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11.1 Atmospheric Basics 271

Dew FormationDiscovery LabDiscovery Lab

OBJECTIVES

• Describe the composi-tion of the atmosphere.

• Compare and contrastthe various layers of theatmosphere.

• Identify three methodsof transferring energythroughout the atmo-sphere.

VOCABULARY

Imagine living in the blazing heat of the Sahara desert, near the equa-tor. Then imagine living in the frozen vastness above the arctic circle.Why are these places so different? The answer lies in how solarenergy interacts with the atmosphere, and how the interactions com-bine to produce weather and climate.

ATMOSPHERIC COMPOSITIONThe ancient Greeks thought that air was one of the fundamental ele-ments that could not be broken down into anything else. Today, weknow that air is a combination of many gases, each with its ownunique characteristics. Together, these gases form Earth’s atmo-sphere, which extends from Earth’s surface to outer space.

About 99 percent of the atmosphere is composed of nitrogen andoxygen, with the remaining one percent consisting of small amountsof argon, hydrogen, carbon dioxide, water vapor, and other gases. Thepercentages of the main components, nitrogen and oxygen, are criti-cal to life on Earth. If either were to change significantly, life as weknow it could not exist. Among the lesser-percentage gases, however,

Atmospheric Basics 11.111.1

Dew forms when moist air nearthe ground cools and the water vaporin the air changes into water droplets.In this activity, you will model theformation of dew.

1. Fill a glass about two-thirds full ofwater. Record the temperature ofthe room and the water.

2. Add ice cubes until the glass is full.Record the temperature of thewater at 10-second intervals.

3. Observe the outside of the glass.Note the time and the temperatureat which changes occurred on theoutside of the glass.

4. Repeat the experiment outside.Record the temperature of thewater and the air outside.

Observe In your science journal,describe what happened to the out-side of the glass in step 3 and step 4.Relate your observations to theformation of dew. Graph thetemperature of the waterduring both experi-ments. Did the resultsvary with location?Explain.

ozonetropospherestratospheremesospherethermosphere

exosphereradiationconductionconvection

About 99 percent of the atmosphere is composed of nitrogen andoxygen, with the remaining one percent consisting of small amounts

Figure 11-1 Nitrogenmakes up 78 percent of thegases in Earth’s atmosphere.Oxygen makes up 21 per-cent. The remaining onepercent consists of smallamounts of various othergases.

there is some variability, particularly in water vapor and carbon diox-ide. Figure 11-1 shows the composition of the atmosphere.

Key Atmospheric Gases The amount of water vapor in theatmosphere at any given time or place changes constantly. It can beas much as four percent of the atmosphere or as little as almost zero.The percentage varies with the seasons, with the altitude of a par-ticular mass of air, and with the surface features beneath the air. Airover deserts, for instance, is drier than air over oceans. Carbon diox-ide, another variable gas, makes up under one percent of the atmo-sphere. Why is it necessary to even mention these seeminglyinsignificant gases?

The level of both carbon dioxide and water vapor are criticalbecause they play an important role in regulating the amount ofenergy the atmosphere absorbs. Water vapor, the gaseous form ofwater, is the source of clouds, rain, and snow. In addition, water is theonly substance in the atmosphere that exists in three states: solid, liq-uid, and gas. This is important because when water changes from onestate to another, heat is either absorbed or released, and this heatgreatly affects the atmospheric motions that create weather and cli-mate.

The atmosphere also contains solids in the form of tiny particlesof dust and salt. Dust is carried into the atmosphere by wind. Salt ispicked up from ocean spray. Dust and salt play a role in cloud for-mation, as you’ll learn later. Ice is the third solid found in the atmo-

272 CHAPTER 11 Atmosphere

Percentages of Gases That Make Up Earth's Atmosphere

Argon

0.93%

Carbon dioxide

0.03%

Water vapor

0.0 to 4.0%

Trace gases

0.01%

Neon

Helium

Methane

Krypton

Hydrogen

Ozone

Xenon

Oxygen

21%

Nitrogen

78%

The level of both carbon dioxide and water vapor are criticalbecause they play an important role in regulating the amount ofenergy the atmosphere absorbs. Water vapor, the gaseous form ofwater, is the source of clouds, rain, and snow. In addition, water is theonly substance in the atmosphere that exists in three states: solid, liq-uid, and gas. This is important because when water changes from onestate to another, heat is either absorbed or released, and this heatgreatly affects the atmospheric motions that create weather and cli-mate.

of dust and salt. Dust is carried into the atmosphere by wind. Salt ispicked up from ocean spray. Dust and salt play a role in cloud for-mation, as you’ll learn later. Ice is the third solid found in the atmo-

sphere, usually in the form of hail and snow.

Ozone Another component of the atmosphere, ozone (O3), isa gas formed by the addition of a third oxygen atom to an oxygenmolecule (O2). Ozone exists in small quantities mainly in a layer wellabove Earth’s surface. It is important because it absorbs ultravioletradiation from the Sun. If ozone did not control the amount of ultra-violet radiation reaching Earth’s surface, our fragile skin could nottolerate exposure to the Sun for long. Evidence indicates that theozone layer is thinning. You’ll learn more about this issue in theScience in the News feature at the end of this chapter and in laterchapters.

STRUCTURE OF THE ATMOSPHEREThe atmosphere is made up of several different layers, as shown inFigure 11-2. Each layer differs in composition and temperature.


EnvironmentalConnection

Figure 11-2 The five mainlayers of the atmospherevary in temperature andchemical composition.

Exosphere

Satellite

Meteor

Thermosphere

Mesosphere

Stratosphere

Troposphere

700

(km)

600

500

400

300

200

100

50

10

0

Aurora caused by particles from the Sun interacting with Earth’s atmosphere

Ozone layer

Weather balloon

ozone (O(O3), is), isa gas formed by the addition of a third oxygen atom to an oxygenmolecule (Omolecule (O2). Ozone exists in small quantities mainly in a layer wellabove Earth’s surface. It is important because it absorbs ultravioletradiation from the Sun. If ozone did not control the amount of ultra-violet radiation reaching Earth’s surface, our fragile skin could nottolerate exposure to the Sun for long. Evidence indicates that the

Figure 11-3 Differences inchemical composition causeair temperatures to varythroughout the atmosphere.

