdeflections to the right in the northern hemisphere and deflections to the leftin the southern hemisphere. Since theCoriolis force appears to cause these deflections, wind will not blow directlyperpendicular to isobars; wind will not blow directly from high pressure towardlow pressure.
A thermal is a parcel of airin the Earth’s atmosphere. If we have alow-pressure thermal surrounded by high pressure, the pressure gradient forcewill push wind from the surrounding high-pressure air inward toward thelow-pressure thermal. At the same time,the Coriolis force will cause deflections to the right in the northernhemisphere and to the left in the southern hemisphere. The net result of the pressure gradient forcetogether with the Coriolis force is an inward circulation of wind. Winds will blow inward while circulatingcounterclockwise in the northern hemisphere, and winds will blow inward whilecirculating clockwise in the southern hemisphere. If we instead have a high-pressure thermalsurrounded by low pressure, the pressure gradient force will push wind from thehigh-pressure thermal outward toward the surrounding low-pressure air. At the same time, the Coriolis force willcause deflections to the right in the northern hemisphere and to the left inthe southern hemisphere. The net resultof the pressure gradient force together with the Coriolis force is an outwardcirculation of wind. Winds will blowoutward while circulating clockwise in the northern hemisphere, and winds willblow outward while circulating counterclockwise in the southern hemisphere. The weather pattern around a low-pressurethermal is called a cyclone. Tornadoes and hurricanes are extreme examplesof cyclones, as we will discuss. Theweather pattern around a high-pressure thermal is calledan anticyclone. A beautiful clear day isan extreme example of an anticyclone, as we will discuss. In summary, a cyclone is the weather patternaround a low-pressure thermal, where winds blow inward while circulatingcounterclockwise in the northern hemisphere and clockwise in the southern hemisphere;an anticyclone is the weather pattern around a high-pressure thermal, wherewinds blow outward while circulating clockwise in the northern hemisphere andcounterclockwise in the southern hemisphere.
At the center of a cyclone isa low-pressure thermal; this low-pressure thermal has a low density and istherefore buoyed upward by the surrounding air.As an alternative argument, the low pressure is causedby hot temperatures, and hot air must rise.Either argument leads us to conclude that the low-pressure thermal atthe center of a cyclone rises. At thecenter of an anticyclone is a high-pressure thermal; this high-pressure thermalhas a high density and therefore sinks downward into the surrounding air. As an alternative argument, the high pressureis caused by cold temperatures, and cold air mustsink. Either argument leads us toconclude that the high-pressure thermal at the center of an anticyclonesinks. As the low-pressure thermal atthe center of a cyclone rises, the surrounding air pressure at higherelevations decreases in accord with the law of atmospheres. Therefore, the rising low-pressure thermalexpands as its own pressure pushes the surrounding lower-pressure air. As the high-pressure thermal at the center ofan anticyclone sinks, the surrounding air pressure at lower elevationsincreases in accord with the law of atmospheres. Therefore, the high-pressure thermalcontracts as the surrounding high-pressure air compresses the sinkingthermal. In summary, a cyclone is theweather pattern around a low-pressure thermal, where winds blow inward whilecirculating counterclockwise in the northern hemisphere and clockwise in thesouthern hemisphere; the low-pressure thermal then rises to higher elevationsand expands. Conversely, an anticycloneis the weather pattern around a high-pressure thermal that sinks to lowerelevations and contracts, then the winds blow outward while circulatingclockwise in the northern hemisphere and counterclockwise in the southernhemisphere.
The most obvious way tochange the temperature of a gas is through the addition or extraction ofheat. If we add heat to a gas, we expectit to become warmer; if we extract heat from a gas, we expect it to becomecooler. However, it is possible tochange the temperature of a gas without adding or extracting heat. If a gas expands, it must become cooler evenif no heat was extracted. This is because the expanding gas pushes thesurrounding gas. This requires work, andwork is a form of energy. The gasextracts this energy from its own internal energy content, and so the gasbecomes cooler. Converselyif a gas contracts, it must become warmer even if no heat was added. This is because the surrounding gas performswork on the gas while compressing the gas.This work is added to the internal energy contentof the gas, and so the gas becomes warmer.This is remarkable; we can actually change the temperature of a gaswithout adding or extracting any heat.We can demonstrate this by performing all of these experiments while thegas is wrapped within a thermal insulator, which willnot permit any heat to be added or extracted.Nevertheless, the gas becomes warmer when we compress it, and the gasbecomes cooler when we expand it. We are forced to conclude that heat and temperature are twocompletely different physical concepts.Most people naïvely believe that heat and temperature are essentiallythe same thing. After all, when we addheat to an object it often becomes warmer, and when weextract heat from an object it often becomes cooler. Nevertheless, we have just discussedcirc*mstances where we can actually change the temperature of a gas without theaddition or extraction of any heat. Infact, it is possible for a gas to become warmer under certain circ*mstanceswhen we have extracted heat from the gas!It is also possible for a gas to become cooler under certaincirc*mstances when we have added heat to the gas! These extraordinary examples persuade us thatheat and temperature are indeed two completely different physicalconcepts. Therefore, we must usedifferent terms to describe one process where there is no temperature changeand another process where there is no heat exchanged, since heat andtemperature are two completely different physical concepts. A process where there is no temperature changeis called an isothermal process. A process where there is no heat exchanged iscalled an adiabatic process. These aretwo different processes. Simply becausea process is adiabatic (no heat exchanged) does notmean that it is necessarily isothermal (no temperature change). Simply because a process is isothermal (notemperature change) does not mean that it is necessarily adiabatic (no heatexchanged). We just discussed twoexamples of adiabatic processes that are not isothermal. A gas that expands adiabatically (no heatexchanged) becomes cooler; since the temperature is changing, this is not anisothermal process. A gas that contractsadiabatically (no heat exchanged) becomes warmer; since the temperature ischanging, this is not an isothermal process either. These are two examples of processes that areadiabatic yet not isothermal, meaning the temperature changes even though noheat was exchanged.There are other examples of processes that are isothermal yet notadiabatic, meaning heat was exchanged even though thetemperature did not change. Obviously,there are processes that are neither adiabatic nor isothermal. Also note that aprocess where there is no pressure change is called an isobaric process.
Air is a poor conductor ofheat. A spectacular illustration of thepoor conduction of heat through air is the moderate heat we feel from theintense temperature of charcoal during a barbecue. When a piece of charcoal begins glowing red,its temperature is a couple of thousand degrees! Yet, we can place our hand within just a fewinches of the charcoal; although we feel moderate heat, our hand is not indanger from the extreme temperature of the charcoal. How can our hand be within a few inches of anobject at a couple of thousand degrees of temperature and yet not be in anydanger? We conclude that the air betweenour hand and the hot charcoal is a poor conductor of heat. Since air is such a poor conductor of heat,we always assume thermals in the Earth’s atmosphere neither gain heat fromtheir surroundings nor lose heat to their surroundings. That is, we always assume thermals in theEarth’s atmosphere do not exchange heat with their surroundings. This is called theadiabatic approximation. Of course,thermals do exchange some heat with their surroundings, but air is such a poorconductor of heat we can ignore the small amounts of heat that are exchanged between a thermal and its surroundings. In other words, the adiabatic approximationis an excellent approximation for analyzing most meteorological processes. As we discussed, the air at the center of acyclone rises and expands. As anexcellent approximation, the rising thermal expands adiabatically, by theadiabatic approximation. If the thermalexpands adiabatically, then it must cool.As we discussed, the air at the center of an anticyclone sinks andcontracts. As an excellentapproximation, the sinking thermal contracts adiabatically, by the adiabaticapproximation. If the thermal contractsadiabatically, then it must warm. Insummary, a cyclone is the weather pattern around a low-pressure thermal wherewinds blow inward while circulating counterclockwise in the northern hemisphereand clockwise in the southern hemisphere; the low-pressure thermal then rises,expands adiabatically, and cools.Conversely, an anticyclone is the weather pattern around a high-pressurethermal that sinks, contracts adiabatically, and warms, then the winds blowoutward while circulating clockwise in the northern hemisphere andcounterclockwise in the southern hemisphere.
The low-pressure air at thecenter of a cyclone rises, expands adiabatically, and cools, but cold air is ata high pressure. We conclude that low pressure air at lower elevations in the tropospheretransitions to high pressure air at higher elevations in the troposphere. In brief, low air pressure near mean sealevel becomes high air pressure aloft.The word aloft simply means up toward the sky. The loft of a house is the highest room inthe house (often the attic), the choir loft of a church is high above the pews,and lofty dreams are high goals. Ifwinds blow inward toward the low air pressure near mean sea level, these windsmust eventually blow outward from the high air pressure aloft. This circulation of air is a necessaryfeature of convection, as we discussed earlier in the course. Cyclones may initially form from low airpressure at lower elevations in the troposphere (near mean sea level) or fromhigh air pressure at higher elevations in the troposphere (aloft), but generallycyclones initially form from both occurring in conjunction with each other,from the appropriate vertical motion of air throughout the troposphere. Similarly, we conclude that high pressure air at lower elevations in the tropospheretransitions to low pressure air at higher elevations in the troposphere. In brief, high air pressure near mean sealevel becomes low air pressure aloft. Ifwinds blow outward from the high air pressure near mean sea level, these windsmust blow inward toward the low air pressure aloft. Again, this circulation of air is a necessaryfeature of convection, as we discussed earlier in the course. Anticyclones may initially form from high airpressure at lower elevations in the troposphere (near mean sea level) or fromlow air pressure at higher elevations in the troposphere (aloft), but generallyanticyclones initially form from both occurring in conjunction with each other,from the appropriate vertical motion of air throughout troposphere.
