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atmosphere

World Encyclopedia | Date: 2005

atmosphere Envelope of gases surrounding the Earth and shielding it from the harsh environment of space. The gases it contains are vital to life. About 95% by weight of the Earth's atmosphere lies below the 25km (15mi) altitude; the mixture of gases in the lower atmosphere is commonly called air. The atmosphere's composition by weight is: nitrogen 78.09%, oxygen 20.9%, argon 0.93%, 0.03% of carbon dioxide, plus 0.05% of hydrogen, the inert gases and varying amounts of water vapour. The atmosphere can be conceived as concentric shells; the innermost is the troposphere, in which dust and water vapour create clouds and weather. The stratosphere extends from 10–55km (8–36mi) and is cooler and clearer and contains ozone. Above, to a height of 70km (43mi), is the mesosphere in which chemical reactions occur, powered by sunlight. The temperature climbs steadily in the thermosphere, which gives way to the exosphere at c.400km (250mi), where helium and hydrogen may be lost into space. The ionosphere ranges from c.50km (30mi) out into the Van Allen radiation belts.


© World Encyclopedia 2005, originally published by Oxford University Press 2005.