How to Define Atmosphere

A cloudy sky

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The term "atmosphere" has multiple meanings in science:

Atmosphere Definition

Atmosphere refers to the gases surrounding a star or planetary body held in place by gravity. A body is more likely to retain an atmosphere over time if gravity is high and the temperature of the atmosphere is low.

The composition of the Earth's atmosphere is about 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, with water vapor, carbon dioxide, and other gases. The atmospheres of other planets have a different composition.

The composition of the Sun's atmosphere consists of about 71.1 percent hydrogen, 27.4 percent helium, and 1.5 percent other elements.

Atmosphere Unit

An atmosphere is also a unit of pressure. One atmosphere (1 atm) is defined to be equal to 101,325 Pascals. A reference or standard pressure is commonly 1 atm. In other cases, "Standard Temperature and Pressure" or STP is used.