Activity 64: Earth’s Atmosphere
BACKGROUND INFORMATION
Name:
The atmosphere is the “sea of gas” that surrounds the earth. Just like the ocean, it has different layers and
pressures. The layer that humans live in is called the troposphere, and it is where most weather occurs.
In the troposphere temperature and pressure generally decrease with altitude. About 80% of the earth’s
gases are in the troposphere.
The layers of the atmosphere are defined mostly by changes in temperature. The temperature of the
layers depends largely on the gases in that layer. The stratosphere contains ozone (O3), which absorbs
some of the sun’s radiation and is, therefore, warmer than the upper troposphere. The stratosphere
contains another 19% of earth’s gases, leaving only 1% in the mesosphere and thermosphere. Once in the
thermosphere, there is such a low concentration of gases that it is hard to say where the layer ends. The
thermosphere starts at 80 km, but different sources will list different upper limits for this and the
uppermost layer of the atmosphere, the exosphere. Some sources say that the thermosphere ends at
around 100 km; others say the space shuttle orbits in it at 200-plus km. It is important to remember that
while there is a much lower concentration of gases at these altitudes, gravity is still very strong. The force
of gravity at 200 km is still about 94% of what it is on the surface of the earth. The uppermost layer of the
atmosphere is the exosphere. It is beyond this layer that true “outer space” begins. This extremely thin
layer begins approximately 10,000 km from earth’s surface. It is in the exosphere that some man-made
satellites orbit the earth.
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The temperature within the troposphere decreases with height because the concentration of air
molecules decreases with height.
However, the temperature of the stratosphere increases with height because of the ozone layer in the
upper stratosphere that absorbs ultraviolet radiation and heats up.
The temperature of the mesosphere decreases with height for the same reason as in the troposphere
– because the concentration of air molecules decreases.
Then temperature increases again in the thermosphere due to the tremendous absorption of solar
energy by the gases of the thermosphere, which makes the molecules of these gases move very fast.
Oddly enough, although the temperature is high, it would not feel hot. Even though the molecules in
the thermosphere are moving very fast, there are too few of them to actually transfer much energy to
a person.