Relative humidity is a conceptthat everyone believes that they understand, but in actualityalmost no one correctly understands this concept of relative humidity. Most people believe that the relativehumidity of air is the amount of moisture in the air. This is such a gross simplification of thecorrect definition of relative humidity that it is actually an incorrectunderstanding. Firstly, air is only ableto hold a maximum amount of moisture. Wemay demonstrate this with the following experiment. We place a lid upon a cup of water; a simplepiece of paper will serve as a satisfactory lid. Liquid water continuously evaporates intowater vapor, but the lid will confine the water vapor to the trapped airbetween the lid and the liquid water.When this confined air holds the maximum amount of water vapor that itis able to hold, some of the water vapor must condense back into liquid waterso that additional liquid water may evaporate into water vapor. Some of this water will condense back intothe liquid within the cup, but some of this water will condense into drops ofliquid water on the sides of the cup and even underneath the lid. When the air holds the maximum amount ofmoisture that it is able to hold, the air is said to besaturated. To saturate anythingmeans to fill it to capacity; the air is saturatedwhen it holds the maximum moisture that it is able to hold. The saturation amount of air is itself afunction of temperature. Warm air has agreater saturation amount since warm air is able to hold a greater quantity of moisture,while cold air has a lesser saturation amount since cold air is not able tohold as much moisture as warm air. Thestrict definition of the relative humidity of air is the amount of moisture inthe air as a fraction of the saturation amount at the given temperature. Let us devote some time tocarefully understand this definition.Firstly, the relative humidity of air is directlyrelated to the amount of moisture in the air. Adding moisture to air increases the relativehumidity, while subtracting moisture from air decreases the relativehumidity. However, it is possible tochange the relative humidity of air without adding or subtracting water. By simply changing the temperature of theair, we change the saturation amount of the air and thus we change the fractionof the moisture to the new saturation amount.If air becomes warmer, the saturation amount is greater, making theamount of moisture that is actually within the air a smaller fraction of that greatersaturation amount. In other words,warming air decreases its relative humidity.If air becomes colder, the saturation amount is lesser,making the amount of moisture that is actually within the air a greaterfraction of that lesser saturation amount.In other words, cooling air increases its relative humidity. An analogy would be helpful to understandthese processes. Imagine a large bucketand a small cup. A large bucket is ableto hold a large amount of water, while a small cup is only able to hold a smallamount of water. The large bucket isanalogous to hot air, since hot air has a large saturation amount, meaning itis able to hold more moisture. The smallcup is analogous to cold air, since cold air has a small saturation amount,meaning it is not able to hold as much moisture as hot air. Now suppose the small cup is mostlyfull. If we pour this water into a largeempty bucket, the large bucket will be mostly empty. If we take a large bucket that is mostlyempty and pour this water into a small cup, the small cup will be mostlyfull. This is remarkable: it is the sameamount of water in the small cup and the large bucket. Nevertheless, this same amount of water makesthe small cup mostly full and makes the large bucket mostly empty. We always keep in mind that the small cup isanalogous to cold air and the large bucket is analogous to hot air. If we take cold air and warm it, thisdecreases the relative humidity, since this is analogous to taking a small cupthat is more full and pouring its water into a largebucket that will now be more empty. Ifwe take warm air and cool it, this increases the relative humidity, since thisis analogous to taking a large bucket that is more emptyand pouring its water into a small cup that will now be more full. This is remarkable: we have not changed theamount of moisture in the thermal. Weare changing its relative humidity without adding or extracting water; we arechanging its relative humidity by changing its temperature. We now realize that regarding relativehumidity as simply the amount of moisture in the air is a grossly incorrectunderstanding of relative humidity. Asanother remarkable example, consider two thermals both at fifty percentrelative humidity. Does this mean theyhold the same amount of moisture? Isn’t fifty percent equal to one-half? Therefore, are not both thermals holdingmoisture equal to half of their respective saturation amounts? This is certainly true, but two thermals willalmost always have two different temperatures. The hotter thermal has a greater saturationamount, while the colder thermal has a lesser saturation amount. Half of a greater amount is greater, and halfof a lesser amount is lesser. Thus, thewarmer thermal actually holds more moisture and the cooler thermal actuallyholds less moisture, even though both thermals have the same relativehumidity! This is analogous to a largebucket and a small cup that are both half full.The large bucket holds more water and the small cup holds less water,even though both are half full! We always keep in mind that the small cup isanalogous to cold air and the large bucket is analogous to hot air. If a large bucket and a small cup are bothhalf full and yet the large bucket holds more water and the small cup holdsless water, we conclude that two thermals both at fifty percent relativehumidity hold different quantities of moisture.The warmer thermal holds more moisture (analogous to the large bucket),while the cooler thermal holds less moisture (analogous to the small cup). As an extreme example, consider two thermals:one at ninety percent relative humidity and the other at ten percent relativehumidity. Which thermal holds moremoisture, and which thermal holds less moisture? We are tempted to conclude that surely itmust be the ninety-percent humid thermal that holds more moisture, and we aretempted to conclude that surely it must be the ten-percenthumid thermal that holds less moisture.In actuality, we cannot draw any conclusions about the moistures of thetwo thermals without knowing their temperatures. Again, we imagine a large bucket and a smallcup. Suppose the large bucket is onlyten-percent full, while the small cup is ninety-percent full. Nevertheless, suppose the large bucket is solarge that it still holds more water at ten-percent capacity than the small cupat ninety-percent capacity. We alwayskeep in mind that the small cup is analogous to cold air and the large bucketis analogous to hot air. Suppose we havetwo thermals: one at ninety percent relative humidity and the other at tenpercent relative humidity. Now supposethat the ten-percent-humid thermal is so warm that its saturation amount is solarge that ten percent of that large saturation is nevertheless more moisturethan the ninety-percent-humid cold thermal.Therefore, it is possible for a ten-percent-humid thermal to hold moremoisture than a ninety-percent-humid thermal if the ten-percent-humid thermalis sufficiently warm and if the ninety-percent-humid thermal is sufficientlycold.
Since warming air decreasesthe relative humidity, the least humid time of the day is typically in the lateafternoon just before sunset, since the Sun has spent the entire daytimewarming the air. Since cooling airincreases the relative humidity, the most humid time of the day is typically inthe very early morning hours just before sunrise, since the air has spent theentire nighttime cooling in the darkness.Just before sunrise, the air may have cooled sufficiently for the relativehumidity to increase to one hundred percent.That is, the air has become saturated withwater vapor. At one hundred percentrelative humidity (saturation), water vapor must condense into liquid water sothat additional liquid water may evaporate.The temperature to which we must cool air until it becomes saturated is called the dew point, since the water vapor thatcondenses into liquid water is called dew.In the early morning, we may see the leaves of trees and the surface ofour car covered with water as if it had rained overnight. In actuality, it became sufficiently coldovernight that the dew point was achieved. The air became saturated, and water vaporbegan condensing into liquid water. Evenin the summertime, the nighttime air may become sufficiently cold that the dewpoint is achieved, thus forming dew.
Condensation is the changingof state from water vapor to liquid water.The condensing water must liberate heat to its surroundings to condense;this liberated heat warms the surroundings.Therefore, condensation is a warming process. Evaporation is the changing of state fromliquid water to water vapor. The evaporating water must extract heat fromits surroundings to evaporate; this extracted heat cools the surroundings. Therefore, evaporation is a cooling process. This is why we feel chilly immediately aftertaking a shower. Our bodies are covered with water that evaporates; the evaporatingwater extracts the heat needed for that evaporation from our bodies, thuscooling our bodies. Drying our bodieswith a towel removes water that would have evaporated; hence, we feel lesschilly whenever we dry ourselves with a towel.This is also why humans and some animalsperspire (sweat). The act of perspiring(sweating) covers our bodies with water that evaporates; the evaporating waterextracts the heat needed for that evaporation from our bodies, thus cooling ourbodies. Suppose the surrounding air isvery humid, perhaps close to saturation.Some of the water vapor in the air must condense to liquid water so thatadditional liquid water may evaporate.Whereas perspiration (sweat) on our skin may evaporate which cools ourbodies since evaporation is a cooling process, some water vapor in thesurrounding air condenses to liquid water onto our skin, adding heat back toour bodies since condensation is a warming process. In this case, our bodies cannot cooleffectively, and we feel uncomfortable.As a result, humid air feels warmer than its actual temperature. We can convert this discomfort into aneffective air temperature that is warmer than the actual air temperature. This effective air temperature is called the heat stress index (or the heat index forshort). For example, a meteorologist mayreport in the summertime that the actual temperature today will be ninetydegrees fahrenheit, but itwill feel like ninety-five degrees fahrenheit. The ninety degrees fahrenheit is the true air temperature, while theninety-five degrees fahrenheit is the heat stressindex (or simply the heat index). In thewintertime, wind makes the air feel colder than its actual temperature. We can convert this discomfort into anothereffective air temperature that is colder than the actual air temperature. This effective air temperature is called the windchill. For example, a meteorologist may report inthe wintertime that the actual temperature today will be thirty-five degrees fahrenheit, but it will feel liketwenty-five degrees fahrenheit. The thirty-five degrees fahrenheit is the true air temperature, while thetwenty-five degrees fahrenheit is the windchill. We mayuse all of these principles to construct a hygrometer, a device that measuresthe relative humidity of the air. Ahygrometer is simply two thermometers.One thermometer is wrapped in a wet cloth; thisis called the wet-bulb of the hygrometer.The other thermometer that is not wrapped in awet cloth is called the dry-bulb of the hygrometer. Water will evaporate from the wet-bulbthermometer. Since evaporation requiresheat, the evaporating water will extract heat from the wet-bulb thermometer,giving it a colder temperature than the dry-bulb thermometer. Again, evaporation is a cooling process. From the difference in temperature betweenthe wet-bulb of the hygrometer and the dry-bulb of the hygrometer, we cancalculate the relative humidity of the air.
