Nature Bulletin No. 542-A November 2, 1974
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation
Until after the time of Christopher Columbus, most people believed that
this world was flat, like a table top. Some of the ancient Greeks,
however, had reasoned that the earth was round and that the canopy of
air above them was part of another sphere surrounding this planet. They
used two words to describe it -- atmos (vapor) and sphaira (sphere). We
know now that they were right. There is a gaseous envelope which, held
by gravity, clings to the earth and follows its every movement. We
commonly call it "the air. .
The air we breathe is the commonest thing on earth and, although free,
the most precious. There is an old proverb: "A man can live three weeks
without food, three days without water, but only three minutes without
air. " None of us, nor any of the animals and plants, could exist without
our atmosphere. It furnishes us with those elements necessary for all
life: oxygen, nitrogen, carbon dioxide and water vapor. It filters and
protects us from the killing rays of the sun. We live beneath the roof of
a gigantic greenhouse.
Without our atmosphere there would be no winds, no clouds, no rain,
no colorful sunrises and sunsets -- in other words, no weather at all and
consequently no climate. In daytime, as on the moon, the sky would be
black -- not blue -- and the temperature would zoom to 230 F. or more.
At night it would plunge to possibly 300 below zero. This would be a
Near the earth's surface, perfectly dry air contains about 78% nitrogen,
21% oxygen, and one percent of argon -- a peculiar inactive gas, small
amounts of carbon dioxide and hydrogen, and tiny amounts of five rare
gases: neon, krypton, helium, ozone and xenon. Normally, however, air
also contains water vapor in quantities varying from a mere trace to
about 5 percent on hot humid days, and it is responsible for the clouds,
fogs, rain, snow, hail, dew and frost. At altitudes of more than 5 miles
there is very little of it because of the low temperatures. In addition, our
air contains various amounts of impurities such as dust, soot, pollen,
spores, bacteria, salt particles from sea spray and -- especially near
industrial areas -- poisonous gases.
The density of the atmosphere diminishes rapidly with altitude. Six
miles up it is so thin that a man cannot breathe; at 12 miles there is not
enough oxygen to keep a candle burning; at 400 miles, in the almost
airless Mesosphere, the main ingredients are probably hydrogen and
helium, the two lightest gases.
Data obtained from radio waves, balloons and missiles indicate that, for
convenience, the atmosphere may be divided into five layers or
concentric spheres around the earth. They are chiefly distinguished by
the temperatures in them but each merges gradually into the one above
it. The lowest layer, roughly 7 miles high, is the Troposphere where
most of our weather is generated and which contains the air we breathe.
Above that is the Stratosphere, extending to an altitude of about 20
miles and then the Chemosphere where the sun's ultraviolet rays are
screened from the earth by the ozone gas they produce. The space
between altitudes of 50 and 250 is called the Ionosphere. Above it, and
extending to possibly 600 miles above the earth, is the Mesosphere.
Beyond that is the Exosphere merging into outer space where, like
Columbus, men now carry on exploratory work.
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Update: June 2012