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/ ætməsfɪr /


ETYM Greek atmos vapor; akin to Skr. âtman breath, soul, German athem breath; + spheros sphere: cf. French atmosphčre. Related to Sphere.
Mixture of gases surrounding a planet. The Earth's atmosphere is prevented from escaping by the pull of the Earth's gravity. Atmospheric pressure decreases with height in the atmosphere. In its lowest layer, the atmosphere consists of nitrogen (78%) and oxygen (21%), both in molecular form (two atoms bonded together). The other 1% is largely argon, with very small quantities of other gases, including water vapor and carbon dioxide. The atmosphere plays a major part in the various cycles of nature (the water cycle, carbon cycle, and nitrogen cycle). It is the principal industrial source of nitrogen, oxygen, and argon, which are obtained by fractional distillation of liquid air.
The lowest level of the atmosphere, the troposphere, is heated by the Earth, which is warmed by infrared and visible radiation from the Sun. Warm air cools as it rises in the troposphere, causing rain and most other weather phenomena. However, infrared and visible radiations form only a part of the Sun’s output of electromagnetic radiation. Almost all the shorter-wavelength ultraviolet radiation is filtered out by the upper layers of the atmosphere. The filtering process is an active one: at heights above about 50 km/31 mi ultraviolet photons collide with atoms, knocking out electrons to create a plasma of electrons and positively charged ions. The resulting ionosphere acts as a reflector of radio waves, enabling radio transmissions to “hop” between widely separated points on the Earth’s surface.
Waves of different wavelengths are reflected best at different heights. The collisions between ultraviolet photons and atoms lead to a heating of the upper atmosphere, although the temperature drops from top to bottom within the zone called the thermosphere as high-energy photons are progressively absorbed in collisions. Between the thermosphere and the tropopause (at which the warming effect of the Earth starts to be felt) there is a “warm bulge” in the graph of temperature against height, at a level called the stratopause. This is due to longer-wavelength ultraviolet photons that have survived their journey through the upper layers; now they encounter molecules and split them apart into atoms. These atoms eventually bond together again, but often in different combinations. In particular, many ozone molecules (oxygen atom triplets) are formed. Ozone is a better absorber of ultraviolet than ordinary (two-atom) oxygen, and it is the ozone layer that prevents lethal amounts of ultraviolet from reaching the Earth’s surface.
Far above the atmosphere, as so far described, lie the Van Allen radiation belts. These are regions in which high-energy charged particles traveling outward from the Sun (as the so-called solar wind) have been captured by the Earth’s magnetic field. The outer belt (at about 1,600 km/1,000 mi) contains mainly protons, the inner belt (at about 2,000 km/1,250 mi) contains mainly electrons. Sometimes electrons spiral down toward the Earth, noticeably at polar latitudes, where the magnetic field is strongest. When such particles collide with atoms and ions in the thermosphere, light is emitted. This is the origin of the glows visible in the sky as the aurora borealis (northern lights) and the aurora australis (southern lights).
A fainter, more widespread, airglow is caused by a similar mechanism.
During periods of intense solar activity, the atmosphere swells outward; there is a 10–20% variation in atmosphere density. One result is to increase drag on satellites. This effect makes it impossible to predict exactly the time of reentry of satellites.
Other ingredients are found in particular localities: gaseous compounds of sulfur and nitrogen in towns, salt over the oceans; and everywhere dust composed of inorganic particles, decaying organic matter, tiny seeds and pollen from plants, and bacteria.
1. A particular environment or surrounding influence; SYN. ambiance, ambience.
2. A unit of pressure: the pressure that will support a column of mercury 760 mm high at sea level and 0 degrees centigrade; SYN. atm.
3. The envelope of gases surrounding any celestial body.
4. The mass of air surrounding the Earth.
5. The weather or climate at some place; SYN. atmospheric state.


air · ambiance · ambience · atm · atmospheric state · aura · standard atmosphere · standard pressure

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