We now apply everything wehave discussed about relative humidity to cyclones and anticyclones. As we discussed, the low-pressure,low-density thermal at the center of a cyclone rises, expands adiabatically,and cools. Since it cools, its relativehumidity increases. As we alsodiscussed, the high-pressure, high-density thermal at the center of ananticyclone sinks, contracts adiabatically, and warms. Since it warms, its relative humiditydecreases. In summary, the thermal atthe center of a cyclone becomes more humid as it rises, while the thermal atthe center of an anticyclone becomes less humid (or more dry) as it sinks. If the thermal at the center of a cyclonebecomes more humid as it rises, the dew point could be achieved,causing water vapor to condense into liquid water. However, even if the dew point is achieved, water vapor cannot condense into liquid waterin midair. The liquid water requires asurface upon which to condense, such as the surface of the leaves of trees orthe surface of our car. Fortunately, theatmosphere is not just air; there are tiny pieces of dust and silt and salt inthe atmosphere. When the dew point is achieved, the water vapor can condense into liquid wateraround these tiny pieces of dust and silt and salt, forming a microscopic dropof water around each tiny piece of dust or silt or salt. For this reason, this tiny piece of dust orsilt or salt is called a condensation nucleus, sinceit is at the center of the microscopic drop of water. The center of anything iscalled its nucleus. For example,the center of a biological cell is called the cellularnucleus, the center of an atom is called the atomic nucleus, and the center ofan entire galaxy is called the galactic nucleus. If the dew point is achieved causing watervapor to condense into microscopic drops of water around these condensationnuclei, the thermal becomes opaque.Ordinarily, air is transparent, as we know from our daily experience. Almost every second of every day of ourlives, we effortlessly see through the air around us, since air is ordinarilytransparent. However, liquid water isopaque. Actually, a small quantity ofliquid water is transparent. We can seethrough a glass of water for example.However, larger and larger quantities of liquid water become less andless transparent and more and more opaque.It is rather difficult seeing through a fish tank for example, and it ishopeless trying to see through the ocean to the seafloor. Therefore, when the dew point is achievedcausing water vapor to condense into liquid water, the thermal does indeedbecome opaque. We can no longer seethrough the air; the thermal has turned from invisible to visible. This is so remarkable that the thermaldeserves a special name. A thermal thathas achieved the dew point causing its water vapor to condense into liquidwater and thus the thermal has turned from transparent to opaque (frominvisible to visible) is called a cloud.We conclude that clouds form when thermals rise, expand adiabatically,cool, and become more humid until the dew point is achieved. This dew point is a specifictemperature. Therefore, thermals must be lifted to a specific elevation to cool to the dewpoint. This elevation iscalled the lifting condensation level (or the condensation level forshort). We can almostalways see the lifting condensation level (or simply the condensationlevel) with our own eyes, since clouds often have flat bottoms. This flat bottom is the lifting condensationlevel (or simply the condensation level).Below this elevation, the dew point has not beenachieved, and the thermals are still transparent (invisible). Above this elevation, the dew point has been achieved, and the thermals are opaque (visible)clouds.
We can categorize clouds intothree broad types: cumulus clouds, cirrus clouds, and stratus clouds. Cumulus clouds have the appearance ofcauliflower or puffs of cotton. Cirrusclouds have the appearance of individual wisps or feathers. Finally, if there are so many clouds in thesky that they all blend together to form one giant layer of cloud covering theentire sky, this is a stratus cloud. Theword stratus is derived from a Latin word meaninglayer. As we discussed, the wordstratify (meaning layered) is derived from the same Latin word. The word stratum (a layer of sedimentaryrock) also derives from the same Latin word, as we discussed earlier in thecourse. Note that there are other cloudtypes in addition to these three. Forexample, many cirrus clouds that seem to almost blend together into one giantlayer of cloud covering the entire sky is called a cirrostratus cloud, meaningintermediate between cirrus clouds and stratus clouds. As another example, many cumulus clouds thatseem to almost blend together into one giant layer of cloud covering the entiresky is called a stratocumulus cloud, meaning intermediate between cumulusclouds and stratus clouds. If it isprecipitating (raining or snowing) out of a cumulus cloud, then it is called acumulonimbus cloud. If it isprecipitating (raining or snowing) out of a stratus cloud, then it is called animbostratus cloud. If the air becomessufficiently cold that it achieves the dew point without having to be pushed up to higher elevations, a cloud will form at ornear the ground. This type of cloud is called fog.
We have already discussedenough meteorology to somewhat reliably predictweather over a timescale of a few hours using only a barometer. The rising or the falling of the air pressureas indicated by the barometer is called the barometrictendency. If the barometric tendency isfalling, then low-pressure, low-density thermals must be rising, expandingadiabatically, cooling, and becoming more humid. The relative humidity may increasesufficiently for the dew point to be achieved, formingclouds and perhaps even precipitation (rain or snow). Conversely, if the barometric tendency isrising, then high-pressure, high-density thermals mustbe sinking, contracting adiabatically, warming, and becoming less humid. The relative humidity may decreasesufficiently for liquid water to evaporate back into water vapor. In other words, thermals will turn fromopaque (visible) clouds to transparent (invisible) air; we will have a clearday. In summary, a falling barometrictendency is an indication of what is commonly considered tobe bad weather, while a rising barometric tendency is an indication ofwhat is commonly considered to be good weather.
The rate at which a risingthermal cools before it becomes a cloud is called thedry adiabatic rate of cooling (or the dry adiabatic rate for short). The rate at which a rising thermal coolsafter it becomes a cloud is called the wet adiabaticrate of cooling (or the wet adiabatic rate for short). A thermal becomes a cloud when water vaporcondenses into liquid water. Thisliberates heat, thus making the thermal warmer.Again, condensation is a warming process. Therefore, the wet adiabatic rate of coolingis always more shallow than the dry adiabatic rate of cooling. Stated the other way around, the dryadiabatic rate of cooling is always more steep than the wet adiabatic rate ofcooling. The rate at which thesurrounding atmosphere cools with rising elevation is calledthe environmental lapse rate of cooling (or the environmental lapse rate forshort). Suppose the environmental lapserate is more shallow than the wet adiabatic rate whichitself must be more shallow than the dry adiabatic rate. In other words, both adiabatic rates are more steep than the environmental lapse rate. In this case, either before or after athermal becomes a cloud, its rate of cooling is very steep. The rate of cooling of the thermal may besufficiently steep that the thermal becomes so cold and so dense that it is forced to sink to lower elevations. This is called absolute stability, and whatis commonly considered to be bad weather such asclouds or precipitation (rain or snow) will be less likely. Conversely, suppose the environmental lapserate is steeper than the dry adiabatic rate which itself must be steeper thanthe wet adiabatic rate. In other words,both adiabatic rates are more shallow than theenvironmental lapse rate. In this case,either before or after a thermal becomes a cloud, its rate of cooling isshallow. Thermals will probably not coolsufficiently to become dense enough to sink to lower elevations. In other words, thermals are more likely torise to higher elevations. This iscalled absolute instability, and what is commonly considered tobe bad weather such as clouds and perhaps even precipitation (rain orsnow) will be more likely. It is alsopossible for the environmental lapse rate to be steeper than the wet adiabaticrate but more shallow than the dry adiabaticrate. In this case, a thermal before itbecomes a cloud may cool to attain sufficiently high density to sink, resultingin what is commonly considered to be goodweather. However, if the thermal reachesthe lifting condensation level and becomes a cloud, its rate of coolingslows. Hence, the thermal will continueto rise, resulting in what is commonly considered to be badweather. This iscalled conditional instability, and either good weather or bad weathermay result under these circ*mstances.
Our discussion leads us toconclude that weather is strongly determined by lifting, the rising ofthermals. There arethree mechanisms that could cause lifting: orographic lifting, convergencelifting, and frontal wedging.Orographic lifting is caused by mountainspushing air aloft. This term is derived from the Greek root oro-for mountain, as we discussed earlier in the course. When winds encounter a mountain, much of theair blows up over the mountain. As theair rises, it expands adiabatically, cools, and becomes more humid. If the dew point isachieved, clouds form, and precipitation may occur. Therefore, we expect a humid climate on thewindward side of a mountain range. Thewindward side of anything is the side that faces the wind. The opposite of the windward side of anythingis its leeward side, which faces away from the wind. The adjective leeward isderived from the noun lee, which means shelter. For example, the lee of a building or the leeof a rock faces away from the wind and hence provides shelter from thewind. On the leeward side of a mountainrange, air sinks, contracts adiabatically, warms, and becomes less humid. Therefore, we expect an arid (dry) climate onthe leeward side of a mountain range.Actually, we expect an arid (dry) climate for an additional reason: anymoisture that was in the air probably precipitated out of the air on thewindward side of the mountain range.With moisture subtracted and in addition warming temperatures fromsinking air, we expect an extremely arid (dry) climate on the leeward side ofmountain ranges. These are called rainshadowdeserts. For example, the contiguousUnited States is at the midlatitudes, and theprevailing winds at the midlatitudes blow from thewest, as we will discuss shortly.Therefore, the west side of the Rocky Mountains is its windward side,while the east side of the Rocky Mountains is its leeward side. The leeward side (the east side) of the RockyMountains is the Great Plains of the United States, which is a rainshadow desert.Although there is agriculture in the Great Plains, the soil is not asproductive as the farmland of the midwestern UnitedStates, which is further east of the Great Plains. Convergence lifting iscaused by crowded winds pushing air aloft. Consider an island or a peninsula surroundedon many sides by water. Every day, seabreezes will blow from the surrounding waters toward the island or peninsula,as we discussed. These breezes becomecrowded and thus push each other upward.As the air rises, it expands adiabatically, cools, and becomes morehumid. If the dew point is achieved, clouds form, and precipitation may occur. Therefore, we expect islands and peninsulasto have humid climates. Actually, weexpect the climate to be extremely humid, since the winds originally came fromthe surrounding waters, where evaporation added significant moisture to the seabreezes. With moisture added and inaddition cooling temperatures from rising air, we expect extremely humidclimates on islands and peninsulas. Forexample, Florida is a peninsula in the southeastern United States. Every day, a sea breeze blows from the Gulfof Mexico from the west towards Florida.Every day, a sea breeze blows from the Atlantic Ocean from the easttowards Florida. Every day, a sea breezeblows from the Caribbean Sea from the south towards Florida. These sea breezes were already humid, sincethey came from bodies of water where evaporation added moisture to thewinds. In addition, these sea breezesbecome crowded over Florida and thus push each other upward. The air rises, expands adiabatically, cools,and becomes even more humid. The dewpoint is achieved, clouds form, and rain occurs. This explains why Florida has an extremelyhumid climate. In fact, the entirepeninsula is infested with amphibians and reptiles as aresult of this extreme humidity.Frontal wedging is caused by one air masspushing another air mass aloft. This isthe most important type of lifting that determines weather patterns, as we willdiscuss shortly.
