In meteorology, the atmospheric layer, constituting essentially all of the atmosphere above the mesosphere (that is, above about 80-90 km altitude), in which temperature increases with height; includes the exosphere and most or all of the ionosphere.

In the area above the mesosphere, the temperature increases with altitude up to about 200 km, and above that it varies widely depending on the degree of solar activity with temperatures as high as 2000 degrees C are said to be possible.