Stratosphere

The stratosphere is one of the layers of the atmosphere of the planet Earth. It extends from about 8 km above the poles (18 km above the equator) to about 50 km. In the stratosphere, the temperature rises with increasing height. This makes the stratosphere very stable. This temperature gradient is caused by the ozone layer which absorbs some UV radiation from sunlight. The absorbed radiation is converted to heat. 
This differs from the lower layer, the troposphere, where temperature declines with higher altitude, and from the next higher layer, the mesosphere, where the temperature again declines.
Airplanes that are going far usually fly in the lower stratosphere, because the engines are more efficient in cold air and drag is small in low air density air.