The lightest type of liquidprecipitation is called mist. Heavier than mist is drizzle, and theheaviest liquid precipitation is called rain.The lightest freezing precipitation is calledsnow. Heavier than snow is freezingdrizzle. Heavier than freezing drizzleis called sleet. Even heavier than sleetis called graupel, and hail is the heaviest freezingprecipitation. Hail is quite dangerous;many people have been killed from falling hail. Snow is very light because it is composed ofindividual snowflakes, which are themselves composed of mostly air. Since clouds formaloft (at higher elevations) in the troposphere where the air temperature iscolder, precipitation almost always begins in the frozen state, such as snow orsleet. On its way down to lowerelevations, the precipitation warms and melts into liquid precipitation such asrain. This is usually the case even inthe summertime; warm rain in the summertime most likely began as snow or sleetfrom clouds at higher elevations in the troposphere that melted into rain onits way down toward lower elevations in the troposphere. We can personally experience this extremetemperature difference between the lower troposphere and the upper tropospherewith a sufficiently tall mountain. As weclimb the mountain, the air temperature becomes colder and colder until wereach the summit of the mountain, where it may be so cold that it issnowing. This is usually the case evenin the summertime. The air temperatureat the bottom of the mountain may be quite hot in the summertime. Nevertheless, the air temperature becomescolder and colder as we climb the mountain until (if the mountain issufficiently tall) the air temperature is so cold that it is snowing at thesummit of the mountain, even in summertime when the base of the mountain isstill quite hot!
Global (Large-Scale) Meteorological Dynamics
The Coriolis force caused bythe Earth’s rotation causes the global circulation of air in the atmosphere tobe complex. In order to emphasize thecomplications caused by the rotation of the Earth, let us first suppose thatthe Earth were not rotating. In thiscase, the global circulation of air in the atmosphere would be simple. Since the equator is hot throughout theentire year, the air at the equator is at low pressure. Since the poles are cold throughout theentire year, the air at the poles is at high pressure. The pressure gradient force would then push airfrom high pressure at the poles toward low pressure at theequator. The result is that windswould blow from the north in the northern hemisphere and from the south in thesouthern hemisphere. Since we alwaysname wind based on the direction it is blowing from, the winds in the northern hemispherewould be a north wind, while winds in the southern hemisphere would be a southwind. These are knownas the prevailing winds. Caution: winddoes not always blow in the directions of these prevailing winds; variations inpressure may cause winds to blow in various different directions. The prevailing winds are the directions inwhich the wind generally or usually blows, not the directions in which the windalways blows. If the Earth were notrotating, winds in the northern hemisphere would generally or usually be anorth wind from the north pole toward the equator, and winds in the southernhemisphere would generally or usually be a south wind from the south poletoward the equator. At the equator, thelow-pressure, low-density air would rise, becoming high pressure aloft. The high pressure at the poles is lowpressure aloft. Again, the pressuregradient force pushes air from high pressure toward low pressure. Hence, the pressure gradient force would pushthe risen air at the equator toward the poles, where the air would sink untilthe pressure gradient force pushes the air back toward the equator. This overall motion iscalled a circulation cell. Noticethere would be only one circulation cell in each hemisphere if the Earth werenot rotating. This discussion completelysummarizes the global circulation of air in the atmosphere if the Earth werenot rotating.
Of course, the Earth isrotating, causing a Coriolis force and hence tremendous complications to thesimplistic model we have just presented.The low-pressure, low-density air still rises at the equator, and thepressure gradient force still pushes this risen air toward the poles. However, by the time the air reaches roughlythirty degrees latitude in each hemisphere, the air has cooled sufficiently tosink. The pressure gradient force thenpushes this air back toward the equator, completing the tropical circulationcells. However, the Coriolis forcecauses rightward deflections in the northern hemisphere and leftwarddeflections in the southern hemisphere.The net result of the pressure gradient force together with the Coriolisforce is that the prevailing winds (near mean sea level) from roughly 30°N latitude to 0° latitude (the equator) blow from thenortheast; these are called the northeast trade winds, since we always namewind based on the direction it is blowing from.The prevailing winds (near mean sea level) from roughly 30°S latitude to 0° latitude (the equator) blow from thesoutheast; these are called the southeast trade winds, since we always namewind based on the direction it is blowing from.The term trade wind is used since these windsfacilitated trade between the Old World and the New World by pushing sailingships across the Atlantic Ocean from Europe and Africa toward North America andSouth America. The high-pressure,high-density air still sinks at the poles, and the pressure gradient forcestill pushes this air toward the equator.However, by the time the air reaches roughly sixty degrees latitude ineach hemisphere, the air has warmed sufficiently to rise. The pressure gradient force still pushes thisrisen air toward the poles where it sinks, completing the polar circulationcells. However, the Coriolis forcecauses rightward deflections in the northern hemisphere and leftwarddeflections in the southern hemisphere.The net result of the pressure gradient force together with the Coriolisforce is that the prevailing winds (near mean sea level) from 90°N latitude (the north pole) to roughly 60°N latitude blow from the northeast; these are called thepolar northeasterlies, since we always name windbased on the direction it is blowing from.The prevailing winds (near mean sea level) from 90°Slatitude (the south pole) to roughly 60°S latitudeblow from the southeast; these are called the polar southeasterlies,since we always name wind based on the direction it is blowing from. Notice that air sinks at roughly thirtydegrees latitude in each hemisphere, while air rises at roughly sixty degreeslatitude in each hemisphere. Sinking airis high-density, high-pressure air, while rising air is low-density,low-pressure air. Therefore, we havehigh pressure at roughly thirty degrees latitude in each hemisphere, and wehave low pressure at roughly sixty degrees latitude in each hemisphere. The pressure gradient force pushes air fromhigh pressure toward low pressure.Hence, wind will blow from roughly thirty degrees latitude to roughlysixty degrees latitude in each hemisphere, where the air rises and is pushedback to thirty degrees latitude where it sinks, completing the midlatitude circulation cells. However, the Coriolis force causes rightwarddeflections in the northern hemisphere and leftward deflections in the southernhemisphere. The net result of thepressure gradient force together with the Coriolis force is that the prevailingwinds (near mean sea level) from roughly 30°Nlatitude to roughly 60°N latitude blow from thesouthwest; these are called the southwesterlies,since we always name wind based on the direction it is blowing from. The prevailing winds (near mean sea level)from roughly 30°S latitude to roughly 60°S latitude blow from the northwest; these are called thenorthwesterlies, since we always name wind based onthe direction it is blowing from. Tosummarize, there are three prevailing winds in each hemisphere, and there arethree circulation cells in each hemisphere.In the northern hemisphere, the prevailing winds (near mean sea level)are the northeast trade winds near the equator, the southwesterliesat the midlatitudes, and the polar northeasterlies near the north pole. In the southern hemisphere, the prevailingwinds (near mean sea level) are the southeast trade winds near the equator, thenorthwesterlies at the midlatitudes,and the polar southeasterlies near the south pole. Thecirculation cells are called the two Hadley cells (one in each hemisphere) nearequator, named for the British meteorologist George Hadley, the two Ferrel cells (one in each hemisphere) at the midlatitudes, named for the American meteorologist William Ferrel, and the two polar cells (one in each hemisphere)near the poles. If the Earth rotatedfaster, the Coriolis force would be stronger, thus causing more prevailingwinds and more circulation cells in each hemisphere. If the Earth rotated slower, the Coriolisforce would be weaker, thus causing fewer prevailing winds and fewercirculation cells in each hemisphere. Ifthe Earth stopped rotating, the Coriolis force would vanish, and there would beonly one prevailing wind and only one circulation cell in each hemisphere, aswe discussed with our simplistic non-rotating model. A spectacular example of the effects of astrong Coriolis force on the global circulation of air is the planet Jupiter,which rotates more than twice as fast as the Earth. In fact, Jupiter is the fastest rotatingplanet in the Solar System. Therefore,Jupiter has the strongest Coriolis force out of all the planets in the SolarSystem. This very strong Coriolis forcehas divided Jupiter’s atmosphere into many prevailing winds and manycirculation cells. We can actually seethese winds in photographs of Jupiter.We can even see these winds if we look at Jupiter with our own eyesthrough a sufficiently powerful telescope.