Lower Atmospheric Layers Thelayer closest to Earth’s surface, the troposphere, contains most of themass of the atmosphere, includingwater vapor. This is the layer in whichmost weather takes place and most airpollution collects. The troposphere ischaracterized by a general decrease intemperature from bottom to top. Theupper limit of the troposphere, calledthe tropopause, varies in height. It’sabout 16 km above Earth’s surface inthe tropics and about 9 km or less at thepoles. The tropopause is where thegradual decrease in temperature stops.

Above the tropopause is the stratosphere, a layer made up primarilyof concentrated ozone. Ozone absorbsmore ultraviolet radiation than does airin the troposphere. As a result, thestratosphere is heated, and air graduallyincreases in temperature to the top ofthe layer, called the stratopause, locatedabout 50 km above Earth’s surface.

Upper Atmospheric Layers Above the stratopause is themesosphere. There is no concentrated ozone in the mesosphere, sothe temperature decreases once again, as shown in Figure 11-3. Thetop of this layer, the mesopause, is the boundary between the meso-sphere and the next layer, the thermosphere. The thermospherecontains only a minute portion of the atmosphere’s mass. What airdoes exist in this layer increases in temperature once again, this timeto more than 1000°C. In the thermosphere, however, the moleculesthat make up air are so sparse and widely spaced that, despite thehigh temperature, this layer would not seem warm to a human pass-ing through it.

The ionosphere is part of the thermosphere. It is made up of elec-trically charged particles and layers of progressively lighter gases.The exosphere is the outermost layer of Earth’s atmosphere. Lightgases such as helium and hydrogen are found in this layer. Above theexosphere lies outer space. There is no clear boundary between theatmosphere and space. There are simply fewer and fewer moleculeswith increasing altitude until, for all practical purposes, you haveentered outer space.


Temperature (°C)

Thermosphere

Stratosphere

Tropopause

Troposphere

Mesosphere

Mesopause

Stratopause

Highest

concentration

of ozone

10

–80 –60 –40 –20 0 20 40 60 80

20

30

40

50

60

70

80

90

100

110

120

Altitude (km)

0

Temperature Variations with Altitude

6% reflected by

the atmosphere

25% reflected

from clouds

50% directly or indirectly

absorbed by Earth’s surface

15% absorbed

by the

atmosphere

4% reflected from

Earth’s surface

SOLAR FUNDAMENTALSThe Sun is the source of all energy in the atmosphere. This energy istransferred to Earth and throughout the atmosphere in three ways.

Radiation The Sun is shining on, and therefore warming, someportion of Earth’s surface at all times. This method of energy trans-fer is called radiation. Radiation is the transfer of energy throughspace by visible light, ultraviolet radiation, and other forms of elec-tromagnetic waves. All substances that have temperatures aboveabsolute zero emit radiation. The higher the temperature of a sub-stance, the shorter the wavelength it emits.

While Earth is absorbing solar radiation, it is also continuouslysending energy back into space. As you can see from Figure 11-4,about 35 percent of incoming solar radiation is reflected into spaceby Earth’s surface, the atmosphere, or clouds. Another 15 percent isabsorbed by the atmosphere itself. This means that only about 50percent of incoming solar radiation is absorbed directly or indirectlyby Earth’s surface. The rate of absorption for any particular areavaries depending on the physical characteristics of the area and theamount of solar radiation it receives. Different areas absorb energyand heat up at different rates. For example, water heats up and coolsdown more slowly than land. And, as a general rule, darker objectsabsorb energy faster than lighter ones.


Figure 11-4 Over thecourse of a year, Earth sends back into space justabout as much energy as itreceives from the Sun. Thisis fortunate: if Earth sentback too much, it wouldgradually cool off, while if it sent back too little, itwould warm up to poten-tially dangerous levels.

The Sun is shining on, and therefore warming, someportion of Earth’s surface at all times. This method of energy trans-fer is called radiation. Radiation is the transfer of energy throughspace by visible light, ultraviolet radiation, and other forms of elec-tromagnetic waves. All substances that have temperatures above

While Earth is absorbing solar radiation, it is also continuouslysending energy back into space. As you can see from

absorbed by the atmosphere itself. This means that only about 50percent of incoming solar radiation is absorbed directly or indirectlyby Earth’s surface. The rate of absorption for any particular area

amount of solar radiation it receives. Different areas absorb energyand heat up at different rates. For example, water heats up and coolsdown more slowly than land. And, as a general rule, darker objectsabsorb energy faster than lighter ones.absorb energy faster than lighter ones.rgy gh

For the most part, solar radiation does not heat air directly. How,then, does air become warm? Most of the solar radiation that trav-els through the atmosphere does so at short wavelengths. The atmo-sphere does not easily absorb short wavelengths, so much of thesolar radiation passes through the atmosphere and is absorbed byEarth’s surface. The surface then radiates energy, but the radiation itgives off has a longer wavelength than the energy coming from theSun. The energy radiated by Earth’s surface does not pass backthrough the atmosphere. Rather, it is absorbed by the atmosphereand warms air through the processes of conduction and convection,which, along with radiation, make up the three methods of energytransfer illustrated in Figure 11-5.

Conduction To understand how the energy radiated by Earth’ssurface warms the atmosphere, think about what happens when youturn on a burner on the stove. The hot burner radiates energy muchlike Earth’s surface does.

Now, imagine that you place a pot of water on the burner. Throughconduction, which is the transfer of energy that occurs when mol-ecules collide, energy is transferred from the bottom of the pot into thelowest part of the water. In the same way, energy is transferred fromthe particles of air near Earth’s surface to the particles of air in the low-est layer of the atmosphere. For conduction to occur, substances mustbe in contact with one another. That’s why conduction affects only avery thin atmospheric layer near Earth’s surface.

Figure 11-5 Energy istransferred throughout the atmosphere by theprocesses of conduction,convection, and radiation.

