ATMOSPHERE

The atmosphere of Earth is a layer of gases surrounding the planetEarth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultravioletsolar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperatureextremes between day and night(the diurnal temperature variation).

Atmospheric stratification describes the structure of the atmosphere, dividing it into distinct layers, each with specific characteristics such as temperature or composition. The atmosphere has a mass of about 5×10¹⁸ kg, 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 the name given to atmosphere used in breathing andphotosynthesis. Dry air contains roughly (by volume) 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039%carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. While air content and atmospheric pressurevary at different layers, air suitable for the survival ofterrestrial plants and terrestrial animals is currently only known to be found in Earth's troposphere and artificial atmospheres.