LESSON Earth’s · PDF file15.1 LESSON PLAN PREVIEW ... Earth’s atmosphere is divided into four main layers, ... Earth’s surface and contains the oxygen we need to live

452 Lesson 1

LESS

ON 1 Earth’s Atmosphere

When people Watch the weather report on TV, what informa-tion are they most interested in? Chances are they want to know whether the weather is cloudy or sunny, whether it is likely to rain, and what the air temperature is. Most people also probably pay close attention to the forecast for the weather tomorrow and the next few days. Any weather report, whether you watch it on TV, hear it on the radio, read it in a newspaper, or check it on the Internet, has the same function. A weather report describes the condition of Earth’s atmosphere.

properties of the atmosphere Properties of the atmosphere include its composition, relative

humidity, temperature, and air pressure.

The atmosphere is the thin layer of gases that surrounds Earth. To get an idea of the size of the atmosphere, imagine that Earth were the size of an apple. You breathe on the apple, and a film of water forms on its surface. Compared to the apple, the film of water is very thin. The atmosphere is about that thin compared to the size of Earth.

We live at the bottom of the atmosphere, which provides us with oxy-gen, protects us from the most harmful rays in sunlight, and transports and recycles water. It also burns up incoming meteors and helps control climate. To understand the atmosphere, you need to know its composi-tion. Humidity, temperature, and air pressure are also important proper-ties of the atmosphere.

Composition of the Atmosphere You may be surprised to learn that the air you breathe is made up mostly of nitrogen, not oxygen. The atmosphere consists of roughly 78 percent nitrogen gas and 21 percent oxygen gas. The remaining 1 percent is composed of several other gases, as shown in Figure 1. Air also contains water vapor, which is water in the form of a gas.

• Describe the properties of the atmosphere.• Identify the four main layers of the atmosphere.• Explain heat transfer and the interaction of air

masses in the troposphere.

Reading Strategy When you read about the layers of the atmosphere, make a compare/contrast table that compares the four layers. Construct the table with one row for each layer. Include columns for height, temperature characteristics, and other characteristics.

Vocabulary atmosphere, relative humidity, air pressure, troposphere, stratosphere, ozone layer, mesosphere, thermosphere, radiation, conduction, convection, convection current, air mass, front

Guiding Question: How can we describe Earth’s atmosphere?

FOCUS Write the term atmosphere in the center of the board and circle it. Have students create a class cluster diagram that describes what they already know about Earth’s atmosphere. As you work through the lesson, revise and add to the diagram.

GUIDING QUESTION

15.1 LESSON PLAN PREVIEWReal World Relate the concept of air pressure to flying in an airplane.Differentiated Instruction Less proficient readers use a visual to help them understand the layers of the atmosphere.Inquiry Students model the transfer of heat through radiation.

15.1 RESOURCESLesson 15.1 Worksheets • Lesson 15.1 Assessment • Chapter 15 Overview Presentation

Gases in Dry Air

Argon

Carbon dioxide

Neon

Helium

Methane

Krypton

Hydrogen

Other Gases

0.93

0.033

0.0018

0.00052

0.0002

0.00011

0.00005

Percentage by Volume

Source: Data from National Weather Service, NOAA

All othergases 1%

Nitrogen78%

Oxygen21%

▶ Nitrogen In the atmosphere, nitrogen gas occurs as a molecule with the chemical formula N2. This chemical formula indicates that a molecule of nitrogen gas consists of two nitrogen atoms. All organisms, or living things, contain nitrogen. However, only certain kinds of bacteria can use nitro-gen in the form in which it occurs in the atmosphere. These bacteria take nitrogen in and convert it to chemical compounds in a process called nitrogen fixation. These chemicals then become available for other organisms to use.

▶ Oxygen Oxygen makes up only about one fifth of the atmosphere. The most common form of oxygen in the atmosphere occurs as a molecule having the chemical formula O2. Therefore, most molecules of oxygen are made up of two atoms of oxygen. Most living things cannot survive without oxygen. In addition, oxygen is necessary for combustion, or burning. During combustion, oxygen combines chemically with a fuel, such as gasoline, wood, or paper (Figure 2). The products of combustion are carbon dioxide and water.

