15.1 The Atmosphere

Lesson Objectives

• Explain why Earth’s atmosphere is important.
• Describe the composition of the atmosphere.
• List properties of the atmosphere.

Vocabulary

• air pressure
• altitude
• sound

Introduction

Why is Earth the only planet in the solar system known to have life? The main reason is Earth’s atmosphere. The atmosphere is a mixture of gases that surrounds the planet. We also call it air. The gases in the air include nitrogen, oxygen, and carbon dioxide. Along with water vapor, air allows life to survive. Without it, Earth would be a harsh, barren world.

Why the Atmosphere Is Important

We are lucky to have an atmosphere on Earth. The atmosphere supports life, and is also needed for the water cycle and weather. The gases of the atmosphere even allow us to hear.

The Atmosphere and Living Things

Most of the atmosphere is nitrogen, but it doesn't do much. Carbon dioxide and oxygen are the gases in the atmosphere that are needed for life.

• Plants need carbon dioxide for photosynthesis. They use sunlight to change carbon dioxide and water into food. The process releases oxygen. Without photosynthesis, there would be very little oxygen in the air.
• Other living things depend on plants for food. These organisms need the oxygen plants release to get energy out of the food. Even plants need oxygen for this purpose.

The Atmosphere and the Sun’s Rays

The atmosphere protects living things from the Sun’s most harmful rays. Gases reflect or absorb the strongest rays of sunlight. Figure below models this role of the atmosphere.

The atmosphere shields Earth from harmful solar rays.

The Atmosphere and Earth’s Temperature

Gases in the atmosphere surround Earth like a blanket. They keep the temperature in a range that can support life. The gases keep out some of the Sun’s scorching heat during the day. At night, they hold the heat close to the surface, so it doesn’t radiate out into space.

The Atmosphere and Earth’s Water

Figure below shows the role of the atmosphere in the water cycle. Water vapor rises from Earth’s surface into the atmosphere. As it rises, it cools. The water vapor may then condense into water droplets and form clouds. If enough water droplets collect in clouds they may fall as rain. This how freshwater gets from the atmosphere back to Earth's surface.

The atmosphere is a big part of the water cycle. What do you think would happen to Earth’s water without it?

The Atmosphere and Weather

Without the atmosphere, there would be no clouds or rain. In fact, there would be no weather at all. Most weather occurs because the atmosphere heats up more in some places than others.

The Atmosphere and Weathering

Weather makes life interesting. Weather also causes weathering. Weathering is the slow wearing down of rocks on Earth’s surface. Wind-blown sand scours rocks like sandpaper. Glaciers of ice scrape across rock surfaces like a file. Even gentle rain may seep into rocks and slowly dissolve them. If the water freezes, it expands. This eventually causes the rocks to crack. Without the atmosphere, none of this weathering would happen.

The Atmosphere and Sound

Sound is a form of energy that travels in waves. Sound waves can’t travel through empty space, but they can travel through gases. Gases in the air allow us to hear most of the sounds in our world. Because of air, you can hear birds singing, horns tooting, and friends laughing. Without the atmosphere, the world would be a silent, eerie place.

Composition of Air

Air is easy to forget about. We usually can’t see it, taste it, or smell it. We can only feel it when it moves. But air is actually made of molecules of many different gases. It also contains tiny particles of solid matter.

Gases in Air

Figure below shows the main gases in air. Nitrogen and oxygen make up 99 percent of air. Argon and carbon dioxide make up much of the rest. These percentages are the same just about everywhere in the atmosphere.

This graph identifies the most common gases in air.

Air also includes water vapor. The amount of water vapor varies from place to place. That’s why water vapor isn’t included in Figure above. It can make up as much as 4 percent of the air. Ozone is a molecule made of three oxygen atoms. Ozone collects in a layer in the stratosphere.

Particles in the Air

Air includes many tiny particles. The particles may consist of dust, soil, salt, smoke, or ash. Some particles pollute the air and may make it unhealthy to breathe. But having particles in the air is very important. Tiny particles are needed for water vapor to condense on. Without particles, water vapor could not condense. Then clouds could not form and Earth would have no rain.

Properties of Air

We usually can’t sense the air around us unless it is moving. But air has the same basic properties as other matter. For example, air has mass, volume and, of course, density.

Density of Air

Density is mass per unit volume. Density is a measure of how closely molecules are packed together. The closer together they are, the greater the density. Since air is a gas, the molecules can pack tightly or spread out.

The density of air varies from place to place. Air density depends on several factors. One is temperature. Like other materials, warm air is less dense than cool air. Since warmer molecules have more energy, they are more active. The molecules bounce off each other and spread apart. Another factor that affects the density of air is altitude.

Altitude and Density

Altitude is height above sea level. The density of air decreases with height. There are two reasons. At higher altitudes, there is less air pushing down from above. Also, gravity is weaker farther from Earth's center. So at higher altitudes, air molecules can spread out more. Air density decreases. You can see this in Figure below.

This drawing represents a column of air. The column rises from sea level to the top of the atmosphere. Where does air have the greatest density?

Air Pressure

Because air is a gas, its molecules have a lot of energy. Air molecules move a lot and bump into things. For this reason, they exert pressure. Air pressure is defined as the weight of the air pressing against a given area.

At sea level, the atmosphere presses down with a force of about 1 kilogram per square centimeter (14.76 pounds per square inch). If you are standing at sea level, you have more than a ton of air pressing against you. Why doesn’t the pressure crush you? Air presses in all directions at once. Other molecules of air are pushing back.

Altitude and Air Pressure

Like density, the pressure of the air decreases with altitude. There is less air pressing down from above the higher up you go. Look at the bottle in Figure below. It was drained by a hiker at the top of a mountain. Then the hiker screwed the cap on the bottle and carried it down to sea level. At the lower altitude, air pressure crushed it. Can you explain why?

At sea level, pressure was greater outside than inside the bottle. The greater outside pressure crushed the bottle.

Lesson Summary

• Gases in the atmosphere are needed by living things. They protect life from the Sun’s harmful rays. They also keep temperatures in a range that can support life. Gases in air play a major part in the water cycle, weather, and weathering. They are also needed to transmit most sounds.
• Nitrogen and oxygen make up about 99 percent of the air. Argon and carbon dioxide make up much of the rest. The air also contains water vapor. The amount of water vapor varies from place to place.
• Air has mass and volume. It also has density and exerts pressure. Both the density and pressure of air decrease with altitude.

Lesson Review Questions

Recall

1. State how living things interact with the atmosphere.

2. How does the atmosphere keep Earth warm at night?

3. What role does the atmosphere play in the water cycle?

4. Why does weathering on Earth’s surface depend on the atmosphere?

5. Describe the composition of air.

Apply Concepts

6. Create a graph that shows how air pressure changes with altitude. Use the data in Table below as a guide.

Air Pressure (atm)	Altitude (m)	Altitude (ft)
1	0	0
3/4	2,750	7,902
1/2	5,486	18,000
1/3	8,376	27,480
1/10	16,132	52,926
1/100	30,901	101,381
1/1,000	48,467	159,013
1/10,000	69,464	227,899
1/100,000	86,282	283,076

Think Critically

7. Explain how and why the density of air changes with altitude.

8. Review Figure above and its caption. What would the bottle look like if the hiker hadn't screwed on the cap before returning to sea level? Explain your answer.

Points to Consider

In this lesson, you read that air density and pressure change with altitude. The temperature of the air also changes with altitude. Air temperature measures the heat energy of air molecules.

• What heats the atmosphere? Where does air gets its energy?
• What causes the atmosphere to lose energy and become cooler?