We 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 outer pressure.
 
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.