Cold surface

Radiation warms

Earth's surface

A few centimeters of air

near Earth's surface are

heated by conduction

Cold air pushes warm air

upward, creating a

convection current


For the most part, solar radiation does not heat air directly. How,then, does air become warm? Most of the solar radiation that trav-els through the atmosphere does so at short wavelengths. The atmo-sphere does not easily absorb short wavelengths, so much of thesolar radiation passes through the atmosphere and is absorbed byEarth’s surface. The surface then radiates energy, but the radiation itgives off has a longer wavelength than the energy coming from theSun. The energy radiated by Earth’s surface does not pass backthrough the atmosphere. Rather, it is absorbed by the atmosphereand warms air through the processes of conduction and convection,which, along with radiation, make up the three methods of energytransfer illustrated in

lowest part of the water. In the same way, energy is transferred fromthe particles of air near Earth’s surface to the particles of air in the low-est layer of the atmosphere. For conduction to occur, substances mustbe in contact with one another. That’s why conduction affects only a


Figure 11-6 Many differ-ent factors, including con-vection currents, cause thedifferent types of weathershown here.

Convection Once the energy has made its way into the lower partof the atmosphere, can it ever move higher? Recall the pot of water.Energy has been transferred by conduction to the lowest layer ofwater molecules. This heated water expands, becomes less dense, andforms bubbles that rise. The rising bubbles bring the warm water tothe top. The water at the top then cools, causing pockets of coolwater to sink and become reheated when they come into contactwith the bottom of the pot. This process is known as convection, thetransfer of energy by the flow of a heated substance—in this case, thewater. A similar process takes place in the atmosphere. Pockets of airnear Earth’s surface are heated, become less dense than the sur-rounding air, and rise. As the warm air rises, it expands and starts tocool. When it cools below the temperature of the surrounding air, itincreases in density and sinks. As it sinks, it warms again and theprocess starts anew. Convection currents, as these movements of airare called, are among the main mechanisms responsible for the ver-tical motions of air, which in turn cause the different types ofweather shown in Figure 11-6.

1. Describe the importance of water vaporin the atmosphere.

2. Why does temperature increase withheight through the stratosphere?

3. Rank the main atmospheric gases in thetroposphere in order from most abundantto least abundant. Do not include tracegases.

4. Thinking Critically Based on what youknow about radiation and conduction,what conclusion might you make about

summer temperatures in a large city com-pared with those in the surroundingcountryside?

SKILL REVIEW

5. Predicting Of the three main processes of energy transfer throughout the atmos-phere, which do you think plays thegreatest role in warming the upper troposphere? Why? For more help, refer to the Skill Handbook.

earthgeu.com/self_check_quiz

convection, thetransfer of energy by the flow of a heated substance—in this case, thewater. A similar process takes place in the atmosphere. Pockets of airnear Earth’s surface are heated, become less dense than the sur-rounding air, and rise. As the warm air rises, it expands and starts tocool. When it cools below the temperature of the surrounding air, itincreases in density and sinks. As it sinks, it warms again and theprocess starts anew. Convection currents, as these movements of airare called, are among the main mechanisms responsible for the ver-tical motions of air, which in turn cause the different types ofweather shown in

11.211.2 State of the Atmosphere

When people talk about the weather by saying that it’s sunny orcloudy or cold, they’re describing the current state of the atmo-sphere. Scientists describe the atmosphere, too, using words such astemperature, air pressure, wind speed, and the amount of moisture inthe air. These are atmospheric properties that describe weather con-ditions. We’ll examine each in turn, beginning with temperature.

TEMPERATURE VERSUS HEATMost of us tend to think of heat and temperature as being essentiallythe same thing. They are, in fact, two different concepts. Temperatureis a measurement of how rapidly or slowly molecules move around.More molecules or faster-moving molecules in a given space generatea higher temperature. Fewer molecules or slower-moving moleculesgenerate a lower temperature and cause a substance—air, forinstance—to cool.Heat, on the other hand, is the transfer of energythat occurs because of a difference in temperature between sub-stances. The direction of heat flow depends on temperature. Heatflows from an object of higher temperature to an object of lowertemperature. How does this relate to the atmosphere? Heat is thetransfer of energy that fuels atmospheric processes, while tempera-ture is used to measure and interpret that energy.

Measuring Temperature Temperaturecan be measured in degrees Fahrenheit(°F), in degrees Celsius (°C), or in kelvins(K). Fahrenheit is the scale most com-monly used in the United States. Celsius,the scale used in this book, is convenientbecause the difference between its freezingand boiling points is exactly 100 degrees.The kelvin is the SI unit of temperature.The Kelvin scale measures the number ofkelvins above absolute zero, which corre-sponds to approximately �273°C and�523°F. This scale is a more direct mea-sure of molecular activity, because atabsolute zero, molecular motion theoreti-cally stops. Because nothing can be colderthan absolute zero, there are no negativenumbers on the Kelvin scale. Figure 11-7compares the different temperature scales.

OBJECTIVES

• Describe the variousproperties of the atmo-sphere and how theyinteract.

• Explain why atmosphericproperties change withchanges in altitude.

VOCABULARY

temperatureheatdew pointcondensationlifted condensation leveltemperature inversionhumidityrelative humidity


32°F

212°F

Absolute

zero

–523°F

Freezing

point of

water

Boiling

point of

water

Fahrenheit

0°C

100°C

–273°C

Celsius

273 K

373 K

0 K

Kelvin

Figure 11-7 The Kelvin scale starts at 0 K, whichcorresponds to �273°C and �523°F.

sphere. Scientists describe the atmosphere, too, using words such astemperature, air pressure, wind speed, and the amount of moisture inthe air. These are atmospheric properties that describe weather con-ditions. We’ll examine each in turn, beginning with temperature.

Temperature

is a measurement of how rapidly or slowly molecules move around.More molecules or faster-moving molecules in a given space generatea higher temperature. Fewer molecules or slower-moving molecules

Heat, on the other hand, is the transfer of energythat occurs because of a difference in temperature between sub-stances. The direction of heat flow depends on temperature. Heatstances. The direction of heat flow depends on temperature. Heatflows from an object of higher temperature to an object of lowertemperature. How does this relate to the atmosphere? Heat is themperature. How does this relate to the atmosphere? Heat is thetransfer of energy that fuels atmospheric processes, while tempera-ture is used to measure and interpret that energy.

