atmosphere (earth science)

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Mixture of gases surrounding a planet. Planetary atmospheres are prevented from escaping by the pull of gravity. On Earth, atmospheric pressure decreases with altitude. In its lowest layer, the atmosphere consists of nitrogen (78%) and oxygen (21%), both in molecular form (two atoms bonded together) and argon (1%). Small quantities of other gases are important to the chemistry and physics of the Earth's atmosphere, including water, carbon dioxide, and traces of other noble gases (rare gases), as well as ozone. The atmosphere plays a major part in the various cycles of nature (the water cycle, the carbon cycle, and the nitrogen cycle). It is the principal industrial source of nitrogen, oxygen, and argon, which are obtained by the fractional distillation of liquid air.

The Earth's atmosphere is divided into four regions of atmosphere classified by temperature.

Troposphere
This is the lowest level of the atmosphere (altitudes from 0 to 10 km/6 mi) and it is heated to an average temperature of 15°C/59°F by the Earth, which in turn is warmed by infrared and visible radiation from the Sun. Warm air cools as it rises in the troposphere and this rising of warm air causes rain and most other weather phenomena. The temperature at the top of the troposphere is approximately -60°C/-76°F.

Stratosphere
Temperature increases with altitude in this next layer (from 10 km/6 mi to 50 km/31 mi), from -60°C/-76°F to near 0°C/32°F.

Mesosphere
Temperature again decreases with altitude through the mesosphere (50 km/31 mi to 80 km/50 mi), from 0°C/32°F to below -100°C/-148°F.

Thermosphere
In the highest layer (80 km/50mi to about 700 km/450 mi), temperature rises with altitude to extreme values of thousands of degrees. The meaning of these extreme temperatures can be misleading. High thermosphere temperatures represent little heat because they are defined by motions among so few atoms and molecules spaced widely apart from one another.

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