The layer of the Earth's atmosphere directly above the troposphere and below the mesosphere. It extends from approximately 7 miles above the earth's surface to about 30 miles. This is where many jet aircraft fly. It is also home to the ozone layer.

To expand on what vectormane said:

The stratosphere is the section of Earth's atmosphere that lies immediately above the troposhere, the area of the atmosphere where we live, and where weather happens. These two layers are seperated by the tropopause, a thin layer of the atmosphere. The stratosphere is bordered on the top by the stratopause.

Unlike the troposhere, temperatures in the stratosphere actually increase with altitude. In the bottom half of the stratosphere, temperature increases slowly, beginning at -70 degrees farenheit. In the upper half, temperatures increase relatively quickly, eventually reaching almost 40 degrees farenheit in some places. The reason for this is the fact that the ozone is located in this upper half and so tt absorbs most of the sun's ultraviolet radiation.

Of the earth's total breathable air, 99% is located in the stratosphere and the troposhere, with most actually in the troposhere. The pressure here is still very low, far less that at any place in the troposhere. The air here tends to be very dry, and no clouds exist in this layer. And, unlike the troposhere, where there are powerful convection currents (the kind that create Cumulonimbus and other clouds) that flow upward, this layer has only currents that move horizontally.

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