Stratosphere
The stratosphere' is the layer of the Earth's atmosphere between the troposphere and the ionosphere. The lower boundary is at a height of around 9km near the poles and 17km near the equator, and the upper boundary is at a height of 50-55km. At a height of around 25km, most of the gas is ozone; this provides a valuable screen, stopping dangerous ultraviolet light from the Sun reaching the ground.
Most long-distance commercial aircraft fly in the stratosphere, as the lower air density means that there is less turbulence (hence a smoother flight) and less fuel is needed.
Within the stratosphere, the temperature generally increases with height, from around -55C (-67F) to 0C (32F).