Temperaturecan be measured in degrees Fahrenheit(°F), in degrees Celsius (°C), or in kelvins(K). Fahrenheit is the scale most com-monly used in the United States. Celsius,the scale used in this book, is convenientbecause the difference between its freezingand boiling points is exactly 100 degrees.The kelvin is the SI unit of temperature.The Kelvin scale measures the number ofkelvins above absolute zero, which corre-sponds to approximately �273°C and�523°F. This scale is a more direct mea-523°F. This scale is a more direct mea-

11.2 State of the Atmosphere 279

Dew Point Another atmospheric measurement is the dew point.The dew point is the temperature to which air must be cooled atconstant pressure to reach saturation. Saturation is the point atwhich the air holds as much water vapor as it possibly can. The dewpoint is important because until air is saturated, condensation can-not occur. Condensation occurs when matter changes state from agas to a liquid. In this case, water vapor changes into liquid water andeventually falls as rain. Given its role in this process, the dew point isoften called the condensation temperature.

VERTICAL TEMPERATURE CHANGESThe temperature on a mountaintop is cooler than at lower elevationsbecause the temperature of the lower atmosphere decreases withincreasing distance from its heat source—Earth’s surface. Individualmasses of air moving upward through the atmosphere experience achange in temperature, too. An air mass that does not exchange heatwith its surroundings will cool off by about 10°C for every 1000-mincrease in altitude. This is called the dry adiabatic lapse rate—therate at which unsaturated air to which no heat is added or removedwill cool. If the air is able to continue rising, eventually it will cool toits condensation temperature. The height at which condensationoccurs is called the lifted condensation level (LCL). As shown inFigure 11-8, clouds form when water vapor condenses into waterdroplets, so the height of the LCL often corresponds to the base ofclouds. Above the LCL, air becomes saturated and cools more slowly.The rate at which saturated air cools is called the moist adiabaticlapse rate. This rate ranges from about 4°C/1000 m in very warm airto almost 9°C/1000 m in very cold air.

Temperature (°C)

Moist adiabatic

lapse rate

(6 C° per 1000 m)

Dry adiabatic

lapse rate

(10 C° per 1000 m)

Lifted

condensation

level

1000

2000

3000

4000

5000

0–10 10 20 30 40

Altitude (m)

Earth's

surface

Adiabatic Lapse RatesFigure 11-8 Condensationoccurs at the lifted conden-sation level (LCL). Air abovethe LCL is saturated andthus cools more slowly thanair below the LCL.

dew point is the temperature to which air must be cooled atconstant pressure to reach saturation. Saturation is the point atwhich the air holds as much water vapor as it possibly can. The dewpoint is important because until air is saturated, condensation can-not occur. Condensation occurs when matter changes state from agas to a liquid. In this case, water vapor changes into liquid water andeventually falls as rain. Given its role in this process, the dew point is

The temperature on a mountaintop is cooler than at lower elevationsbecause the temperature of the lower atmosphere decreases withincreasing distance from its heat source—Earth’s surface. Individualmasses of air moving upward through the atmosphere experience achange in temperature, too. An air mass that does not exchange heat

cool. If the air is able to continue rising, eventually it will cool toits condensation temperature. The height at which condensationoccurs is called the lifted condensation level (LCL). As shown in

clouds form when water vapor condenses into waterdroplets, so the height of the LCL often corresponds to the base ofclouds. Above the LCL, air becomes saturated and cools more slowly.

AIR PRESSURE AND DENSITYJust like water in the ocean, air has mass and constantly exerts pres-sure on our bodies. Why? The gravitational attraction between Earthand atmospheric gases causes particles of gas to be pulled toward thecenter of Earth. You don’t notice this pressure because you have spentyour whole life under it and are accustomed to it. A fish living deepin the ocean exists under pressure that would crush our bodies, butthe fish survives because its body is adapted to such pressure. Just aswater pressure increases with depth in the ocean, pressure increasesas you near the bottom of the atmosphere because of the greater massof the atmosphere above you. Conversely, atmospheric pressuredecreases with height because there are fewer and fewer gas particlesexerting pressure.

The density of air is proportional to the number of particles of airoccupying a particular space. As Table 11-1 shows, the density of airincreases as you get closer to the bottom of the atmosphere. This isbecause gases at the top of the atmosphere press down on the airbelow, thereby compressing the particles and increasing the densityof the air. Thus, at the top of a mountain, temperature, pressure, anddensity are all less than they are at lower elevations.

PRESSURE-TEMPERATURE-DENSITY RELATIONSHIPThe previous discussion raises an important point about the atmo-sphere: temperature, pressure, and density are related, as shown inTable 11-2. In the atmosphere, temperature is directly proportionalto pressure. So, if an air mass maintains a certain density—that is,the number of gas particles in a fixed volume remains the same—as temperature increases or decreases, pressure does, too. By the


Table 11-1 Density Changes With Altitude

Altitude Density Altitude Densitykm g/L km g/L

0 1.23 30 0.018

2 1.01 40 0.004

4 0.82 50 0.001

6 0.66 60 0.0003

8 0.53 70 0.00009

10 0.41 80 0.00002

15 0.19 90 0.000003

20 0.09 100 0.0000005

Table 11-2

Atmospheric

Relationships

As T ↑, P ↑

As T ↓, P ↓

As T ↓, D ↑

As T ↑, D ↓

T = Temperature

P = Pressure

D = Density

↑ = Increases

↓ = Decreases

Just like water in the ocean, air has mass and constantly exerts pres-sure on our bodies. Why? The gravitational attraction between Earthand atmospheric gases causes particles of gas to be pulled toward thecenter of Earth. You don’t notice this pressure because you have spentyour whole life under it and are accustomed to it. A fish living deepyyin the ocean exists under pressure that would crush our bodies, butthe fish survives because its body is adapted to such pressure. Just aswater pressure increases with depth in the ocean, pressure increasesas you near the bottom of the atmosphere because of the greater massof the atmosphere above you. Conversely, atmospheric pressuredecreases with height because there are fewer and fewer gas particlesexerting pressure.

because gases at the top of the atmosphere press down on the airbelow, thereby compressing the particles and increasing the densityof the air. Thus, at the top of a mountain, temperature, pressure, anddensity are all less than they are at lower elevations.


same token, as pressure increases or decreases, temperature does,too. You will further explore this relationship in the GeoLab at theend of this chapter.

The relationship between temperature and density, on the otherhand, is inversely proportional. So, if an air mass maintains a certainpressure, as temperature increases, density decreases, and as temper-ature decreases, density increases. This is why air rises when its tem-perature increases—it becomes less dense.

