In Earth's atmosphere, the limits of the atmospheric layers are approximate and variable, …—© Merriam-Webster Inc.

Gaseous envelope that surrounds the Earth. Near the surface it has a well-defined chemical composition (see air). In addition to gases, the atmosphere contains solid and liquid particles in suspension. Scientists divide the atmosphere into five main layers: in ascending order, the troposphere (surface to 6–8 mi, or 10–13 km); the stratosphere (4–11 mi, or 6–17 km, to about 30 mi, or 50 km); the mesosphere (31–50 mi, or 50–80 km); the thermosphere (50–300 mi, or 80–480 km); and the exosphere (from 300 mi and gradually dissipating). Most of the atmosphere consists of neutral atoms and molecules, but in the ionosphere a significant fraction is electrically charged. The ionosphere begins near the top of the stratosphere but is most distinct in the thermosphere. See also ozone layer.

This entry comes from Encyclopædia Britannica Concise.
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