The Flyer's Environment
live at the bottom of an ocean of air, the atmosphere, which is
necessary to support life on earth. Not only does it provide oxygen but
it also filters out harmful radiation from the sun. The presence of the
atmosphere prevents excessive heat loss in both plants and animals, and
maintains their surface temperature within the range required for
survival. The exact upper limit of the earth's atmosphere has not been
determined, but estimates have varied from a few hundred miles to
several thousand miles.
This large volume of air has tremendous weight. At sea level, it exerts
a pressure of about 15 lb./in. squared (pounds per square inch) upon the
body - or a total of about 20 tons for the average man. This weight
sounds formidable, but at sea level it is quite compatible with man's
existence because the body's inner pressure equalizes the surrounding
As a pilot rises into the atmosphere, he experiences a decrease in
pressure. Close to the earth, the air is most compressed, and,
therefore, most dense, because of the weight of the air above it
pressing down. During ascent from the earth's surface, pressure is lost
rapidly, becoming one half as great at 18,000 feet as at sea level.
Besides adapting to the rarefied air at altitude, the pilot's body must
adjust to dropping temperatures. Even in summer, the temperature of the
air at 18,000 feet is near the freezing point. On some days, it is much
lower. In this abnormal habitat, survival depends upon the ability of
the body to make adaptive changes.