1. The thin layer of gasses surrounding the earth that is the medium for weather and climate.
2. An envelope of gas surrounding a planet, star, or other celestial body, where the gravitational field is strong enough to restrain the gases.

In physics, a unit of pressure equal to 760 torr, 1013.25 millibars, or 1.01325 x 10⁵ newtons per square meter (the terms used here are located in this unit).

The actual pressure exerted by he atmosphere fluctuates around this value, which is assumed to be standard at sea level and zero degrees centigrade/32 degrees Fahrenheit, and is used when dealing with very high pressures.

A mixture of gases surrounding the earth and such atmosphere is prevented from escaping because of the pull of gravity.

On earth, atmospheric pressure decreases with altitude and in its lowest layer, earth's atmosphere consists of nitrogen (78%) and oxygen (21%), both in molecular form (two atoms bonded together) and 1% argon.

Small quantities of other gases are important to the chemistry and physics of the earth's atmosphere, including water and carbon dioxide.

The earth's atmosphere is divided into four regions of atmosphere classified by temperature and the thermal structure of the earth's atmosphere is the result of the complex interaction of the following:

• Electromagnetic radiation from the sun.
• Radiation reflected from the earth's surface.
• Molecules and atoms in the atmosphere.