At the equator, there islittle to no wind, since the air is rising; this is calledthe equatorial low, since low-pressure, low-density air rises. This rising air expands adiabatically, cools,and becomes more humid. The dew point may be achieved, forming clouds and rain. Indeed, there is a perpetual band of cloudsat the equator, and the perpetual rain from these clouds causes tropicalrainforests at and near the equator, such as the Amazon rainforest in northernSouth America, the Congo rainforest in central Africa, and the Indonesianrainforests. Sailing ships that foundthemselves at the equatorial low would become stuck, since there are no windsto push ships. For this reason, theequatorial low is also called the doldrums. Sailors would pray that their ship happens todrift slightly to the north or slightly to the south to catch one of the tradewinds that would push them again. Atleast the sailors could drink the perpetual rainwater while stuck at theequatorial low (the doldrums). As wediscussed earlier in the course, the lack of wind at the equatorial low permitsthe oceanic equatorial countercurrents to flow virtually unhindered against thedirection of other surface ocean currents near the equator. At roughly thirty degrees latitude in bothhemispheres, there is also little to no wind, but for the opposite reason. The air is sinking; these arecalled the subtropical highs, since high-pressure, high-density airsinks. This sinking air contractsadiabatically, warms, and becomes less humid (more dry). Hence, we do not have clouds or rain. Indeed, there is a perpetual band of clearskies free of clouds at roughly thirty degrees latitude in bothhemispheres. Theperpetual lack of rain causes hot deserts at and near roughly thirty degreeslatitude in both hemispheres, such as the Basin and Range in southwesternUnited States and northwestern Mexico (including the Mojave Desert, the SonoranDesert, and the Chihuahuan Desert), the Sahara innorthern Africa, the Arabian Desert in the Arabian peninsula, the Gobi in Chinaand Mongolia, the Patagonian Desert in Argentina, the Kalahari in southernAfrica, and the Great Australian Desert in Australia (including the GreatVictoria Desert, the Great Sandy Desert, the Tanami Desert, the Simpson Desert,and the Gibson Desert). Sailingships that found themselves at the subtropical high in either hemisphere wouldbecome stuck, since there are no winds to push ships. There would also be no rain for the sailorsto drink. Therefore, not only wouldsailors pray that their ship happens to drift slightly to the north or slightlyto the south to catch prevailing winds that would push them again, but thesailors would also kill their horses to stretch out their limited supply ofdrinking water. For this reason, thesubtropical highs are also called the horse latitudes. At roughly sixty degrees latitude in bothhemispheres, there is little to no wind, since the air is rising; these are called the subpolar lows, since low-pressure,low-density air rises. This rising airexpands adiabatically, cools, and becomes more humid. The dew point may beachieved, forming clouds and rain.Indeed, there is a perpetual band of clouds at roughly sixty degreeslatitude in both hemispheres, and the perpetual rain from these clouds causesboreal forests (cold forests or taigas) at and near roughly 60°Nlatitude, including the Canadian boreal forests, the Scandinavian borealforests, and the Russian boreal forests.Theoretically, there would be boreal forests (cold forests or taigas) atand near roughly 60°S latitude if there were land atthese latitudes. At the poles, there isalso little to no wind since the air is sinking; these are called the polarhighs, since high-pressure, high-density air sinks. This sinking air contracts adiabatically,warms, and becomes less humid (more dry).Hence, we do not have clouds or rain.Indeed, there is a perpetual area free of clouds at and near the polesin both hemispheres. To summarize, wehave the equatorial low (the doldrums) at the equator, we have the subtropicalhighs (the horse latitudes) at roughly thirty degrees latitude in bothhemispheres, we have the subpolar lows at roughly sixty degrees latitude inboth hemispheres, and we have the polar highs at ninety degrees latitude inboth hemispheres. At the lows, we haverising air, causing humid climates from perpetual clouds and rain. At the highs, we have sinking air, causingarid (dry) climates from the perpetual absence of clouds and rain. Actually, we expect arid climates at thehighs for an additional reason: any moisture that was in the air precipitatedout of the rising air at the equatorial low and at both subpolar lows before being pushed toward the highs where the air sinks. With moisture subtracted and in additionwarming temperatures from sinking air, we expect extremely arid (dry) climatesat both subtropical highs and at both polar highs.
An air mass is an enormousmass of air that has roughly the same temperature and pressure throughout itsvolume at a given elevation. We canclassify air masses based on their temperature.An air mass that forms near the equator will be warm; these are called tropical air masses, which we label with theuppercase (capital) letter T for tropical.An air mass that forms near the poles will be cold; these are called polar air masses, which we label with theuppercase (capital) letter P for polar.We can also classify air masses based on their moisture. An air mass that forms over the ocean or anybody of water will be humid, since evaporating water will add moisture to theair mass; these are called maritime air masses, whichwe label with the lowercase letter m for maritime. An air mass that forms over a continent willbe dry, since there is little water on the continent to evaporate to addmoisture to the air mass; these are called continentalair masses, which we label with the lowercase letter c for continental. To summarize, there are four different typesof air masses. An air mass that formsover a body of water near the equator will be humid and warm; these are called maritime tropical air masses, which we label withthe symbol mT.An air mass that forms over a body of water near the poles will be humidand cold; these are called maritime polar air masses,which we label with the symbol mP. An air mass that forms over a continent nearthe equator will be dry and warm; these are calledcontinental tropical air masses, which we label with the symbol cT. Finally, an airmass that forms over a continent near the poles will be dry and cold; these are called continental polar air masses, which we label withthe symbol cP.We must emphasize that once an air mass is born of a certain type, itdoes not remain that type permanently.In other words, the particular type of an air mass can change. For example, an air mass that forms near theequator will be warm. This would be atropical air mass, but if this air mass happens to movetoward one of the poles, it may become colder and colder until we mustreclassify it as a polar air mass. Thereverse can occur. An air mass that formsnear one of the poles will be cold. Thiswould be a polar air mass, but if this air mass happens to move toward theequator, it may become warmer and warmer until we must reclassify it as atropical air mass. As another example,an air mass that forms over a continent will be dry. This would be a continental air mass, but ifthis air mass happens to move over the ocean or any body of water, it maybecome more and more humid as evaporating water adds more and more moisture tothe air mass. Eventually, we mustreclassify it as a maritime air mass.The reverse can occur. An airmass that forms over the ocean or any body of water will be humid. This would be a maritime air mass, but ifthis air mass happens to move over a continent, it may lose more and moremoisture through precipitation that will not be replenished,since there is little water on the continent to evaporate. The air mass becomes less and less humiduntil we must reclassify it as a continental air mass. A spectacular example of the changing of anair mass is lake-effect snow. The fiveGreat Lakes are between the United States and Canada, two countries in theNorth American continent. Cities on thewindward side of the Great Lakes may experience very little snow, since airmasses that form over either Canada or the United States would be continentalair masses (dry air masses). However, acity on the leeward side (the opposite side of the windward side) of the GreatLakes may experience enormous amounts of snow.This is because a continental air mass that moves over the Great Lakeswill become more and more humid as water evaporates from the Great Lakes. By the time the air mass has crossed theGreat Lakes, the air mass has become so humid that it is now a maritime airmass. The humid maritime air mass thenprecipitates snow onto these cities on the opposite side of the Great Lakesfrom cities that experienced no snow from the same air mass when it wasformerly a dry continental air mass before crossing the Great Lakes. As a result, two cities that are notparticularly distant from each other may nevertheless experience vastlydifferent amounts of precipitation, since these two cities are on two oppositesides of a large body of water.
The Bjørgvin Theory of Meteorology
The fundamentaltheory of meteorology was formulated by the Norwegian meteorological physicist Vilhelm Bjerknes and othermeteorologists in Bjørgvin, Norway. Consequently, we will refer to thefundamental theory of meteorology as the BjørgvinTheory of Meteorology. According to the Bjørgvin Theory of Meteorology, the Earth’s troposphere(the lowest layer of the atmosphere) is divided intomany pieces called air masses. These airmasses are pushed by the prevailing winds, and muchmeteorological activity (commonly known as weather) occurs at theboundary between two air masses, which is called a front. This BjørgvinTheory of Meteorology, the fundamental theory of meteorology, is remarkablysimilar to the Theory of Plate Tectonics, the fundamental theory of geology. As we discussed earlier in the course, theTheory of Plate Tectonics states that the Earth’s lithosphere (the uppermostlayer of the geosphere) is divided into many piecescalled tectonic plates. These tectonicplates are pushed by convection cells in the asthenosphere (underneath thelithosphere), and much geological activity occurs at the boundary between twotectonic plates. These two fundamentaltheories have further similarities. Justas there are different types of tectonic plate boundaries that cause differenttypes of geological activities as we discussed earlier in the course, there aredifferent types of fronts (air mass boundaries) that cause different types ofmeteorological activities (commonly known as weather). A cold air mass pushing on a warm air mass is called a cold front.The symbol for a cold front on a weather map is triangles along thefront pointing in the direction in which the cold front is moving. A warm air mass pushing a cold air mass is called a warm front.The symbol for a warm front on a weather map is semicircles along thefront again pointing in the direction in which the warm front is moving. Cold fronts move faster than warm fronts, aswe will discuss shortly. Therefore, afaster-moving cold front can catch up to and merge with a slower-moving warmfront. This is calledan occluded front. We will discuss themeaning of the term occluded shortly.The symbol for an occluded front on a weather map is both triangles andsemicircles along the front again pointing in the direction in which theoccluded front is moving. A front thatdoes not move for several days or perhaps even a couple of weeks is called a stationary front. The symbol for a stationary front on aweather map is both triangles and semicircles along the front, but thetriangles and the semicircles point in two opposite directions.
The meteorological term frontis borrowed from military terminology. Vilhelm Bjerknes and other meteorologists formulated the Bjørgvin Theory of Meteorology during and shortly after theGreat War (commonly known as the First World War or World War I) roughly onehundred years ago. The Great War (theFirst World War or World War I) was the most global and most horrific war inhuman history up to that time, compelling many people throughout the world to often draw military analogies. A military front is the boundary between twoopposing armies. If one army advancesover (or pushes) the other army, the military front will move with the advancingarmy. If two armies areequally matched, the military front will not move. This is called astationary military front, the textbook example being the western front of theGreat War (the First World War or World War I).The western front remained stationary for most of the years of the GreatWar since the combined British and French armies on the western side of thewestern front equally matched the German army on the eastern side of thewestern front. The western front did notmove until the United States joined the British and the French toward the endof the Great War. The combined British,French, and American armies now had sufficient momentum to advance upon theGerman army, finally pushing the western front eastward. As Vilhelm Bjerknes and other meteorologists formulated the Bjørgvin Theory of Meteorology during and shortly after theGreat War, they imagined air masses pushing each other as if they were opposingarmies. It is for this reason thatmeteorologists to the present day refer to the boundary between two air massesas a front. The term for a meteorological front that does not move, astationary meteorological front, was literally copiedfrom the term for a military front that does not move, which is again astationary military front.
