Man lives at the bottom of an ocean of air. We may ordinarily take the atmosphere for granted and focus much more concern on the weather. This ocean of air, however, 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 downward at all times. At a higher altitude, however, both the pressure and the density of air decrease. This explains why passenger jets, which often fly near 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 take in enough oxygen with each breath.

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 [16.1–48.3 km...