There are five layers in our atmosphere, each with its own
name and set of characteristics. We live and play in the troposphere, the lowest layer of our atmosphere. Our
weather occurs in this layer and most aircraft fly in it.
The troposphere extends to an altitude of around 50,000
feet at a boundary called the tropopause. The exact altitude of the tropopause depends on the time of the year
and latitude.
The next highest layer is called the stratosphere.
Residing in the stratosphere is the ozone layer that protects
us from the sun's harmful ultraviolet radiation. Very few aircraft can fly in this layer. For the most part, we are very
unaware of the stratosphere. The stratosphere extends to
an altitude of around 160,000 feet at a boundary called the
stratopause.
Above the stratopause is a layer of the atmosphere
called the mesosphere. Only rockets and meteors travel
through this layer. Above the mesosphere is the thermosphere and then the exosphere. There is a boundary
between the thermosphere and exosphere (called, you
guessed it, the thermopause), but no top boundary to the
exosphere. Outer space exists in the two topmost layers of
our atmosphere.
Environmental Conditions in
Near Space
Now that you're familiar with the structure of our
atmosphere, we can put near space into perspective by
discussing its location and environmental conditions. First,
where is near space located? I define near space as those
altitudes in the stratosphere and mesosphere between
75,000 feet and 330,000 feet. I selected the lower boundary because of the environmental conditions found at this
altitude and the higher boundary because it is the internationally defined boundary for outer space. For an altitude
comparison, many of you have flown in commercial
aircraft, which fly at an altitude between 30,000 and
40,000 feet.
Now, what's it like in near space? There are a number
of unique conditions found in near space. The first is its
temperature. Let's launch a near spacecraft and see what
it tells us about temperature (please see the charts accompanying this article). As our near spacecraft ascends in the
troposphere, we find that the air temperature continuously
decreases. The troposphere cools with altitude because it's
warmed by its contact with the ground. You might think
that the troposphere should be warmed by sunlight, but,
the troposphere is very transparent to sunlight, so sunlight
shines right through it without warming it. Once our near
spacecraft passes through the tropopause, sensors find
that the air temperature stops cooling. During the summer,
the tropopause occurs at an altitude of 50,000 feet for mid-latitudes and the air temperature is a chilly - 60° Fahrenheit.
In the winter, the tropopause lowers to an altitude of 40,000
feet and its temperature can drop to an even colder - 90°
Fahrenheit.
Once our near spacecraft enters the stratosphere, we
FEBRUARY 2004
Near Space
Layers of the Earth’s atmosphere.
find that the air temperature increases with altitude. At an
altitude of 100,000 feet — which is easily reached by our
near spacecraft — the air temperature can warm to 20°
Fahrenheit or warmer. The stratosphere warms with
increasing altitude because of its ozone content. The sun's
ultraviolet radiation is blocked by our ozone layer. Recall
that energy cannot be created or destroyed; it may be in
another place and possibly in a new form. In the stratosphere, energy from the sun's ultraviolet radiation is eventually converted into higher air temperatures. As our near
spacecraft gets closer to the sun, there is more ultraviolet
radiation for the ozone to block, and, therefore, warmer air
temperatures.
The second condition found in near space is reduced
air pressure. As opposed to the up and down changes in
air temperature that our near spacecraft detected, air pressure can only decrease with increasing altitude. Our near
spacecraft sees air pressure dropping by a factor of two for
Air temperature cycles as altitude increases.
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