In most atmospheric interactions, however, neither density norpressure remains unchanged, and this muddles the relationshipamong temperature, pressure, and density. Earlier, for example, wenoted that both temperature and density decrease with increasingaltitude in the troposphere. If density decreases with height, how cantemperature decrease as well if it is inversely proportional to density?The answer lies in the fact that temperature varies with changes inboth pressure and density. In this case, temperature is proportionalto the ratio of pressure to density, which decreases with increasingaltitude.

Temperature Inversions In the atmosphere, the relationshipbetween temperature and pressure is not always fixed.Although tem-perature and pressure in the overall troposphere decrease withheight, there is an exception to this rule known as a temperatureinversion. A temperature inversion is an increase in temperaturewith height in an atmospheric layer. It’s called a temperature inver-sion because the temperature-altitude relationship is inverted, orturned upside down. This can happen in several ways. We’ll considerone that involves the rapid cooling of landon a cold, clear, winter night when the windis calm. Under these circumstances, thelower layers of the atmosphere are notreceiving heat from Earth’s surface—they’re losing heat. As a result, the lowerlayers of air become cooler than the airabove them, so that temperature increaseswith height and forms a temperature inver-sion. In some cities, such as the one shownin Figure 11-9, a temperature inversion canworsen air-pollution problems by actinglike a lid to trap pollution under the inver-sion layer. In all cases, the presence orabsence of inversions can have a profoundeffect on weather conditions, as you’ll learnin the next chapter.

Figure 11-9 A temperatureinversion in Long Beach,California, traps air pollu-tion above the city.

In the atmosphere, the relationshipbetween temperature and pressure is not always fixed.Although tem-perature and pressure in the overall troposphere decrease withheight, there is an exception to this rule known as a temperatureinversion. A temperature inversion is an increase in temperaturewith height in an atmospheric layer. It’s called a temperature inver-sion because the temperature-altitude relationship is inverted, orturned upside down. This can happen in several ways. We’ll considerone that involves the rapid cooling of landon a cold, clear, winter night when the windis calm. Under these circumstances, thelower layers of the atmosphere are notreceiving heat from Earth’s surface—they’re losing heat. As a result, the lowerlayers of air become cooler than the airabove them, so that temperature increaseswith height and forms a temperature inver-sion. In some cities, such as the one shown

WIND

You may have entered a large, air-conditioned building on a hotsummer day. As you opened the door, a sudden rush of cool airgreeted you. This happened because the air conditioner created animbalance between the warm, less-dense air outside the building andthe cool, more-dense air inside. The cool air, being denser, had set-tled toward the bottom of the building. When the door opened, thecool, dense air rushed out to try to relieve the imbalance. The rush ofair that you experienced is commonly known as wind.

In essence, the atmosphere works much like an air-conditionedbuilding. Cool air, being more dense, sinks and forces warm, less-dense air upward. In the lower atmosphere, air generally moves fromareas of high density to areas of low density. The air moves inresponse to density imbalances created by the unequal heating andcooling of Earth’s surface. These imbalances, in turn, create areas ofhigh and low pressure. In its simplest form, wind can be thought ofas air moving from an area of high pressure to an area of low pres-sure. Wind is usually measured in miles per hour or kilometers perhour. Ships at sea usually measure wind in knots. One knot is equalto 1.85 km/h.

Like temperature and pressure, wind changes with height in theatmosphere. Why? Near Earth’s surface, wind is constantly disruptedby the friction that results from its contact with trees, buildings, andhills—even the surface of water affects air motion. Farther up fromEarth’s surface, air encounters less friction, and wind speeds increase.Look at Figure 11-10. Would you expect the wind to blow morestrongly over the ocean or across the dunes?

Figure 11-10 When windblows over these sand dunesin Namibia, it encountersmore friction than when itblows over water.


You may have entered a large, air-conditioned building on a hotYYsummer day. As you opened the door, a sudden rush of cool airgreeted you. This happened because the air conditioner created angreeted ygreeted yimbalance between the warm, less-dense air outside the building andthe cool, more-dense air inside. The cool air, being denser, had set-tled toward the bottom of the building. When the door opened, thecool, dense air rushed out to try to relieve the imbalance. The rush ofair that you experienced is commonly known as wind.

building. Cool air, being more dense, sinks and forces warm, less-dense air upward. In the lower atmosphere, air generally moves fromareas of high density to areas of low density. The air moves inresponse to density imbalances created by the unequal heating andcooling of Earth’s surface. These imbalances, in turn, create areas ofhigh and low pressure. In its simplest form, wind can be thought ofas air moving from an area of high pressure to an area of low pres-sure. Wind is usually measured in miles per hour or kilometers per

Like temperature and pressure, wind changes with height in theatmosphere. Why? Near Earth’s surface, wind is constantly disruptedby the friction that results from its contact with trees, buildings, andhills—even the surface of water affects air motion. Farther up fromEarth’s surface, air encounters less friction, and wind speeds increase.


RELATIVE HUMIDITYJust for fun, reach out and grab a handful of air. You may not knowit, but you also grabbed some water vapor. Air in the lower portionof the atmosphere always contains at least some water vapor, eventhough that amount may be very small. The amount of water vaporin air is referred to as humidity.

Imagine now that you take your handful of air—and its watervapor—into a room full of dry air and let it go. Would that roomfulof air have the same humidity as your handful? No, because the watervapor in that handful would be very small relative to how muchwater vapor that roomful of air could actually hold. The ratio ofwater vapor in a volume of air relative to how much water vapor thatvolume of air is capable of holding is called relative humidity.As thegraph in the Problem-Solving Lab shows, relative humidity varieswith temperature. Warm air is capable of holding more moisturethan cool air. Thus, if the temperature of a room increased, the air in

Using Numbers At20°C, a cubic meterof air can hold atotal of 17 g of watervapor. What is theair’s relative humidityif it holds only 6 g ofwater vapor?

Determine relative humidityRelative humidity is the ratio of water

vapor in a given volume of air compared

with how much water vapor that volume of

air can actually hold. Use the graph at the

right to answer the following questions.

Analysis

1. How much water vapor can a cubicmeter of air hold at 25°C?

2. How much water vapor can the samevolume of air hold at 15°C?

Thinking Critically

3.Why do the values in questions 1 and 2 differ?

