The Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night. Dry air contains roughly (by volume) 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide, and trace amounts of other gases.

Air also contains a variable amount of water vapor, on average around 1%. The atmosphere has a mass of about five quadrillion (5x1015) tonnes, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space.

An altitude of 120 km (75 mi) is where atmospheric effects become noticeable during atmospheric reentry of spacecraft. The Kármán line, at 100 km (62 mi), also is often regarded as the boundary between atmosphere and outer space. Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the "major gases" of the atmosphere.

The remaining gases often are referred to as "trace gases," among which are the greenhouse gases such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other chemical compounds. Many natural substances may be present in tiny amounts in an unfiltered air sample, including dust, pollen and spores, sea spray, volcanic ash, and meteoroids.

Various industrial pollutants also may be present, such as chlorine (elementary or in compounds), fluorine (in compounds), elemental mercury, and sulfur (in compounds such as sulfur dioxide [SO2]).

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