Ask A Scientist© Weather Archive

name         Roberto B.
status       student
age          16

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

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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