Over Earth’s long history, the amount of oxygen in the atmosphere has changed. The atmosphere of early Earth contained almost no oxygen. The early atmosphere was mostly made up of carbon dioxide, nitrogen, carbon monoxide, and water vapor. Oxygen gas began to build up when tiny photosynthetic microorganisms first appeared. These microorgan-isms produced oxygen during photosynthesis and released it into the atmosphere.

▶ Water Vapor Air contains water vapor. The chemical formula for water—whether it is solid, liquid, or gas—is H2O, indicating that a water molecule consists of two hydrogen atoms and one oxygen atom.

FIGURE 1 Composition of Dry Air Dry air is composed of many gases, as shown in the graph and table.

FIGURE 2 Combustion Without oxygen, wood could not burn and there would be no bright, crackling campfire.

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454 Lesson 1

Relative Humidity Air does not always hold the same amount of water. Relative humidity is the ratio of water vapor the air contains to the maximum amount it could have at that temperature. Average daytime relative humidity in Phoenix, Arizona, is only 31 percent. This means that, in general, the air in Phoenix contains less than a third of the water vapor that it could contain. In contrast, on the tropical island of Guam, the relative humidity rarely drops below 88 percent.

On some hot days, you may have heard people complaining about the humidity. People are sensitive to changes in relative humidity because perspiration cools our bodies. When humidity is high, sweat does not readily evaporate, and the body cannot cool itself efficiently.

▶ Condensation In general, warm air can hold more water vapor than cooler air. Suppose warm air contains all the water vapor it can hold, and then the air cools down. When this happens, the water vapor becomes liquid water or ice in a process called condensation. If the temperature is above freezing, water droplets form. Ice crystals form when the tempera-ture is below freezing. Dew and frost are both examples of condensation.

▶ Cloud Formation In order for condensation to occur, there must be a surface on which water vapor can condense. Dew and frost, for example, form on surfaces such as blades of grass or flower petals, as shown in Figure 3. Like dew and frost, clouds are the result of condensation. During the formation of clouds, water vapor condenses on tiny particles in the air. These particles include salt crystals, smoke, and dust.

Air Temperature Like the air’s water content, the temperature of the air also varies from place to place, and from time to time. Temperature varies over Earth’s surface because the sun’s rays strike some areas more directly than others. In the next chapter, you will learn more about factors that affect air temperature.

ReadingCheckpoint

Why can’t clouds form unless there are tiny particles in the air?

FIGURE 3 Condensation After the air becomes cooler, beads of dew form on the flower petals. The process of dew formation, which is also called condensation, occurs when cooled air contains more water vapor than it can hold at that temperature.

ANSWERS

Reading Checkpoint In order for condensation to occur, there must be a surface on which water vapor can condense. For cloud formation, this surface consists of tiny particles in the air.

Air pressure

Mercury

Lower Air Pressure

Higher Air Pressure

Air pressure

The Atmosphere 455

Air Pressure Air is made up of individual mol-ecules of nitrogen and other gases. Each molecule, tiny as it is, weighs something. The weight of a column of air pushes down on the area beneath it, the way the weight of your body pushes down on the ground that you stand on. The force with which something pushes on an area is called pres-sure. Air pressure, or atmospheric pressure, is the force exerted by air on the area below it.

▶ Measuring Air Pressure A barometer is an instrument that measures air pressure. There are two common types of barometers: mercury and aneroid. In a mercury barometer, which is shown in Figure 4, air pressure pushes a column of mercury upward in a tube. The greater the air pressure, the higher the mercury rises. When a mercury barometer is used, air pressure is usually expressed in inches because the height of the mercury column in the barometer is measured in inches. An aneroid barometer has a metal chamber whose walls bend inward when air pressure is high. The walls bulge out when air pressure is low. The bending of the chamber walls moves a dial, and the dial indi-cates the changing air pressure. In an aneroid barometer, air pressure is expressed in units called millibars.

▶ Altitude and Air Pressure In general, the lower the altitude, or height above sea level, the higher the air pressure. To understand why this is true, think of a stack of books. The one at the bottom of the pile is under the most pressure because it is bearing the weight of all the books above it. Similarly, the air at the bottom of a column of air is under greater pressure than the air higher up. Air pressure at sea level is about 1000 millibars. In contrast, at the top of Mount Everest, air pressure is just over 300 millibars.

