We live at the bottom of an ocean of air. We take the atmosphere pretty much for granted; we are generally much more concerned with the weather. But this ocean of air has profound consequences for life on Earth.

The surface density of air is about 0.074 lb/ft³ (1.184 g/l) and surface pressure is about 14 lb/ft² (1 atm). This mass of air presses down upon us at all times. But at a higher altitude, both the pressure and the density of air decrease. This is why passenger jets, which fly at 40,000 ft (12,192 m) to take advantage of the thin or low density air, require pressurized cabins. Without them, passengers would not be able to breath properly.

The atmosphere is generally divided into four zones or layers. Starting at sea level and increasing in altitude, they are the troposphere (0-10 mi [0-16.1 km]), the "stratosphere"(10-30 mi...