by the National Weather Service

“ The Earth's Atmosphere Planet Earth is unique in that its atmosphere sustains life as we know it. Weather—the state of the atmosphere—at any given time and place strongly influences our daily routine as well as our general life patterns. Virtually all of our activities are affected by weather, but of all man's endeavors, none is influenced more intimately by weather than aviation. ”

“ The decrease in air density with increasing height has a physiological effect which we cannot ignore. The rate at which the lungs absorb oxygen depends on the partial pressure exerted by oxygen in the air. The atmosphere is about one-fifth oxygen, so the oxygen pressure is about one-fifth the total pressure at any given altitude. Normally, our lungs are accustomed to an oxygen pressure of about 3 pounds per square inch. But, since air pressure decreases as altitude increases, the oxygen pressure also decreases. ”

“ Pressure the atmosphere exerts on the surface is the result of the weight of the air above. Thus, air near the surface is much more dense than air at high altitudes. This decrease of density and pressure with height enters frequently into our discussions in later chapters. ”