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7.2: Composition of the Atmosphere

Difficulty Level: Basic / At Grade Created by: CK-12
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Just what is 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.

Water Vapor

Humidity is the amount of water vapor in the air. Humidity varies from place to place. It also varies in the same place from season to season. On a summer day in Atlanta, Georgia, humidity is high. The air feels very heavy and sticky. On a winter day in Flagstaff, Arizona, humidity is low. The air sucks all the moisture out of your nose and lips. Humidity can change rapidly if a storm comes in. Humidity can vary over a short distance, such as near a lake. Even when humidity is at its highest, water vapor makes up only about 4% of the atmosphere.

Greenhouse Gases

Greenhouse gases trap heat in the atmosphere. This is essential so that Earth has a more moderate temperature. Without greenhouse gases, nighttime temperatures would be frigid. Natural greenhouse gases include carbon dioxide, methane, water vapor, and ozone. CFCs and some other man-made compounds are also greenhouse gases. Human activities may increase the amount of greenhouse gases like carbon dioxide in the atmosphere.

Particles in the Air

Air includes many tiny particles. The particulates may consist of dust, soil, salt, smoke, or ash. Some particulates 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.


  • humidity: The amount of water vapor held in the air.
  • particulates: Solid particles in the air, such as dust, salt, or ash.


  • The major atmospheric gases are nitrogen and oxygen. The atmosphere also contains minor amounts of other gases, including carbon dioxide.
  • Greenhouse gases trap heat in the atmosphere. These gases include carbon dioxide, methane, water vapor, and ozone.
  • Not everything in the atmosphere is gas. Particulates are particles that are important as the nucleus of raindrops and snowflakes.


Use this resource to answer the questions that follow.

Learn About Planet Earth-Earth's Atmosphere at http://www.youtube.com/watch?v=fyfN9t_E0w8 (5:42)

  1. What does the atmosphere contain?
  2. How much nitrogen does the atmosphere contain?
  3. How much oxygen does the atmosphere contain?
  4. What does the atmosphere do?
  5. How many layers does the atmosphere have?
  6. List the layers of the atmosphere and two characteristics of each layer.


  1. What are the two major atmospheric gases? What are the important minor gases?
  2. What are particulates? Why are they important?
  3. What is humidity? How much humidity is in the air on a tremendously hot and humid day?

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Difficulty Level:



6 , 7 , 8

Date Created:

Jan 04, 2013

Last Modified:

Jan 07, 2014
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