Chapter 14 The Atmosphere470

1Characteristics of the AtmosphereKey Concept Earth’s atmosphere absorbs solar energyand transports energy around Earth’s surface.

The atmosphereatmosphere is a mixture of gases that surrounds Earth.The atmosphere contains the oxygen you need to live. It alsoprotects you from the sun’s damaging rays. Every breath youtake, every tree that is planted, and every automobile you ride inaffect the atmosphere’s composition.

The Composition of the AtmosphereThe mixture of gases that makes up the atmosphere is

commonly called air. As you can see in Figure 1, most of thismixture is nitrogen. About 78% of the atmosphere is nitrogen.Oxygen makes up about 21% of the atmosphere. The other 1%of the atmosphere is made of other gases, such as argon, carbondioxide, and water vapor. Water vapor is an invisible gas thatforms when water reaches a certain temperature. Sometimes,water vapor can make up as much as 4% of the air.

The atmosphere also contains liquids and solids. Liquidwater (water droplets) and solid water (snow and ice crystals)are found in clouds. The atmosphere also contains small par-ticles, such as dust, volcanic ash, sea salt, dirt, and smoke. Youcan turn off the lights at night and shine a flashlight to seesome of these tiny particles floating in the air.

What You Will Learn• Earth’s atmosphere is a mixture

of gases that surrounds Earthand absorbs solar radiation.

• Pressure and temperature in theatmosphere change as distancefrom Earth’s surface increases.

Why It MattersThe atmosphere is a protectivelayer that allows life to survive onEarth’s surface.

Vocabulary• atmosphere • stratosphere• air pressure • mesosphere• troposphere • thermosphere

Graphic Organizer In your ScienceJournal, create Combination Notesthat express information about thelayers of the atmosphere in wordsand in pictures or diagrams.

Nitrogen, the most commonatmospheric gas, is releasedwhen dead plants and deadanimals break down andwhen volcanoes erupt.

Oxygen, the second mostcommon atmospheric gas,is made by living things,such as plants.

Argon, carbon dioxide,water vapor, and othergases make up the remain-ing 1% of the atmosphere.

Nitrogen78%

Oxygen21%

1%

atmosphereatmosphere (AT muhs FIR) a mixtureof gases that surrounds a planet ormoon

air pressureair pressure (ER PRESH uhr) themeasure of the force with which airmolecules push on a surface Composition of the AtmosphereFigure 1

Quick Lab

Section 1 Characteristics of the Atmosphere 471

Air Pressure and TemperatureWhat would carrying a column of air that is 500 km high

feel like? You carry this load every day! You do not notice theload because your body is used to it. At sea level, a squareinch of surface area is under almost 7 kg (15 lb) of air.

Altitude and Air PressureGravity pulls gas molecules in the atmosphere toward

Earth’s surface. As a result, there are a lot of air moleculesnear Earth’s surface. When a large number of air molecules arecontained in a small space, those molecules exert a lot of pres-sure on one another and on surfaces around them. Air pressureAir pressureis the measure of the force with which air molecules push ona surface. The air pressure at any point in the atmosphere isequal to the weight of the air directly above that point. Airpressure is greatest at Earth’s surface because there is a lot ofair above Earth’s surface. A human pyramid models how airpressure changes in the atmosphere, as shown in Figure 2.

Why is air pressure highest near Earth’s surface?

6.4.e

Atmospheric Composition and Air TemperatureAir temperature also changes as altitude increases. The tem-

perature differences result from the way that energy is absorbedby gases in the atmosphere. Some parts of the atmosphere arewarmer because they contain higher concentrations of gasesthat absorb energy from the sun or from Earth’s surface.

Figure 2 In the atmosphere,air pressure is greatest atEarth’s surface. In a humanpyramid, pressure is greatestat the bottom.

Lowerpressure

Higherpressure

6.4.b Students know solar energy reaches Earththrough radiation, mostly in the form of visible light.6.4.e Students know differences in pressure,heat, air movement, and humidity result in changesof weather.

Modeling AirPressure1. Fill a plastic cup to

the brim with water.2. Firmly hold an index card

over the mouth of the cup.3. Quickly invert the glass

over a sink.4. Release the paper

square, andobserve whathappens.