Layers of the Atmosphere The main layers of the atmosphere are the troposphere,

stratosphere, mesosphere, and thermosphere.

Earth’s atmosphere is divided into four main layers, primarily on the basis of changes in temperature. The layers of the atmosphere are the tropo-sphere, stratosphere, mesosphere, and thermosphere.

The Troposphere The lowest layer of the atmosphere, the one directly above the ground, is called the troposphere. The troposphere blankets Earth’s surface and contains the oxygen we need to live. The movement of air within the troposphere is also largely responsible for Earth’s weather. Almost all clouds are found in the troposphere. The troposphere contains three quarters of the atmosphere’s mass, even though it is thin compared to the atmosphere’s other layers. The troposphere averages about 11 kilometers (7 miles) in height. At the poles, it is about 7 kilometers (4 miles) high, and at the equator, it is about 18 kilometers (10 miles) high.

FIGURE 4 Barometers Barometers are instruments that measure atmospheric pressure. The illustration shows how a mercury barometer works. Air pushes down on mercury, as shown by the red arrows. The greater the air pressure, the higher the mercury rises in the tube.

Stratosphere The ozone layer is part of the stratosphere. Ozone in the stratosphere absorbs ultraviolet radiation.

Thermosphere Disturbances in the thermosphere produce the aurora borealis, or northern lights. The aurora borealis is a colorful light display.

Mesosphere Chunks of rocks called meteoroids burn as they zoom through the mesosphere, making �ery trails.

Troposphere We live in the troposphere. Most of Earth’s weather occurs in the troposphere.

THERMOSPHERE

MESOSPHERE

STRATOSPHERE

TROPOSPHERE

500 km

400 km

300 km

200 km

100 km

80 km

50 km

11 km

Find OutMore

Find OutMore

▶ Temperature in the Troposphere Within the troposphere, the higher the air is above Earth, the cooler it becomes. On average, tropospheric air temperature goes down by about 6.5°C for each kilometer in altitude (or 3.6ºF per 1000 feet).

▶ The Top of the Troposphere At the top boundary of the troposphere, the temperature stops going down. A layer at the top of the troposphere acts like a cap, limiting mixing between the troposphere and the atmospheric layer above it.

The Stratosphere The stratosphere is the layer of the atmosphere above the troposphere, as shown in Figure 6. The stratosphere extends 11–50 kilometers (7–31 miles) above sea level. The gases in the strato-sphere do not mix much. Therefore, once substances including pollutants enter it, they usually stay there for a long time.

▶ Temperature in the Stratosphere Unlike the troposphere, the highest part of the stratosphere is warmer than lower levels. However, the strato-sphere is definitely not warm. It reaches a maximum temperature of –3ºC (27ºF) at its highest altitude.

▶ The Ozone Layer The most common form of oxygen is O2, but oxygen also occurs as O3, which is a gas called ozone. Ozone is concentrated in a portion of the stratosphere called the ozone layer. The upper strato-sphere is warmer than the lower stratosphere because ozone gas absorbs and scatters the sun’s ultraviolet (UV) rays. UV light penetrates the upper stratosphere, but most of it fails to reach the lower stratosphere. The ozone layer greatly reduces the amount of UV light that reaches Earth’s surface. UV light can damage an organism’s living tissue and cause harm-ful changes in its DNA. Therefore, the ozone layer’s protective effects are vital for life on Earth.

In the stratosphere, ozone occurs naturally and is beneficial to humans, because it filters out UV light. However, in the lower level of the troposphere, ozone does not occur naturally; in the troposphere, ozone is a pollutant. It harms living tissue, including lung tissue, and can interfere with plant growth.

The Mesosphere and Thermosphere Above the strato-sphere lies the mesosphere, the layer that extends 50–80 kilome-

ters (31–50 miles) above sea level. In this layer, temperatures decrease with altitude, reaching their lowest point at the top of the mesosphere. Air pressure is extremely low.

The thermosphere is the top layer, which begins about 80 kilometers (50 miles) above Earth’s surface and extends upward into space. Air is very thin, and the thermosphere has only a tiny fraction of the atmosphere’s mass. The tem-perature is very high.

ReadingCheckpoint

Which layer of the atmosphere is located directly above the stratosphere?

