The mixture of gases surrounding the Earth or other celestial body, held in place by gravity. It forms distinct layers at different heights. The Earth's atmosphere consists, in ascending order, of the troposphere (containing 90% of the atmosphere's mass), the stratosphere, the mesosphere, the thermosphere, and the exosphere. The atmosphere is composed primarily of nitrogen (78%) and oxygen (21%) and plays a major role in the water cycle, the nitrogen cycle, and the carbon cycle. See more at exosphere, mesosphere, stratosphere, thermosphere, troposphere.
A unit of pressure equal to the pressure of the air at sea level, about 14.7 pounds per square inch, or 1,013 millibars.
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