The layer of gases surrounding the earth, from the surface upward to a height of about 22 miles. The atmosphere consists of a mixture of nitrogen, oxygen, and small quantities of a number of other gases.

The lowest level of the atmosphere, called the troposphere, extends upward to about 36,000 feet. This is the area in which clouds form. Above the troposphere is the stratosphere, where there is no water vapor; therefore clouds cannot form.

The atmosphere presses down on the surface of the earth with a pressure of 29.92 inches (760 millimeters) of mercury. Its standard weight at sea level is 0.07651 pound per cubic foot, and it becomes lighter, or less dense, as its height above the earth increases.

The temperature of the atmosphere drops as its altitude increases, to about 36,000 feet. Here, the temperature stabilizes at -67°F (-55°C), and it remains at this temperature until it reaches an altitude of about 82,000 feet.