Since warm air rises and coldair sinks, the actual front between two air masses is not a perfectlyvertical wall. The actual frontbetween the two air masses is an inclined wall, since the rising warm air willbe above the sinking cold air. In otherwords, the sinking cold air will be below the rising warm air. Therefore, a cold front is inclined backwardas the cold air mass pushes the warm air mass, while a warm front is inclinedforward as the warm air mass pushes the cold air mass. Moreover, since cold air is more dense than warm air, the cold air mass can stronglypush the warm air mass, making the cold front more vertical than a warmfront. That is, a warm front is more shallow than a cold front. Since warm fronts are moreshallow, it takes a longer duration of time for a warm front to moveover any particular location. Since coldfronts are more vertical, it takes a shorter durationof time for a cold front to move over any particular location. Along both cold fronts and warm fronts,rising hot air will expand adiabatically and cool thus becoming more humid; thedew point could be achieved, causing clouds andpossibly precipitation along the front.Since a warm front is more shallow, all of theprecipitation will be spread over a larger area; consequently, theprecipitation along a warm front is often mild.The usual weather associated with a warm front is gentle precipitationover a long duration of time (often many hours) followed by warmer temperaturesas compared with the temperatures before the warm front arrived. Since a cold front is morevertical, all of the precipitation will be concentrated over a smallerarea; consequently, the precipitation along a cold front is often severe. The usual weather associated with a coldfront is intense precipitation over a brief duration of time (often only a fewminutes), followed by colder temperatures as compared with the temperaturesbefore the cold front arrived.
As a concrete application ofthe Bjørgvin Theory of Meteorology, consider weatherpatterns in the contiguous United States, which is at the midlatitudesof the northern hemisphere. Theprevailing winds of the midlatitudes of the northernhemisphere are the southwesterlies. Therefore, weather patterns (both goodweather and bad weather) are pushed from the westtoward the east by these southwesterlies. This explains why weather patterns moveacross the United States from the west toward the east. Philadelphia is west of New York City, andChicago is further west from Philadelphia.A weather pattern in Chicago will move from Chicago toward Philadelphia,and the weather pattern will continue to move from Philadelphia toward New YorkCity. Now consider a lowpressure system being pushed from the west toward east by the southwesterlies.Winds will blow inward toward this moving low pressuresystem. Winds from the south will carrywarmer air, since they are from equatorial latitudes. Winds from the north will carry cooler air,since they are from polar latitudes.Since the United States is in the northern hemisphere, the Coriolisforce deflects all of these winds to the right ultimately circulating themcounterclockwise around this moving low pressuresystem. Hence, the warmer winds from thesouth will be deflected to the east and will collidewith cooler air. Warm air pushing coldair is a warm front; hence, there will be a warm front to the east of themoving low pressure system. Also, the coolerwinds from the north will be deflected to the west and will collide with warmerair. Cool air pushing warm air is a coldfront; hence, there will be a cold front to the west of the moving low pressure system.As this low pressure system is pushed by the southwesterlies, any given geographical region of theUnited States will first be attacked by the warm front, often bringing manyhours of gentle precipitation followed by warmer temperatures. Then, the same geographicalregion will be attacked by the cold front, often bringing only a few minutes ofintense precipitation followed by colder temperatures. Since cold fronts move faster than warmfronts, the cold front to the west of the low pressuresystem may catch up to and merge with the warm front to the east of the lowpressure system, forming an occluded front.The cold air to the west of the former cold front merges with the coldair to the east of the former warm front, thus squeezing the warm air betweenthem and wedging it upward, since warm air rises. Hence, the formation of the occluded frontbegins the dispersion of the entire low pressuresystem. This is the reason these fronts are called occluded fronts.In colloquial English, the verb to occlude means to stopor to obstruct or to close.
As rising air and sinking airrub against each other, electrons are transferred fromone thermal to another. This may createan electric field between the clouds and the ground. Usually, air is a poor conductor ofelectricity; air is usually an electrical insulator. However, all electrical insulators willconduct electricity if subjected to electric fields of sufficientlyenormous strength. The thresholdelectric field at which an electrical insulator becomes an electrical conductoris called the dielectric breakdown of thematerial. The dielectric breakdown ofair is roughly three million volts per meter.If the electric field in air exceeds roughly three million volts permeter, the air actually becomes an electric conductor. In this case, electrons can flow between theclouds and the ground. This flow ofelectrons is called lightning. There is an enormous quantity of energyassociated with lightning. Some of thisenergy is transferred to the air itself, causing aloud, explosive sound called thunder. Inbrief, lightning causes thunder. Thelight from the lightning propagates at the speed of light, which is almost onemillion times faster than the speed of sound.In other words, sound propagates almost one million times slower thanthe speed of light. The speed of lightis so fast that we never notice its propagation in our daily experiences; lightseems to propagate instantaneously fast.However, sound propagates sufficiently slow that we notice itspropagation in some of our daily experiences.For example, some of us notice while sitting near the outfield of abaseball stadium that there is a delay between seeing and hearing a baseballbat crack a baseball. Some of us noticewhile sitting near the infield of a baseball stadium that there is a delaybetween seeing and hearing a baseball land in the baseball mitt of anoutfielder. Some of us notice that thereis a delay between seeing and hearing a hockey stick strike a hockey puck. In all such examples, we see the event first,then we hear the event second. Again,light seems to propagate instantaneously fast, while sound propagates slowenough that the sound arrives noticeably after the light. The speed of sound through air is roughly onemile per five seconds, which we may restate as roughly five seconds permile. We can use this relatively slowpropagation of sound to estimate how far away a storm is occurring from ourlocation. We simply count the number ofseconds starting from when we see lightning until we hear thunder. For every five seconds we count, the storm isroughly one mile distant. For example,if we see lightning and count fifteen seconds until we hear thunder, the stormis roughly three miles distant, since every five seconds we counted correspondsto roughly one mile of distance. If wecount many seconds after seeing lightning but never hear thunder, this meansthat the storm is very far away. Thunderpropagates outwards in all directions, spreading its total energy thinner andthinner. By the time the thunder arrivesat our location, the sound energy was too diluted forour ears to hear. At the oppositeextreme, suppose we see lightning and immediately thereafter we hear thunder;in other words, suppose we did not have the opportunity to count to even onesecond before hearing thunder. Thismeans that the storm is very close; we are probably located within the stormitself.
A tornado is a continentalstorm with fast, circulating winds around an extremely low-pressurethermal. Most tornadoes are a few dozenmeters across. A large tornado could bea couple hundred meters across. Enormoustornados that are one kilometer across are very rare. Most of the tornadoes in the world occur inthe midwestern United States. This is because cPair masses (continental polar air masses) form over Canada, since Canada is inthe North American continent and is near the north pole,while cT air masses (continental tropical air masses)form over Mexico, since Mexico is also in the North American continent but nearthe equator. Moreover, there are twomountain ranges along both coasts of North America: the Rocky Mountains alongthe Pacific coast (the west coast) and the Appalachian Mountains along theAtlantic coast (the east coast). Thesetwo mountain ranges tend to confine air masses between them. Hence, cP airmasses that form over Canada and cT air masses thatform over Mexico tend to collide over the country that is between Canada andMexico; that country is the United States.For all these reasons, most of the tornadoes in the entire world occurin the midwestern United States.
The Fujita scale (or F-scale)is a tornado wind-speed scale, named for the Japanese-American meteorologistTetsuya Theodore Fujita who formulated this scale. The weakest tornadoes are designated F0. More powerfulthan F0 would be called F1followed by F2, F3, and F4. The mostpowerful tornados are designated F5. Even an F0 tornadois powerful enough to destroy entire towns; many people havebeen killed by F0 tornados, the weakest scaleof tornado. We must always seek shelterduring a tornado warning, regardless of the Fujita-scale designation of thetornado.
The largest storms in theentire world form from low-pressure mT air masses(maritime tropical air masses). Thesestorms are called hurricanes if they form in theAtlantic Ocean, and they are called typhoons if they form in the PacificOcean. Other than their oceaniclocation, there is no difference between a hurricane and a typhoon. The development of a hurricane/typhoon is asfollows. A slightly low-pressure mT air mass is called a tropicaldisturbance. If a tropical disturbancehappens to form at or near the equator, the Coriolis force will be too weak tocause any circulation of winds, and the tropical disturbance will quietlydisperse. However, if a tropicaldisturbance happens to form significantly north or south of the equator, theCoriolis force may be strong enough to circulate the winds. When the winds are sufficiently strong, thetropical disturbance becomes a tropical depression. On rare occasions, the winds are so strongthat the tropical depression may become a tropical storm. At this point, the tropical storm is given a human name, as we will discuss. On very rare occasions, the winds become soextraordinarily strong that the tropical storm becomes ahurricane/typhoon. In this case, the hurricane/typhoonretains its tropical-storm human name, as we will discuss. To summarize, first we have a tropicaldisturbance, then we have a tropical depression, then we have a tropical storm,then we have a hurricane/typhoon.
Since a hurricane/typhoon isa low-pressure system, the winds in a hurricane/typhoon circulatecounterclockwise in the northern hemisphere but circulate clockwise in thesouthern hemisphere. The winds circulatearound the eye of the hurricane/typhoon, where there is calm weather and clearskies. When a northern-hemispherehurricane/typhoon attacks a continent, the winds to the right of the eye pushthe ocean waters onto the continent.This is called the storm surge. The winds to the left of the eye push theocean waters away from the continent; thus, there is no storm surge to the leftof the eye. Therefore, most of thedestruction to the left of the eye is from the winds themselves. To summarize, most of the devastation from anorthern-hemisphere hurricane/typhoon is from the storm surge to the right ofthe eye, but most of the devastation from a northern-hemispherehurricane/typhoon is from the winds to the left of the eye. These directions arereversed in the southern hemisphere, but note that hurricanes/typhoonsare rare in the southern hemisphere.This is because winds would circulate clockwise aroundhurricanes/typhoons in the southern hemisphere, causing the winds on thesouthern edge of the storm to blow from the east. However, the prevailing winds at the midlatitudes of the southern hemisphere blow from the west,thus weakening the storm winds and preventing tropical storms fromstrengthening to hurricanes/typhoons. Ofcourse, we could present the same argument for the northern hemisphere. Winds circulate counterclockwise aroundhurricanes/typhoons in the northern hemisphere, causing the winds on thenorthern edge of the storm to again blow from theeast. Again, the prevailing winds at themidlatitudes of the northern hemisphere blow from thewest, which should again weaken the storm winds and prevent tropical stormsfrom strengthening to hurricanes/typhoons.Indeed, this is an important reason why the strengthening of tropicalstorms to hurricanes/typhoons is rare even in the northern hemisphere. However, the Antarctic Circumpolar Current atthe midlatitudes of the southern hemisphere is thestrongest surface ocean current in the entire world, as we discussed earlier inthe course. Although the AntarcticCircumpolar Current is caused by the prevailing winds at the midlatitudes of the southern hemisphere, this surface oceancurrent is so strong that it actually pushes back on the atmosphere, making themidlatitude prevailing winds in the southernhemisphere stronger than the midlatitude prevailingwinds in the northern hemisphere. Thestronger prevailing winds at the midlatitudes of thesouthern hemisphere make the formation of hurricanes/typhoons in the southernhemisphere much less likely than the formation of hurricanes/typhoons in thenorthern hemisphere. In eitherhemisphere, most of the devastation inland from a hurricane/typhoon is fromflooding from rain. As we will discusslater in the course, flooding is the most common and the most destructive ofall natural disasters.
