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10.2: Composition of the Atmosphere

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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

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

    The composition of 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 the figure above. It can make up as much as 4% 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 moisture out of your nose and lips. Humidity can change rapidly if a storm comes in. Humidity can vary over a short distance, like near a lake. Even when humidity is at its highest, water vapor makes up about 4% of the atmosphere at most.

    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.


    • 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.


    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?

    Explore More

    Use the resource below to answer the questions that follow.

    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.

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