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The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earth's gravity.

The thermosphere is the second-highest layer of Earth's atmosphere.Because the thermopause lies at the lower boundary of the exosphere, it is also referred to as the exobase.The lower part of the thermosphere, from 80 to 550 kilometres (50 to 342 mi) above Earth's surface, contains the ionosphere.Filtered air includes trace amounts of many other chemical compounds.Many substances of natural origin may be present in locally and seasonally variable small amounts as aerosols in an unfiltered air sample, including dust of mineral and organic composition, pollen and spores, sea spray, and volcanic ash.Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi).

Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.

The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space.

The Kármán line, at 100 km (62 mi), or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space.

The concentration of water vapor (a greenhouse gas) varies significantly from around 10 ppm by volume in the coldest portions of the atmosphere to as much as 5% by volume in hot, humid air masses, and concentrations of other atmospheric gases are typically quoted in terms of dry air (without water vapor).

among which are the greenhouse gases, principally carbon dioxide, methane, nitrous oxide, and ozone.

The three major constituents of air, and therefore of Earth's atmosphere, are nitrogen, oxygen, and argon.