The human name of a tropicalstorm in the Atlantic Ocean is chosen from six listseach containing twenty-one alphabetized human names. For example, the first tropical storm in theAtlantic Ocean in the year 2023 was named tropical storm Arlene, the second wasnamed tropical storm Bret, the third was namedtropical storm Cindy, and so on and so forth.Notice that the names are in alphabetical order. Although the English alphabet has twenty-sixletters, these six lists each have only twenty-one names because namesbeginning with the five letters Q, U, X, Y, and Z are notused, since human names beginning with any of these five letters arerare. If a tropical storm is promoted to a hurricane, then it retains its humanname. For example, tropical stormFranklin was promoted to hurricane Franklin in theyear 2023. These six lists of humannames are recycled every six years.However, if a hurricane is particularly destructive, then its name ispermanently retired and is forever associated with the hurricane for thatparticular year. A new human namebeginning with the same letter of the English alphabet must then replace thatname for future years. For example, thefourth tropical storm in the year 2013 should have been namedDean, but hurricane Dean was so destructive in the year 2007 that the name Deanwas permanently retired and replaced with the name Dorian. If there happens to be more than twenty-onetropical storms in the Atlantic Ocean in any given year, then the letters ofthe Greek alphabet are used after reaching the end ofthe list of twenty-one names. For example, the twenty-second tropical storm in the Atlantic Oceanin the year 2005 was named tropical storm Alpha, the twenty-third was namedtropical storm Beta (later promoted to hurricane Beta), the twenty-fourth wasnamed tropical storm Gamma, the twenty-fifth was named tropical storm Delta,the twenty-sixth was named tropical storm Epsilon (later promoted to hurricaneEpsilon), and so on and so forth.There are other lists of names for tropical storms in the PacificOcean. Again, there are twenty-one humannames in each list of names for the Atlantic Ocean, and there are twenty-fourletters in the Greek alphabet.Twenty-one plus twenty-four equals forty-five. What do we do if there are more thanforty-five tropical storms in the Atlantic Ocean in a single year? In this case, we run to the nearest church,since it is probably the end of the world!
The Saffir-Simpsonscale is a hurricane/typhoon scale, named for American engineer Herbert Saffir and American meteorologist Robert Simpson whotogether formulated this scale. Theweakest hurricane/typhoon is called Category 1,stronger is called Category 2, even stronger is called Category 3, stronger iscalled Category 4, and the strongest hurricane/typhoon is called Category5. We must keep in mind that even aCategory 1 hurricane/typhoon is stronger and more destructive than a tropicalstorm. For example, hurricane Sandy wasa Category 1 hurricane when it attacked and devastated New Jersey in the year2012. Even tropical storms, which arethemselves weaker than Category 1 hurricanes/typhoons, can destroy entiretowns; many people have been killed by tropicalstorms, themselves weaker than the weakest hurricanes/typhoons. We must always seek shelter during a tropicalstorm warning, and we must certainly always seek shelter during ahurricane/typhoon warning, regardless of the Saffir-Simpson-scaledesignation of the hurricane/typhoon.
Climatology
The study of short-termtrends and variations in the atmosphere is calledmeteorology, and someone who studies short-term trends and variations in theatmosphere is called a meteorologist. Byshort-term, we may mean a few minutes, a few hours, a few days, or a fewweeks. The study of long-term trends andvariations in the atmosphere is called climatology,and someone who studies long-term trends and variations in the atmosphere iscalled a climatologist. By long-term, wemay mean months, years, decades, centuries (hundreds of years), millennia(thousands of years), or even millions of years. The study of the atmosphere (short-termand/or long-term) is called atmospheric sciences, andsomeone who studies the atmosphere (short-term and/or long-term) is called anatmospheric scientist.
Statistics isused to study trends and variations of any kind. Any collection of numbers iscalled data, and the purpose of statistics is to calculate twoquantities about data: the central tendency of the data and the dispersion ofthe data. The central tendency of thedata is a number that is a typical representative of most of the data. The most common way of measuring centraltendency is the average, which we will call the mean. The mean of the data is the sum of thenumbers divided by the number of numbers.The most common way of measuring dispersion is the standard deviation,but in this course we will measure dispersion with therange. The range of the data is thedifference between the largest number and the smallest number. In atmospheric sciences, the daily temperaturemean is the average of the hottest temperature and the coldest temperature inany given day. For example, if thehottest temperature today is eighty degrees fahrenheitand if the coldest temperature today is seventy degrees fahrenheit,then the daily temperature mean for today is seventy-five degrees fahrenheit, since eighty plus seventy is one hundred andfifty, and dividing this by two yields seventy-five. The daily temperature range is the differencebetween the hottest temperature and the coldest temperature in any given day. In the previous example, the dailytemperature range for today would be ten fahrenheitdegrees, since eighty minus seventy equals ten.The monthly temperature mean is the average of all the daily means forthat month. For example, if a particularmonth happens to have thirty days, the monthly temperature mean for that monthwould be the sum of all the daily means for that month divided by thirty. The monthly temperature range is thedifference between the hottest daily mean and the coldest daily mean duringthat month. The annual temperature meanis the average of all the monthly means for that year. In other words, the annual temperature meanis the sum of all the monthly means for that year divided by twelve, sincethere are twelve months in one year. Theannual temperature range is the difference between the hottest monthly mean (almost always July or August in the northern hemisphere) andthe coldest monthly mean (almost always January or February in the northernhemisphere) of that year.
Generally, temperature meansare hotter at the equatorial latitudes, while temperature means are colder atthe polar latitudes. At the midlatitudes, temperature means are hotter duringsummertime and colder during wintertime.However, we must not only specify trends and variations in the temperature,but trends and variations in the precipitation must also bespecified in any climatological analysis. Generally, precipitation means are high atand near the equator due to the equatorial low (the doldrums). Precipitation means are low at and near roughlythirty degrees latitude in both hemispheres due to the subtropical highs (thehorse latitudes). Precipitation meansare high at and near roughly sixty degrees latitude in both hemispheres due tothe subpolar lows. Finally,precipitation means are low at and near the poles due to the polar highs.
Temperature ranges aresmaller at coasts and shores due to the marine effect: the oceans stabilizetemperatures due to the relatively large heat capacity of water. Conversely, temperature ranges are largerinland due to the continental effect: continents do not stabilize temperaturesdue to the relatively small heat capacity of land. In other words, inland winters tend to becolder and inland summers tend to be hotter as compared with coasts and shoreswhere both winters and summers tend to be relatively mild. Extending this logic across the entireplanet, temperature ranges are generally smaller in the water hemisphere (thesouthern hemisphere), since the abundance of southern-hemisphereoceans stabilize temperatures in that hemisphere. Conversely, temperature ranges are generallylarger in the land hemisphere (the northern hemisphere), since the abundance ofnorthern-hemisphere continents do not stabilizetemperatures in that hemisphere. Inother words, winters in the northern hemisphere tend to be colder and summersin the northern hemisphere tend to be hotter as compared with the southernhemisphere, where both winters and summers tend to be relatively mild.
For most of the history ofplanet Earth, the hot temperatures at the equatorial latitudes and the coldtemperatures at the polar latitudes have been moderatedby the oceans due to the relatively large heat capacity of water. However, the moving tectonic plates of thelithosphere slowly change the configuration of the continents and the oceansover enormous timescales (millions of years).If there happens to be relatively isolated continents and/ormicrocontinents at the poles, the relatively small heat capacity of theselandmasses will permit the temperatures at the poles to become extremelycold. The result is an ice age, anextremely long period of time (millions of years) when the temperature at thepoles is so cold that enormous icecaps cover these landmasses. To our knowledge, there have been only fiveice ages throughout the entire history of the Earth, each lasting many millionsof years. As we discussed earlier in thecourse, the Current Ice Age began roughly thirty million years ago when SouthAmerica ripped off of Antarctica, completely isolatingAntarctica at the South Pole and establishing the Antarctic Circumpolar Currentsurrounding Antarctica. Hence,Antarctica became extremely cold, and our entire planet Earth plunged into theCurrent Ice Age. The Current Ice Agebegan roughly thirty million years ago and continues to the present day. The Current Ice Age will last many moremillions of years as long as Antarctica remains isolated at the South Polesurrounded by the Antarctic Circumpolar Current that further isolatesAntarctica. There areonly two scenarios that can end the Current Ice Age. In one scenario, Antarctica may move off of the South Pole, which would make it less cold. This would also interrupt the AntarcticCircumpolar Current, which would contribute to warming temperatures. In the other scenario, another continent maymove to the South Pole and collide with Antarctica. This would end the isolation of Antarctica,making it less cold. This would alsointerrupt the Antarctic Circumpolar Current, again contributing to warmertemperatures. Whether Antarctica movesaway from the South Pole or another continent moves toward the South Pole, ittakes millions of years for tectonic plates to move significantly, as wediscussed earlier in the course.Therefore, the Current Ice Age will continue for millions of more yearsto come. During the Current Ice Age, thesouthern icecap covers the continent Antarctica, and the northern icecap coversthe microcontinent Greenland.
