The atmosphere is composed of a shell of gases that
surrounds the Earth. This shell is remarkably thin, only about
several hundred kilometers thick. While this may seem large,
it is extremely small when you consider it in comparison with
the size of the Earth.
The atmosphere contains many gases, but the 2 most abundant
by far are Nitrogen and Oxygen. Together, these
2 gases make up approximately 99% of the air. The chart on
of the Earth Science Reference Tables shows this. The chart
refers to the "Troposphere", which is the bottom
layer of the atmosphere.
Layers of the
The temperature of the atmosphere changes
with altitude. It is not a simple direct or inverse
relationship. If you were to begin at the Earth's
surface and travel upwards, you would find that the
temperature decreases with altitude up to a
point, then begins to increase with altitude.
This trend reverses itself two more times as you travel
toward outer space. Scientists have divided the
atmosphere into layers based on these temperature
changes, as shown in the chart on
of the Earth Science Reference Tables.
This chart can be very confusing!
Notice that we are first concentrating on only the
left side of the chart. This is the section that
describes the temperature changes, and gives the names
of the different layers.
||As you travel upward from sea level, the temperature
steadily drops. This layer is called the Troposphere.
Unless you fly in a plane at very high altitudes, you
spend your entire life in this layer! The top of the
Troposphere is called the Tropopause, and it
marks the end of this layer and the beginning of the
next layer: the Stratosphere.
|In the Stratosphere, the temperature rises
with altitude. This is caused mainly by the presence of
the ozone layer, which absorbs ultraviolet radiation.
The end of the Stratosphere is marked by the
Stratopause, and this is also the beginning of the
next layer: the Mesosphere.
||In the Mesosphere, the temperature again
decreases with altitude. This continues until the end of
the Mesosphere, which is called the Mesopause.
|Following the Mesopause, the last layer of the
atmosphere, the Thermosphere, is characterized by
temperatures that rise dramatically. There is no
"Thermopause", the Thermosphere simply blends
gradually into outer space.
Properties of the Atmosphere
If we look at the right side of the chart, we see
two other variables described: Atmospheric Pressure,
and Water Vapor. These are relatively straight
forward, as they both decrease with altitude (inverse
An important observation here is that pressure drops
dramatically with altitude. Almost all of the mass of
the atmosphere is located at the bottom, due to gravity
pulling the gases down.
Another important thing to note is that all of the
water vapor is contained in the Troposphere. This
is where all weather also occurs.