Download We live at the bottom of an ocean of air - several oceans

We live at the bottom of an ocean of air - several oceans, in fact, as the atmosphere can be
divided into layers distinguished by temperature. Starting at the surface of the earth's crust, the
layer which most apparently affects us is called the troposphere. It extends roughly 5 miles
above the poles and 10 miles above the equator, and contains about 75% of the total mass of
the atmosphere. Due to its closeness to the earth's surface, nearly all water vapor and solid
particles (from forest fires, volcanoes and the burning of fossil fuel, for example) are found in
this sphere. All of earth's plant and animal life exist within the troposphere or the waters
beneath it. Climate and weather are conditions within the troposphere. It is also the layer
where most of the heat exchange between the earth and the atmosphere takes place, and where
heat is distributed through the overturning of air (tropo means "to turn"). Temperature in the
troposphere decreases with height to -76 degrees Fahrenheit.
The stratosphere extends to about 30 miles above
earth, and can itself be subdivided (strato means
"layers") In the lower levels of the stratosphere
the temperature remains the same, but in the
upper levels the temperature actually increases to
roughly the same as that at sea level. This is due
to the ozone layer, where a high concentration of
ozone, a molecular variation of oxygen, absorbs
ultraviolet rays from the sun. Because of this
temperature distribution the over-turning of air is
much less than within the troposphere below, and
the stratosphere becomes a kind of giant lid.
Debris from violent volcanic eruptions, like Mt.
St. Helens in 1980, sometimes enter and remain suspended in the stratosphere for years before
settling back to the earth's surface. The troposphere and stratosphere combined contain 99% of
the total mass of the atmosphere.
In the mesosphere, which extends to about 50 miles, temperature drops again to as low as -173
degrees F. Meteors, small pieces of matter drawn to the atmosphere by earth's gravity, become
visible to the naked eye as they enter the mesosphere and are heated through friction caused by
collisions with air molecules. These "falling stars" usually disintegrate before they reach the
earth's surface. Spectacular meteor showers can be observed at certain times of the year when
the earth, in its orbit, passes through a swarm of particles generated from the breakup of a
comet. The troposphere, stratosphere and mesosphere make up what is called the lower
atmosphere.
Above 50 miles is the upper atmosphere, where air density is
extremely rarefied. The thermosphere extends to 400 miles and
is characterized by large fluctuations of temperature (thermo
means "heat"). At these heights there are relatively few
molecules and heat retention should be low. However, within
the thermosphere solar energy is absorbed and reradiates heat.
At its upper limits the temperature reaches 441 degrees F.