What is the weather today in your area? Observe whether it is clear or cloudy, and whether or not precipi-tation is falling. Then find out the temperature, relative humidity, and air pressure. Use your local news-paper, listen to a TV weather report, or go online to the Web site of the National Oceanic and Atmospheric Administration.

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FIGURE 5 Earth’s Atmosphere From Space If you view Earth from space, you can see the clouds in the atmosphere.

ANSWERS

Find Out More Answers will vary. Encourage students to present and compare their findings about the local weather. Have students note which source, for example the local newspaper or a TV station, had the most accurate forecast.

Reading Checkpoint The meso-sphere

Stratosphere The ozone layer is part of the stratosphere. Ozone in the stratosphere absorbs ultraviolet radiation.

Thermosphere Disturbances in the thermosphere produce the aurora borealis, or northern lights. The aurora borealis is a colorful light display.

Mesosphere Chunks of rocks called meteoroids burn as they zoom through the mesosphere, making �ery trails.

Troposphere We live in the troposphere. Most of Earth’s weather occurs in the troposphere.

THERMOSPHERE

MESOSPHERE

STRATOSPHERE

TROPOSPHERE

500 km

400 km

300 km

200 km

100 km

80 km

50 km

11 km

FIGURE 6 Layers of the Atmosphere The atmosphere has four main layers—the troposphere, stratosphere, mesosphere, and thermosphere.

The Atmosphere 457

Heat transferby radiation

Heat transferby convection

Heat transferby conduction

Heat transfer by

radiation

Conduction(from burner to pan)

Radiation(from burner to air)

Convection(within water)

Heat transferby conduction

458 Lesson 1

The Troposphere and Weather Processes that affect weather in the troposphere include heat

transfer and the interaction of air masses.

Weather and climate each involve properties of the troposphere, such as temperature and humidity. Weather refers to atmospheric conditions over short time periods, typically hours or days, and within relatively small areas. Climate, in contrast, describes the pattern of atmospheric condi-tions in large geographic regions over long periods. For example, London has a moist, temperate climate. But the weather on summer days in London can sometimes be is hot, dry, and sunny.

Heat Transfer in the Troposphere Energy from the sun heats the atmosphere. This energy drives air movement in the troposphere and influences weather and climate. Heat always moves from a warmer substance to something that is cooler. Heat is transferred in three ways—radiation, conduction, and convection. Figure 7 shows how these pro-cesses work in the troposphere.

▶ Radiation On a sunny summer day, the handle of a car door becomes hot because of energy from the sun in the form of sunlight. The car door has been heated by radiation, which is the transfer of energy through space. Heat travels from the sun to Earth’s atmosphere by radiation. When objects are heated by radiation, there is no direct contact between the heat source and the object being heated. Dark objects absorb more radiation than objects that are light in color. Light objects reflect much of the radiation away.

FIGURE 7 Methods of Heat Transfer Radiation, conduction, and convection all help to transfer heat in the troposphere. Heat from the sun and the heat that moves from an electric burner to the air are both transferred by radiation. Heat moves from a burner to water in a pan by conduction. Within the water in the pan, heat is transferred by convection currents.

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▶ Conduction If you touch the heated handle of the car door, you will feel that it is hot. Heat passes from the handle to your hand through conduction. Conduction is the transfer of heat directly between two objects that are in contact with one another. Conduction occurs when mol-ecules collide, and energy is transferred from one molecule to another. In the troposphere, conduction only occurs between Earth’s surface and the molecules in the air directly in contact with it.

▶ Convection Fluids include liquids and gases, such as the gases in the atmosphere. In fluids, molecules are free to move around. Convection is the transfer of heat by the movement of currents within a fluid. Convection is an important method of heat transfer in the troposphere.

▶ Convection Currents and the Movement of Heat The process of convection is related to density. Density is the amount of mass of a substance in a given volume. For example, a brick has a greater density than a block of wood that has the same volume, because there is a greater amount of mass in the brick.

When air near the surface of Earth is heated, it becomes less dense than it was before. The cooler air above it is denser than the warmer air at the surface. Because of this difference in density, the cool air sinks and the warm air rises above it. When the cooler air sinks to ground level, it then picks up heat and begins to rise. Sinking cool air and rising warm air form convection currents. Convection currents cause winds and move heat through the troposphere.

