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Name: Roberto B.
Status: student
Age:  16
Location: N/A
Country: N/A
Date: 2000-2001


Question:
How does the air composition vary at different altitudes?


Replies:
Roberto,

The composition of the atmosphere at different levels partially determines why there are different layers in the atmosphere.

In the lower layer, the Troposphere (up to 8 to 20 kilometers deep; deepest at the equator and shallowest at the poles) and the next higher layer, the Stratosphere (up to 50 kilometers), there is not a great deal of difference in composition.

However, water vapor pressures are generally lower in the Stratosphere (proportional to total atmospheric pressure, which reduces with height throughout the depth of the atmosphere) than in the Troposphere and there is a significant area of ozone in the Stratosphere at about 20-25 kilometers (often called the "ozone layer"). There is generally a greater concentration of ozone throughout the Stratosphere than in the Troposphere. The "ozone layer" strongly absorbs far ultraviolet radiation (very short wavelengths of light from the Sun), resulting in a slight increase in temperature with altitude in the Stratosphere. The Stratosphere is either non-existent or very hard to detect at the poles.

The next layer, the Mesosphere (up to about 90 kilometers),exhibits a decrease in temperature and ozone with altitude. The other atmospheric components are about the same as in the Stratosphere.

The next layer up, the Thermosphere, often synonymous with the "Ionosphere", reaches up to somewhere between 300 and 1000 kilometers (it averages at about 500 kilometers). Temperature increases with altitude. In this layer gases are highly ionized in several layers (leading to communication bounce) and oxygen is dissociated into monatomic oxygen. The bulk of the air is nitrogen and monatomic oxygen.

Above the Thermosphere is the Exosphere, where you can hardly say that there is any atmosphere. Air molecules can easily escape from the Earth's gravity here and is generally thought of as the beginning of "space".

I might add that the depth of the atmosphere and it's layers varies with season and latitude, with the atmosphere being deeper and with better defined layers at the equator than at the poles.

One interesting affect of the ionosphere that I have experienced where I live (near Chicago), was the night nearly 20 years ago that I received WGBH (a PBS station in Boston, where I grew up) on my television. This is a very rare occurrence.

David R. Cook
Atmospheric Section
Environmental Research Division
Argonne National Laboratory


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