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The Earth's Atmosphere

The earth’s atmosphere is a blanket of air that surrounds the planet. The atmosphere is what makes life on earth possible. Without it, there would be no lakes, oceans, or clouds. The temperature of the earth would be unbearable, as would the sun’s rays. The atmosphere helps to shield the earth’s inhabitants from the sun’s rays with ozone, and helps regulate the temperature of the earth with the greenhouse effect. Although the atmosphere is invisible, life on earth depends on it completely.

The Composition of the Atmosphere

The atmosphere is comprised of various gases that are present in different amounts. Some gases, known as permanent gases, comprise a set percentage of the atmosphere. The composition of other gases vary with the processes that take place on earth, these gases are known as variable gases. See the table below for a list of the gases in the atmosphere.

Composition of the Atmosphere Near the Earth’s Surface

Source: Ahrens, C. Donald. Meteorology Today (5^th ed.), West Publishing Company, Minneapolis, 1994.

Vertical Structure of the Atmosphere

The properties of the atmosphere change with increasing distance from the earth’s surface. The atmosphere can be divided into a series of layers defined by temperature, the gases present, as well its electric properties.

In addition to temperature and gaseous composition, air pressure also varies with height. Air pressure is a measure of the weight of the air above a location, thus air pressure decreases with increasing elevation. To illustrate why presure decreases with height, consider the two points (A and B) in the figure below. Notice that a point close to the earth's surface (A) has much more air above it than a point that is higher in the atmosphere (B). Since point B is so much higher in the atmosphere than point A, there is less atmosphere to push down on point B. This results in lower pressure at point B than at point A.

Layers of the atmosphere

The first layer of the atmosphere, the layer closest to the earth, is called the troposphere. The troposphere is where all of the weather on earth occurs. The temperature in the troposphere decreases with height. This is one of the reasons why it is usually colder on top of the mountains than it is at the bottom of the mountains. In the troposphere, the temperature decreases on an average of 6.5 degrees Celsius for every 1000 meters in height, or 3.6 degrees Fahrenheit for every 1000 feet in height. This decrease in temperature with height is known as the temperature lapse rate. Since all of the earth’s weather is contained within the troposphere, the air in this layer is constantly being mixed by air currents.

At about 11 km above the earth’s surface, the air temperature stops decreasing with height. The lapse rate becomes zero. This point marks the end of the troposphere, and the beginning of the tropopause. The tropopause is the boundary between the troposphere and the stratosphere. At about 20 km above the surface, the stratosphere begins. The temperature of the stratosphere increases with height. This increase in temperature with height is called a temperature inversion. This inversion occurs because of the large concentration of ozone in this layer of the atmospheric. As stated earlier, the ozone in the atmosphere helps to shield the earth from the sun’s rays. As the ozone absorbs solar energy, the stratosphere is warmed. If it were not for the ozone in the stratosphere, the temperature lapse rate would probably be similar to that of the troposphere.

At about 50 km, the stratosphere ends and the stratopause begins. The stratopause is the boundary between the stratosphere and the mesosphere. Since the concentration of ozone is lower in this layer, the temperature in the mesosphere once again decreases with height. The atmosphere is beginning to get thinner in the mesosphere. At the top of the mesosphere, the air temperature reaches its coldest average value.

At about 85 km, the mesosphere ends and the mesopause begins. The mesopause is the boundary between the mesosphere and the thermosphere. The thermosphere is a warm layer, because oxygen molecules (O2) absorb solar rays which warms the air. Since the thermosphere has a very low density, the absorption of a small amount of solar energy can cause a large increase in air temperature. At the top of the thermosphere, is the point at which the atmosphere thins and atoms and molecules begin to shoot off into space. This region is sometimes referred to as the exosphere, the upper limit to the earth’s atmosphere.

The Ionosphere

The ionosphere is a region in the upper atmosphere that has a large concentration of ions and free electrons. Ions are atoms and that are electrically charged because they have either lost or gained one or more electrons. The ionosphere plays a major role in radio communications. The ionosphere reflects radio waves back to earth.

See the picture below for a labeled picture of the layers in the atmosphere.

 The picture below depicts the changes of temperature within the layers of the atmosphere.

  Questions

1.    Name two ways in which the atmosphere makes life on earth possible.

2.   What are the two most abundant gases in the earth’s atmosphere?

3.   What is the difference between variable gases and permanent gases?

4.   What characteristics do we use to break the atmosphere into layers?

5.   In which layer of the atmosphere does all weather occur?

6.   What is a temperature lapse rate?

7.   Name the layers of the atmosphere, from the earth up.

8.   What is a temperature inversion?

9.   Why does a temperature inversion exist in the stratosphere?

10.   What is the name given to the upper limit of the earth’s atmosphere?

Source: Ahrens, C. Donald. Meteorology Today (5^th ed.), West Publishing Company, Minneapolis, 1994.

sercc@dnr.state.sc.us