5. How do theeffects of airpressure explainyour observations?

6.4.e

10 min

Chapter 14 The Atmosphere472

tropospheretroposphere (TROH poh SFIR) the lowest layer of the atmosphere, in which temperature decreases at a constant rate as altitude increases Wordwise The root trop- means “to turn.”

stratospherestratosphere (STRAT uh SFIR) the layer of the atmosphere that is above the troposphere and in which temperature increases as altitude increases

mesospheremesosphere (MES oh SFIR) the layer of the atmosphere between the stratosphere and the thermosphere and in which temperature decreases as altitude increases

thermospherethermosphere (THUHR moh SFIR) the uppermost layer of the atmosphere, in which temperature increases as altitude increases

Layers of the AtmosphereGases in Earth’s atmosphere absorb solar energy differently,

which causes temperature gradients. The atmosphere is divided into four main layers based on these temperature differences, as shown in Figure 3.

The Troposphere: The Layer in Which We LiveThe tropospheretroposphere is the layer of the atmosphere that lies

next to Earth’s surface. This layer contains almost 90% of the atmosphere’s total mass. Almost all water vapor, air pollution, and weather are in this layer. Temperature decreases as altitude increases in the troposphere. Differences in air temperature and density cause gases in this layer to mix continuously.

The Stratosphere: Home of the Ozone LayerThe layer above the tropo sphere is called the stratosphere.stratosphere.

The air in this layer is thin and has little moisture. In this layer, temperature rises as altitude increases. This rise happens because a layer of gas called ozone absorbs radiation from the sun, so the air warms. The ozone layer protects life on Earth by absorbing harmful ultraviolet radiation.

Figure 3 The layers of the atmosphere are defined by changes in temperature.

Review

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The Mesosphere: The Middle LayerAbove the stratosphere is the mesosphere. The mesospheremesosphere

is the middle layer of the atmosphere. It is also the coldestlayer. As in the troposphere, temperature decreases as altitudeincreases in the mesosphere.

The Thermosphere: The Edge of the AtmosphereThe uppermost atmospheric layer is called the thermosphere.thermosphere.

In the thermosphere, temperature again increases with altitude.Temperature increases in this layer because atoms of nitrogenand oxygen absorb high-energy solar radiation.

Why are temperatures high in the stratosphereand thermosphere? 6.4.b

Modeling the AtmosphereIn teams, think of a creativeway to make an illustratedscale model of the atmos-phere. You could use side-walk chalk, stakes and string,poster board, or other materi-als approved by your teacher.Assume that the atmosphereis about 500 km thick.

1 Applying Why does thetemperature of different layersof the atmos phere vary?

2 Identifying What is airpressure?

3 Summarizing Why does airpressure decrease as altitudeincreases?

INTERPRETING GRAPHICS Usethe graph below to answer the nexttwo questions.

Composition of the Atmosphere

Argon, Ar0.9%

Oxygen,02 21%

Other0.1%

Nitrogen,N2 78%

4 Identifying What two gasesmake up most of the atmos-phere? What is the combinedpercentage of the two gases?

5 Identifying What percentageof the atmosphere is argon?

6 Applying Concepts Applywhat you know about therelationship between altitudeand air pressure to explainwhy rescue helicopters havea difficult time flying at altitudesabove 6,000 m.

7 Making ComparisonsCompare the four atmosphericlayers. How do temperatures andpressures vary between layers?

8 Making Calculations The tropo-sphere ends at an altitude ofabout 12 km. The mesospherebegins at an altitude of about50 km. What is the approximatethickness of the stratosphere?

• Nitrogen and oxygenmake up most of Earth’satmosphere.

• Air pressure decreases asaltitude increases.

• The composition of atmos-pheric layers affects theirtemperature.

• The troposphere is the low-est atmospheric layer. It isthe layer in which we live.

• The stratosphere containsthe ozone layer, whichprotects us from harmfulultraviolet radiation.

• The mesosphere is thecoldest atmospheric layer.

• The thermosphere is theuppermost layer of theatmosphere.

6.4.b, 6.4.e

Summary

Topic: Composition of theAtmosphere

SciLinks code: HY70328