ReadingCheckpoint

What is convection?

21 3 4 65 7 8 9 Slowly squeeze the dropper bulb so that you release the colored water into the cold water.

21 3 4 65 7 8 9 Observe the beaker from the side to see what happens to the colored water.

Analyze and Conclude1. Observe How did the hot water move in

the beaker? 2. Relate Cause and Effect Why did the hot

water move the way it did? 3. Apply Concepts How is the movement of

the hot water similar to the movement of air in the troposphere?

How Does the Hot Water Move? 21 3 4 65 7 8 9 Obtain two 100-mL beakers. Fill one of them halfway with

cold water.21 3 4 65 7 8 9 Fill the other beaker halfway with hot water. Squirt a few

drops of food coloring into the hot water, until the water is a dark color.

21 3 4 65 7 8 9 Use a dropper to remove some of the hot water that has been colored. Fill the dropper, then wipe it off with a paper towel.

21 3 4 65 7 8 9 Insert the dropper into the cold water so that the dropper’s opening is halfway between the surface of the water and the bottom of the beaker.

FIGURE 8 Conduction When you touch a hot surface, heat is transferred from the surface to your hand.

ANSWERS

Reading Checkpoint Convection is the transfer of heat by currents mov-ing in a fluid.Quick Lab 1. It rose to the top.2. It rose because it is less dense than

cold water.3. Hot water rises just as warm air

rises.

(b) Cold Front

(a) Warm Front

Warm air

Warmfront Light

precipitation

Cold air

Heavyprecipitation

Warm air

Cold front

Cool air

(b) Cold Front

(a) Warm Front

Warm air

Warmfront Light

precipitation

Cold air

Heavyprecipitation

Warm air

Cold front

Cool air

460 Lesson 1

Air Masses and Fronts Throughout a large body of air called an air mass, properties such as temperature, pressure, and humidity are similar. Weather can change when air masses with different properties come together. The boundary between air masses that differ in tempera-ture and moisture is called a front.

▶ Warm Front A warm front is a boundary along which a mass of warmer, moister air pushes against a mass of colder, drier air. Because warm air is less dense than cool air, some of the warm, moist air rises over the cold air mass, as shown in Figure 9a. The warm air then cools. Because cool air can hold less moisture than warm air can, the water vapor in the cooler air condenses and forms clouds. Light rain may fall.

▶ Cold Front A cold front is the boundary along which a colder, drier air mass pushes against a warmer, moister air mass. Because colder air is denser than warmer air, the cold air tends to wedge beneath the warmer air. The warmer air rises. As the air rises, it cools to form clouds. If the ris-ing air contains a lot of water vapor, heavy precipitation—snow or rain—may fall. If there is little water vapor in the air, the cold front may result in clouds only. Cold fronts can produce sudden weather changes, including thunderstorms. Once a cold front passes through an area, the sky usually clears, and the temperature and humidity drop.

1. Use Analogies Think of a swimming pool with a shallow end and a deep end. The pressure of water on the floor at the shal-low end is less than the pressure of water on the floor at the deep end. How is this similar to the way air pressure differs at different altitudes?

2. Relate Cause and Effect Why is the temperature in the upper stratosphere higher than the temperature in the lower stratosphere?

3. Compare and Contrast How is a warm front different from a cold front?

4. Explore the BIGQUESTION Winds occur because of convection currents in the tro-posphere. How do you think winds affect air pollution?

1

FIGURE 9 Warm and Cold Fronts When a warm front approaches (a), warmer air rises over cooler air, and light or moderate precipitation may fall. In a cold front (b), colder air pushes beneath warmer air, resulting in heavy precipitation.

ANSWERS

Lesson 1 Assessment

1. The weight of water above the pool floor is greater at the deep end. Similarly, air pressure at lower altitudes is greater than air pressure at higher altitudes, because the weight of air above points at lower altitudes is greater.

2. In the upper stratosphere, ozone ab-sorbs and scatters UV light, preventing it from reaching the lower stratosphere.

3. In a warm front, a warm air mass pushes against a cold one. In a cold front, a cold air mass pushes against a warm one.

4. Sample answer: Winds move pollut-ants from one area to another and may also clear pollution from an area.