Within theCurrent Ice Age, there have been many periods of time when the Earth has becomeeven colder. These are glacial periods of the Current IceAge. Between two glacial periods is aninterglacial period when the Earth is not as cold. The Earth becomes so cold during a glacialperiod that the icecaps expand beyond the poles and advance onto othercontinents. A major glacial period lastsroughly one hundred thousand years, while a minor glacial period lasts betweenroughly twenty-five thousand years and roughly fifty thousand years. We are currently within an interglacialperiod of the Current Ice Age. Thisinterglacial period began roughly twelve thousand years ago at the end of amajor glacial period that lasted roughly one hundred thousand years. Plate tectonics cannot be responsible for thealternation between glacial periods and interglacial periods within the CurrentIce Age, since tectonic plates do not move appreciably over timescales ofthousands of years. The alternationbetween glacial periods and interglacial periods within the Current Ice Age is caused by the Milankovićcycles, named for the Serbian climatologist and astronomer MulutinMilanković who formulated this theory. The Earth’s orbit around the Sun is presentlyalmost a perfect circle. In other words,the eccentricity of the Earth’s orbit around the Sun is close to zero, but thishas not always been the case nor will it always be the case. Gravitational perturbations (tugs) from theother planets, primarily Jupiter, change the eccentricity of the Earth’sorbit. When the Earth’s orbit isperturbed to become more elliptical, the Earth will besignificantly further from the Sun, causing the Earth to becomesignificantly colder thus causing a major glacial period of the Current IceAge. Calculations show that theeccentricity of the Earth’s orbit changes once every one hundred thousand years(roughly), which is roughly the duration of time of a major glacial periodwithin the Current Ice Age. Minor glacial periods are caused by the precession and the nutationof the Earth’s rotational axis.Precession is the turning of an axis around another axis. Nutation is the nodding of an axis resultingin a change in obliquity. The Earth’srotational axis precesses and nutates, and this changesthe amount of sunlight the Earth receives from the Sun, which in turn causesminor glacial periods within the Current Ice Age. Calculations show that the Earth’s rotationalaxis precesses once every twenty-six thousand years(roughly), and the Earth’s rotational axis nutatesonce every forty-one thousand years (roughly).These are roughly equal to the duration of time of minor glacial periodswithin the Current Ice Age.
Long-term variations inglobal temperature (over millions of years) are caused by slowly movingtectonic plates, resulting in ice ages when continents and/or microcontinentshappen to be relatively isolated at or near the poles, as with the Current IceAge. Intermediate-term variations inglobal temperature (over thousands of years) cause glacial periods andinterglacial periods within the Current Ice Age. These intermediate-term variations in globaltemperature (over thousands of years) are caused bythe Milanković cycles (the variations of theeccentricity of the Earth’s orbit around the Sun, the precession of the Earth’srotational axis, and the nutation of the Earth’s rotational axis). However, there are also short-term variationsin global temperature, from centuries to decades, and even as short as a fewyears. Extremely short-term variationsin global temperature (over a few years) are caused byviolent igneous eruptions. As wediscussed earlier in the course, a single igneous eruption can cause globalcooling for a few years, since igneous eruptions eject ash into the atmosphere,reducing the amount of incoming sunlight to the Earth. Short-term variations in global temperature(from decades to centuries) are caused by acombination of the Pacific Decadal Oscillation (PDO),the Atlantic Multidecadal Oscillation (AMO), and variations in solar activity. As we discussed earlier in the course, thePacific Decadal Oscillation (PDO) is the alternationbetween decades of El Niño domination in the Pacific Ocean and decades of LaNiña domination in the Pacific Ocean.The Atlantic Multidecadal Oscillation (AMO) is a similar oscillation in the Atlantic Ocean, as wealso discussed earlier in the course.Although direct observations of the PDO andthe AMO only stretch back several decades,measurements of the radioactive isotope carbon-fourteen <![if !msEquation]><![if !vml]>
<![endif]><![endif]>within trees have revealed that the PDO and the AMO each undergoroughly sixty-year cycles, where one complete PDO forexample consists of roughly three decades of El Niño domination followed byroughly three decades of La Niña domination.Quantitative observations of solar activity stretch back a fewcenturies, giving us a stronger understanding of how variations in solaractivity affect global temperatures. Attimes, the Sun is more active, radiating more energy that warms planet Earth;at other times, the Sun is more quiet, radiating lessenergy that cools planet Earth. Thesevariations in solar activity manifest themselves through sunspots, regions onthe surface of the Sun where magnetic field strengths are particularly strong. These sunspots have beendirectly observed for roughly four hundred years, since the invention ofthe telescope. Astronomers have observedthat the number of sunspots goes through a roughly eleven-year cycle. In one complete cycle, the number of sunspotsincreases then decreases over a time period of roughlyeleven years. For example, the Sunexperienced a period of increasing activity during the last few years of thetwentieth century and the first few years of the twenty-first century (thecurrent century). This period ofincreasing solar activity was followed by a period ofdecreasing solar activity. During thatparticular period of decreasing solar activity, some of the coldest monthlytemperature means over the last one hundred years occurred. Furthermore, measurements of the radioactiveisotope carbon-fourteen <![if !msEquation]><![if !vml]><![endif]><![endif]>within trees have revealed that this roughlyeleven-year solar cycle itself goes through a roughlytwo-hundred-year cycle. This is the deVries cycle, named for the Dutch physicist Hessel de Vries, one of the pioneersof radiocarbon dating. According to thede Vries cycle, the Sun gradually increases inactivity to what is called a solar maximum thengradually decreases in activity to what is called a solar minimum. Caution: the eleven-year solar cyclescontinue to occur throughout each two-century de Vriescycle. Since one complete de Vries cycle lasts for roughly two centuries, each solarmaximum and each solar minimum lasts for roughly one hundred years. Over the past twelve thousand years (sincethe beginning of the current interglacial period of the Current Ice Age), therehave been roughly sixty complete de Vries cycles,with each de Vries cycle having one solar maximum andone solar minimum. The Modern Maximumoccurred throughout most of the twentieth century, and the Modern Minimum begantoward the beginning of the twenty-first century (the current century). These de Vriescycles have caused variations in global temperatures over the past severalthousand years. Inparticular, a solar maximum contributed to warm temperatures lasting from theancient Late Roman Republic Period to the ancient Early Roman Empire Period, asolar minimum contributed to cold temperatures during the Early Middle Ages, asolar maximum contributed to warm temperatures during the High Middle Ages, anda solar minimum contributed to cold temperatures during the Little Ice Age,lasting from the Late Middle Ages to the Early Modern Ages. Most recently, the Modern Maximum thatoccurred throughout most of the twentieth century contributed to warmingtemperatures during that century, and the Modern Minimum that began toward thebeginning of the twenty-first century (the current century) has already causedcooling temperatures that will continue for the rest of the current century. By combining the roughly sixty-year PDO, the roughly sixty-year AMO,the roughly eleven-year solar cycle, and the roughly two-century de Vries cycle, we obtain a nearly perfect model of globaltemperature variations over the past couple of thousand years. As we just discussed, this model predictsthat there will be gradual global cooling during the current century. Caution: an unexpected violent igneouseruption may cause additional global cooling in addition to these cyclicvariations.
To summarize the climate ofplanet Earth, the average temperature of the atmosphere is determined primarilyby the Earth’s distance from the Sun together with the concentration ofgreenhouse gases in the atmosphere, primarily water vapor, which warms theEarth sufficiently so that it is habitable for life. Temperature means are hot at the equatoriallatitudes, temperature means are cold at the polar latitudes, and temperaturemeans vary at the midlatitudes based on the seasons(warmer in the summertime and cooler in the wintertime). The abundance of liquid water that covers theEarth (the oceans) stabilizes global temperatures due to the large heatcapacity of water. Temperature rangesare smaller in the southern hemisphere (the water hemisphere), whiletemperature ranges are larger in the northern hemisphere (the landhemisphere). Temperature ranges aresmaller at coasts and shores due to the marine effect, while temperature rangesare larger inland due to the continental effect. Precipitation means arehigh at and near the equator due to the equatorial low (the doldrums),precipitation means are low at and near roughly thirty degrees latitude in bothhemispheres due to the subtropical highs (the horse latitudes), precipitationmeans are high at and near roughly sixty degrees latitude in both hemispheresdue to the subpolar lows, and precipitation means are low at and near the polesdue to the polar highs. Long-termvariations in global temperature (over millions of years) are caused by slowlymoving tectonic plates, resulting in ice ages when continents and/ormicrocontinents happen to be relatively isolated at or near the poles, as withthe Current Ice Age that began roughly thirty million years ago and continuesto the present day. Intermediate-termvariations in global temperature (over thousands of years) cause glacialperiods and interglacial periods within the Current Ice Age. These intermediate-term variations in globaltemperature (over thousands of years) are caused bythe Milanković cycles (the variations of theeccentricity of the Earth’s orbit around the Sun, the precession of the Earth’srotational axis, and the nutation of the Earth’s rotational axis). We are currently in an interglacial period ofthe Current Ice Age, and this interglacial period began roughly twelve thousandyears ago. Short-term variations inglobal temperature (over a few decades or a few centuries) arecaused by the roughly eleven-year solar cycle, the roughly two-centuryde Vries cycle, the roughly sixty-year PDO, and the roughly sixty-year AMO. We just began a century of gradual globalcooling resulting from these short-term solar cycles and oceanic cycles. Finally, extremely short-term variations inglobal temperature (over a few years) may result from powerful igneouseruptions.
copyeditor: Michael Brzostek (Spring2023)
LibaridA. Maljian homepage at the Department of Physicsat CSLA at NJIT
Libarid A. Maljian profile atthe Department of Physics at CSLA at NJIT
Department of Physicsat CSLA at NJIT
College of Science andLiberal Arts at NJIT
New Jersey Institute ofTechnology
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