| The envelope of air surrounding a planetary body and bound to it more or less permanently by virtue of gravitational attraction; the system whose chemical properties, dynamic motions, and physical processes constitute (in the case of the planet Earth) the subject matter of meteorology. The atmosphere is gaseous or air portion of the physical environment that encircles a planet. In the case of the earth, it is held more or less near the surface by the earth's gravitational attraction. The divisions of the atmosphere include the troposphere, the stratosphere, the mesosphere, the ionosphere, and the exosphere. |