4. If the relative humidity of the air inquestion 1 was 50 percent, how much

water vapor would it hold?

5. If you wanted to decrease the relativehumidity of a room, would you increase

or decrease its temperature? Explain

your answer.

Interpreting Graphs

Temperature (°C)

–20 –10 0 10 20 30 40

Water vapor (grams per cubic m

eter)

0

10

20

30

40

50

60

70

Humidity Changeswith Temperature

it, but you also grabbed some water vapor. Air in the lower portionof the atmosphere always contains at least some water vapor, eventhough that amount may be very small. The amount of water vaporin air is referred to as humidity.

water vapor that roomful of air could actually hold. The ratio ofwater vapor in a volume of air relative to how much water vapor thatvolume of air is capable of holding is called relative humidity.

ature. Warm air is capable of holding more moisturethan cool air. Thus, if the temperature of a room increased, the air inthan cool air. Thus, if the temperature of a room increased, the air in

the room would be capable of holding more moisture. If no addi-tional water vapor was added to the air, its relative humidity woulddecrease. Conversely, if more water vapor was added to the air, its rel-ative humidity would increase. Do the Problem-Solving Lab on theprevious page to learn more about relative humidity.

Relative humidity is expressed as a percentage. If a certain volumeof air is holding as much water vapor as it possibly can, then its rel-ative humidity is 100 percent. If that same volume of air is holdinghalf as much water vapor as it can, its relative humidity is 50 percent,and so on. Recall that air is saturated when it holds as much watervapor as it possibly can. As you’ll see next, this has important impli-cations for the development of precipitation and clouds such asthose shown in Figure 11-11.

Figure 11-11 Clouds formwhen a mass of rising airbecomes saturated and condenses its water vaporinto large groups of waterdroplets.


1. How is dew point related to saturation?

2. What is the relationship between temper-ature and altitude in a temperature inver-sion?

3. How does atmospheric pressure changewith height in the atmosphere? Why doesit change?

4. Compare and contrast humidity and rela-tive humidity.

5. Thinking Critically Which would meltmore ice—a pot of hot water or a tub ofwarm water? Explain your answer.

SKILL REVIEW

6. Designing an Experiment Design anexperiment that shows how average windspeeds change over different types of sur-faces. For more help, refer to the SkillHandbook.


the room would be capable of holding more moisture. If no addi-tional water vapor was added to the air, its relative humidity woulddecrease. Conversely, if more water vapor was added to the air, its rel-ative humidity would increase. Do the

Relative humidity is expressed as a percentage. If a certain volumeof air is holding as much water vapor as it possibly can, then its rel-ative humidity is 100 percent. If that same volume of air is holdinghalf as much water vapor as it can, its relative humidity is 50 percent,and so on. Recall that air is saturated when it holds as much watervapor as it possibly can. As you’ll see next, this has important impli-

11.3 Moisture in the Atmosphere 285

OBJECTIVES

• Explain how clouds areformed.

• Identify the basic char-acteristics of differentcloud groups.

• Describe the water cycle.

VOCABULARY

condensation nucleiorographic liftingstabilitylatent heatcoalescenceprecipitationwater cycleevaporation

11.311.3 Moisture in the Atmosphere

Would you like to be able to predict the weather? To do so, you’llprobably need to learn more about clouds. Certain types of cloudsare associated with certain types of weather. Before learning aboutcloud types, however, you need to understand how clouds form.

CLOUD FORMATIONYou know that air generally contains some amount of water vaporand that warm, less-dense air rises, while cool, more-dense air sinks.This tendency to rise or sink as a result of differences in density iscalled buoyancy. As you can see in Figure 11-12, clouds form whenwarm, moist air rises, expands, and cools in a convection current. Asthe air reaches its dew point, the water vapor in the air condensesaround condensation nuclei. Condensation nuclei are small parti-cles in the atmosphere around which cloud droplets can form. Theycome from a variety of sources, including sea salt and dust. Whenmillions of these droplets collect, a cloud forms.

Clouds can also form when wind encounters a mountain and theair has no place to go but up. The effect is the same as with any risingair—it expands and cools. This method of cloud formation, shown inFigure 11-13A on the next page, is called orographic lifting.Anothermethod of cloud formation involves the collision of air masses of dif-ferent temperatures, as shown in Figure 11-13B on the next page.Recall that cold, more-dense air is heavier than warm, less-dense air,so it tends to collect near Earth’s surface. As warmer air moves intothe area, some of it will warm up the cold air, but the bulk of it willbe forced to rise over the more-dense, cold air. As the warm air cools,the water vapor in it condenses and forms a cloud.

Figure 11-12 Clouds formwhen warm air is forced upin a convection current.

probably need to learn more about clouds. Certain types of cloudsare associated with certain types of weather. Before learning about

You know that air generally contains some amount of water vaporYYand that warm, less-dense air rises, while cool, more-dense air sinks.

clouds form whenwarm, moist air rises, expands, and cools in a convection current. Asthe air reaches its dew point, the water vapor in the air condensesaround condensation nuclei. Condensation nuclei are small parti-cles in the atmosphere around which cloud droplets can form. Theycome from a variety of sources, including sea salt and dust. Whenmillions of these droplets collect, a cloud forms.

Clouds can also form when wind encounters a mountain and theair has no place to go but up. The effect is the same as with any risingair—it expands and cools. This method of cloud formation, shown inFigure 11-13AFF on the next page, is called orographic lifting.

Recall that cold, more-dense air is heavier than warm, less-dense air,so it tends to collect near Earth’s surface. As warmer air moves intothe area, some of it will warm up the cold air, but the bulk of it willbe forced to rise over the more-dense, cold air. As the warm air cools,the water vapor in it condenses and forms a cloud.


Stability Regardless of how a cloud forms, all rising air expandsand cools. How rapidly any given mass of air cools determines its sta-bility. Stability is the ability of an air mass to resist rising. Imaginean air mass that is warmer than the surface beneath it. Heat flowsfrom the warmer air to the colder surface. The lower layer of the airmass thus loses heat and cools. The cooling air resists rising—it isstable. The rate at which an air mass cools depends in part on thetemperature of the surface beneath the air. The temperature of sur-rounding air masses and the temperature of the air mass itself alsoplay a role in determining the cooling rate.

