Composition of the
Atmosphere
Dana Desonie, Ph.D.
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Printed: April 16, 2015
AUTHOR
Dana Desonie, Ph.D.
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C HAPTER
Chapter 1. Composition of the Atmosphere
1
Composition of the
Atmosphere
• Describe the composition of the atmosphere.
Composition of Air
Several properties of the atmosphere change with altitude, but the composition of the natural gases does not. The
proportions of gases in the atmosphere are everywhere the same, with one exception. At about 20 km to 40 km
above the surface, there is a greater concentration of ozone molecules than in other portions of the atmosphere. This
is called the ozone layer.
Nitrogen and Oxygen
Nitrogen and oxygen together make up 99% of the planet’s atmosphere. Nitrogen makes up 78% of the atmosphere,
but is not involved in geological or biological processes in its gaseous form. Oxygen is extremely important because
it is needed by animals for respiration. It makes up 21% of the atmosphere. The rest of the gases are water vapor,
argon, carbon dioxide, and other "trace" gases. These make up 1% of the atmosphere.
Water Vapor
Humidity is the amount of water vapor in the air. Humidity varies from place to place and season to season. This
fact is obvious if you compare a summer day in Atlanta, Georgia, where humidity is high, with a winter day in
Phoenix, Arizona, where humidity is low. When the air is very humid, it feels heavy or sticky. Dry air usually feels
more comfortable. When humidity is high, water vapor makes up only about 4% of the atmosphere.
Where around the globe is mean atmospheric water vapor higher and where is it lower ( Figure 1.2)? Why? Higher
humidity is found around the equatorial regions because air temperatures are higher and warm air can hold more
moisture than cooler air. Of course, humidity is lower near the polar regions because air temperature is lower.
Greenhouse Gases
Remember that greenhouse gases trap heat in the atmosphere. Important natural greenhouse gases include carbon
dioxide, methane, water vapor, and ozone. CFCs and some other man-made compounds are also greenhouse gases.
Particulates
Some of what is in the atmosphere is not gas. Particles of dust, soil, fecal matter, metals, salt, smoke, ash, and other
solids make up a small percentage of the atmosphere and are called particulates. Particles provide starting points
(or nuclei) for water vapor to condense on and form raindrops. Some particles are pollutants.
Summary
• The major atmospheric gases are nitrogen and oxygen. The atmosphere also contains minor amounts of other
gases, including carbon dioxide.
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FIGURE 1.1
Nitrogen and oxygen make up 99% of
the atmosphere; carbon dioxide is a very
important minor component.
2
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Chapter 1. Composition of the Atmosphere
FIGURE 1.2
Mean winter atmospheric water vapor in the Northern Hemisphere when temperature and humidity are lower than
they would be in summer.
• Greenhouse gases trap heat in the atmosphere and 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.
References
1. User:Mysid/Wikipedia. The composition of the atmosphere .
2. Courtesy of MODIS/NASA. Map of the average atmospheric water vapor . Public Domain
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