atmosphere

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atmosphere
1. Air surrounding the Earth. The atmosphere has no precise upper limit, but for all practical purposes the absolute top can be regarded as being at about 200 km. The density of the atmosphere decreases rapidly with height, and about three-quarters of the mass of the atmosphere is contained within the lowest major layer, the troposphere, whose depth varies between about 10 km and 17 km, being generally smaller further from the equator.

2. Unit of pressure (abbreviation: atm.). Its value is approximately the average pressure of the atmosphere at sea level, the figure adopted being the pressure at sea level in the International Standard Atmosphere (760 mm of mercury, or 1013.25 mb). In SI units, 1 atm. = 101 325 Pa. See also ATMOSPHERIC STRUCTURE.

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