Air can become unstable if it is cooler than the surface beneath it.In this case, heat flows from the warmer surface to the cooler air. Theair warms and becomes less dense than the surrounding air. The less-dense air mass rises. If temperature conditions are right and the airmass rises rapidly, it can produce the type of clouds associated withthunderstorms.

Latent Heat As water vapor in the air condenses, heat is released.Where does this heat come from? It takes energy to change liquidwater into a gaseous state. The energy that is transferred to the gasdoesn’t just go away; it is stored in the water vapor and will not bereleased into the air until condensation occurs. The stored energy iscalled latent heat.Until condensation occurs, latent heat is not avail-able to warm the atmosphere.

When condensation takes place, latent heat is released and warmsthe air. At any given time, the amount of water vapor present in theatmosphere is a significant source of energy because of the latentheat it contains. When condensation occurs, this latent heat can pro-vide energy to a weather system, thereby increasing its intensity.

Warm airCold air

BA

Figure 11-13 Clouds formwhen warm moist air isforced to rise over a moun-tain (A) and when two airmasses of different temper-atures meet (B).

As water vapor in the air condenses, heat is released.Where does this heat come from? It takes energy to change liquidwater into a gaseous state. The energy that is transferred to the gasdoesn’t just go away; it is stored in the water vapor and will not bereleased into the air until condensation occurs. The stored energy iscalled latent heat.Until condensation occurs, latent heat is not avail-able to warm the atmosphere.

When condensation takes place, latent heat is released and warmsthe air. At any given time, the amount of water vapor present in theatmosphere is a significant source of energy because of the latentheat it contains. When condensation occurs, this latent heat can pro-vide energy to a weather system, thereby increasing its intensity.

Table 11-3 Cloud Classification

Height Shape

Prefix Prefix


TYPES OF CLOUDSWhen a mass of rising air reaches its lifted condensation level or LCL,water vapor condenses into droplets of liquid water or ice, depend-ing on the temperature. If the density of these droplets is greatenough, they become visible in the form of a cloud. While this is thebasic principle behind the formation of all clouds, this process cantake place at many different altitudes—sometimes even in contactwith Earth’s surface, in which case it is known as fog. In addition toforming at different heights, clouds form in different shapes,depending on the factors involved in their formation.

Clouds are generally classified according to a system originallydeveloped by English naturalist Luke Howard in 1803. As shown inTable 11-3, the modern system groups clouds by the altitude atwhich they form and by their shape. Low clouds typically form below2000 m. Middle clouds form mainly between 2000 m to 6000 m.High clouds composed of ice crystals form above 6000 m. The finalgroup of clouds includes those that spread throughout all altitudes—at the same time, no less. These are vertical development clouds.

Cirrus

Latin meaning: “hair.”Describes wispy, stringy clouds.

Cumulus

Latin meaning: “pile or heap.”Describes puffy, lumpy-looking clouds.

Stratus

Latin meaning: “layer.”Describes featureless sheets of clouds.

Nimbus

Latin meaning: “cloud.”Describes low, gray rain clouds.

Alto

Describes middle clouds with bases between 2000 m to 6000 m.

Cirro

Describes high clouds with bases starting above 6000 m.

Strato

Refers to low clouds below 2000 m.

Topic: CloudsTo find out more aboutclouds, visit the EarthScience Web Site at earthgeu.com

Activity: Make a poster or media presentationshowing the types ofclouds you observed duringa one-week period.

When a mass of rising air reaches its lifted condensation level or LCL,WWwater vapor condenses into droplets of liquid water or ice, depend-ing on the temperature. If the density of these droplets is greatenough, they become visible in the form of a cloud.While this is thebasic principle behind the formation of all clouds, this process cantake place at many different altitudes—sometimes even in contactwith Earth’s surface, in which case it is known as fog. In addition toforming at different heights, clouds form in different shapes,depending on the factors involved in their formation.

the modern system groups clouds by the altitude atwhich they form and by their shape. Low clouds typically form below

Low Clouds Imagine a warm, summer afternoon. The Sun is beat-ing down, heating Earth’s surface. In areas where the heating is par-ticularly intense, such as fields with dark soil, conduction causes airabove the surface to become warmer than the surrounding air. As thetemperature rises, the air expands. Its density becomes lower thanthat of surrounding air and it begins to rise and cool by furtherexpansion. When it reaches its LCL, it becomes saturated, and thewater vapor it contains condenses into water droplets. These dropletseventually become numerous enough to form a visible cloud. If theair stays warmer than the surrounding air, the cloud will continue togrow. If the air does not stay warmer than the surrounding air, thecloud will flatten out and winds will spread it horizontally into stra-tocumulus or layered cumulus clouds. Another type of low cloud thatforms at heights below 2000 m is a stratus, a layered cloud that coversmuch or all of the sky in a given area. Stratus clouds often form whenfog lifts away from Earth’s surface. Figure 11-14 shows these andother types of clouds.


Freezing level, above which clouds consist of ice crystals

Cirrus

Cirrostratus

Cumulonimbus

Stratocumulus

Altocumulus

Cirrocumulus

Cumulus

Nimbus

Nimbostratus

Altostratus

Stratus

(km)

13

12

11

10

9

8

7

6

5

4

3

2

1

0

Figure 11-14 Clouds form atdifferent heights and in dif-ferent shapes. Compare andcontrast cirrus and stratusclouds.


Middle Clouds Altocumulus and altostratus clouds, which format heights between 2000 m and 6000 m, can be either all liquid or amixture of liquid and ice crystals. This is due to the cooler tempera-tures generally present at the heights at which these clouds form.Middle clouds are usually layered. Altocumulus clouds often resem-ble white fish scales. Altostratus clouds are dark but thin veils ofclouds that sometimes produce mild precipitation.

High Clouds Because they form above heights of 6000 m, wheretemperatures are below freezing, high clouds are made up of ice crys-tals. Thus, some, such as cirrus clouds, often have a wispy, indistinctappearance. Another type of cirriform cloud, called a cirrostratus,forms as a continuous layer that sometimes covers the sky. Cirro-stratus clouds can vary in thickness from being almost transparent tobeing dense enough to block out the Sun or Moon.

