full text of the classic FAA guide
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.
Weather is complex and at times difficult to understand. Our restless atmosphere is almost constantly in motion as it strives to reach equilibrium. These never-ending air movements set up chain reactions which culminate in a continuing variety of weather. Later chapters in this book delve into the atmosphere in motion. This chapter looks briefly at our atmosphere in terms of its composition; vertical structure; the standard atmosphere; and of special concern to you, the pilot, density and hypoxia.