Page 250 - NS-2 Textbook

Our Atmosphere

It is not possible to understand much about weather is the whole area encompassing the mesosphere and the
without having a fundamental knowledge of the atmo- thermosphere. We will talk about each of these important
sphere around us. Actually, we live at the bottom of a layers and transition zones.
vast ocean of air that completely covers the Earth. This
atmosphere has major layers up to about 1,000 miles
TROPOSPHERE
above Earth's surface, though it is believed that traces of
gaseous elements, such as helilun, are present as far out The troposphere is the ocean of air inunediately above the
as 18,000 miles. Earth's surface. It extends to a height of about 11 miles
Our atmosphere is a mixture of different gases. Near above the equatOl; about 7" miles in the temperate
the surface of the Earth, the air is made up of approxi- zones, and only about 5 miles above the poles. Currents,
mately 78 percent nitrogen, 21 percent oxygen, and 1 storms, and -V\Taves occur in this air ocean, much as in the
percent argon and other gases such as carbon dioxide, seas. Air in the troposphere is constantly turning over. 1n
hydrogen, and neon. Within the atmosphere is scaltered fact, tropos is a Greek word meaning "changing" or
about 1 percent water vapOl; called humidity. The amOlmt "hulling." 1n the h'Oposphere, the temperature and com-
of water vapor is greater in equatorial regions and less in position of gases change rapidly.
the polar regions. Nearly all clouds are in the troposphere, so it is here
It is rather interesting to cOlnpare the \vater ocean that weather occurs. Air heated by the Earth rises, in a
·",dth the air "ocean." Water, for instance, is nearly in- process called convection, and is replaced by cooler air de-
compressible. A cubic foot of surface water weighs about scending from higher altihldes. As the hot air rises, the
the same as a cubic foot taken from the bottom of the pressure deCl'eases} and the air expands to become less
Marianas Trench. But this is not the case with a cubic foot dense. When it rises, if it cools sufficiently, it will con-
of air taken from different altihldes. The higher one goes, dense into clouds and then perhaps into rain or snow.
the lighter the air becomes, and consequently the more The whole process is determined by the simplest of the
easily compressed it is. laws of gases: expansion is a cooling process, while com-
The atmosphere thins so rapidly that over half of the pression generates heat.
total atmosphere by weight is in the first 3" miles of at- The average temperahu'e of the air at sea level is
mosphere. It is within this 3,,-mile "air envelope" that al- about 56 degrees F. At the top of the troposphere the tem-
most all of Earth's weather occurs. By the time a balloon perature is about -85 degrees F. The air automatically
has ascended to 20 miles, 99 percent of the atmospheric cools about 5" degrees for each 1,000 feet it travels up-
weight and gases lie below. Beyond 45 miles, only he- vvard; the reverse occurs in descent. This automatic tem-
lium and hydrogen are present, in very tiny amounts. perature change in rising or falling air is called adiabatic
Within the air envelope, then, lies the tempestuous air warming or cooling.
ocean, constantly churning and mixing the gases we Air circulation in the troposphere is of great impor-
breathe. Here are all the \vinds, clouds, rains, and storms tance to us because the circulation of air masses deter-
that make the weather. mines our 'weather. As a result, accurate weather predic-
The atmosphere consists of five principal layers. tion is dependent upon a thorough understanding of air
From the Earth's surface outward into space, they are the movement in the troposphere. Intense shldy of the at-
troposphere, stratosphere, mesosphere, thermosphere, mosphere in recent years has proved that the swift move-
and exosphere. There are also transition zones of ment of cold-air masses about the vast Antarctic conti-
vital importance between some of these layers. The nent is a major factor in determining the vvorld's
tropopause lies between the troposphere and the strato- weather. This is one of the main reasons that several na-
sphere; the chemosphere or ozone layer lies mainly be- tions, including the United States, have had a continuing
tween the stratosphere and mesosphere. The ionosphere interest in Antarctic research.

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