Clouds of Vertical Development If the air that makes up acumulus cloud is unstable enough, the cloud will be warmer than thesurface or surrounding air and will continue to grow. As it rises,water vapor condenses, and the air receives additional warmth fromthe release of latent heat. The cloud can grow through middle alti-tudes as a towering cumulus; if conditions are right, it can reachnearly 18 000 m. Its top is then composed of ice crystals. Strongwinds can spread it into a familiar anvil shape. A puffy, white cumu-lus cloud can thus develop into a full-fledged cumulonimbus, asshown in Figure 11-15. What began as a small mass of moist air isnow an atmospheric giant, capable of producing the torrential rainsand strong winds that are characteristic of thunderstorms.

PRECIPITATIONWhen cloud droplets collide, they jointogether to form a larger droplet in a processcalled coalescence. As the process continues,the droplet eventually becomes too heavy tobe held aloft. At this point, gravity takes overand the droplet falls to Earth as precipita-tion. Precipitation includes all forms ofwater, both liquid and solid, that fall fromclouds. Rain, snow, sleet, and hail are thefour main types of precipitation. Coales-cence is the primary process responsible forthe formation of precipitation from warmclouds. Precipitation from cold clouds gen-erally involves the interaction of ice and

Figure 11-15 Cumulo-nimbus clouds, such as thisone, in Arizona, are associ-ated with thunderstorms.

When cloud droplets collide, they jointogether to form a larger droplet in a processcalled coalescence. As the process continues,the droplet eventually becomes too heavy tobe held aloft. At this point, gravity takes overand the droplet falls to Earth as precipita-tion. Precipitation includes all forms ofwater, both liquid and solid, that fall fromclouds. Rain, snow, sleet, and hail are thefour main types of precipitation. Coales-cence is the primary process responsible forthe formation of precipitation from warmclouds. Precipitation from cold clouds gen-erally involves the interaction of ice and

water molecules in the clouds. Do theMiniLab on this page to model the forma-tion of clouds and precipitation.Why are there so many variations in pre-

cipitation? When precipitation forms at coldtemperatures, it takes the form of ice crystalsor snow. Sometimes, convective currentscarry the droplets up and down throughfreezing and nonfreezing air, thereby form-ing ice pellets or sleet. If that up-and-downmotion is especially strong and takes placeover large stretches of the atmosphere, it canform very large ice pellets known as hail.Figure 11-16 shows a sample of hail.

THE WATER CYCLEThe total amount of water on Earth is con-stant, and probably has been for millions ofyears. More than 97 percent of Earth’s wateris salt water found in oceans. Only three per-cent is freshwater, and two-thirds of this isfrozen in ice caps at the poles. At any onetime, only a small percentage of water is pre-sent in the atmosphere. Still, this water isvitally important because as it continuallymoves between the atmosphere and Earth’ssurface, it nourishes living things. The con-stant movement of water between the atmo-sphere and Earth’s surface is known as thewater cycle.

The water cycle, shown in Figure 11-17,receives its energy from the Sun. Radiationfrom the Sun causes liquid water to changeinto a gas. The process of water changingfrom a liquid to a gas is called evaporation.This is the first step in the water cycle. Waterevaporates from lakes, streams, and oceans

Figure 11-16 Note the dis-tinctive layers in the cross-section of the hailstone. Infer how the layers formed.


What affects the formation of clouds and precipitation?

Model the water cycle.

Procedure

1. Pour about 125 mL of warm water into a

clear, plastic bowl.

2. Loosely cover the top of the bowl with

plastic wrap. Overlap the edges by about

5 cm.

3. Fill a self-sealing plastic bag with ice

cubes, seal it, and place it in the center of

the plastic wrap on top of the bowl. Push

the bag of ice down so that the plastic

wrap sags in the center, but doesn’t touch

the surface of the water.

4. Use tape to seal the plastic wrap around

the bowl.

5. Observe the surface of the plastic wrap

directly under the ice cubes every 10 min-

utes for one-half hour, or until the ice melts.

Analyze and Conclude

1. What formed on the underside of the

wrap? Infer why this happened.

2. Relate your observations to processes in

the atmosphere.

3. Predict what would happen if you

repeated this activity with hotter water.

water molecules in the clouds. Do the

cipitation? When precipitation forms at coldtemperatures, it takes the form of ice crystalsor snow. Sometimes, convective currentscarry the droplets up and down throughfreezing and nonfreezing air, thereby form-ing ice pellets or sleet. If that up-and-downmotion is especially strong and takes placeover large stretches of the atmosphere, it canform very large ice pellets known as hail.

The total amount of water on Earth is con-stant, and probably has been for millions ofyears. More than 97 percent of Earth’s wateris salt water found in oceans. Only three per-cent is freshwater, and two-thirds of this isfrozen in ice caps at the poles. At any onetime, only a small percentage of water is pre-sent in the atmosphere. Still, this water isvitally important because as it continuallymoves between the atmosphere and Earth’ssurface, it nourishes living things. The con-stant movement of water between the atmo-sphere and Earth’s surface is known as thewater cycle.

The water cycle, shown in Figure 11-17,FF

receives its energy from the Sun. Radiationfrom the Sun causes liquid water to changeinto a gas. The process of water changing


1. Explain why a cumulonimbus cloud is notconsidered to be a low, middle, or highcloud.

2. Describe the process that causes a waterdroplet to fall to Earth as precipitation.

3. What determines whether precipitationwill fall as rain or snow?

4. Thinking Critically Based on what youhave learned about latent heat, explainwhy the lapse rate of moist air is less thanthat of dry air.

SKILL REVIEW

5. Concept Mapping Use the following termsto construct a concept map that describesthe processes of the water cycle. For morehelp, refer to the Skill Handbook.

and rises into Earth’s atmosphere. As water vapor rises, it cools andchanges back into a liquid. This process, as you have learned, is calledcondensation, the second step of the water cycle. When water vaporcondenses, it forms clouds.In the third step of the water cycle, water droplets combine to

form larger drops that fall to Earth as precipitation. This water soaksinto the ground and enters lakes, streams, and oceans, or it fallsdirectly into these bodies of water and eventually evaporates, and thewater cycle continues.

waterchanges fromliquid to gas

waterchanges fromgas to liquid

water fallsas rain, snow,sleet, or hail

precipitationevaporationwater cycle condensation

GroundwaterRunoff

Precipitation

Evaporation

Condensation

Figure 11-17 Water movesfrom Earth to the atmos-phere and back to Earth inthe water cycle.


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