The atmosphere is a nearly transparent layer of gases and suspended particles surrounding the Earth, which influences environmental conditions on the planet's surface. Without chemical processes involving several of the atmospheric gases, life could not exist. These physical processes operating in our atmosphere are vitally importance because they are responsible for the Earth's varied climates.

Carbon dioxide makes up 325 parts per million of the atmosphere by volume, and is vital in maintaining the Earth-atmosphere system's heat balance because it absorbs infrared (thermal) radiation. Water vapor, present in variable quantities (0 - 4% by volume), absorbs considerable infrared radiation and is an essential link in the hydrologic cycle. Another trace gas is ozone (the triatomic form of oxygen), which is concentrated in a layer centered at about 25 km (16 ml) above the surface. Ozone absorbs radiation in the ultraviolet region of the spectrum. The ozone layer almost completely shields life on Earth from harmful ultraviolet rays.

The atmosphere is divided into four layers. From the bottom these are the troposphere, the stratosphere, the mesosphere, and the thermosphere, and each possesses a different temperature range. The temperatures decreases with altitude in the troposphere and mesosphere and increases with altitude in the stratosphere and thermosphere.

• The tropopause separates the troposphere and stratosphere, where the level of minimum temperature varies in altitude from about 16 km (10 ml) near the equator to 9 km (5 ml) near the poles.
• The stratosphere and mesosphere are separated by the stratopause, a level of temperature maximum at an altitude near 50 km (30 ml).
• The menopause separates the mesosphere and thermosphere, a temperature minimum which occurs near 80 km (50 ml).