Airplanes typically fly within the lower part of the stratosphere, which is the second layer of Earth’s atmosphere, situated above the troposphere. The stratosphere extends approximately 10 to 50 kilometers above the Earth's surface and is characterized by a gradual increase in temperature with altitude due to the presence of the ozone layer, which absorbs and scatters ultraviolet solar radiation. Commercial jets usually cruise at altitudes between 9,000 and 13,000 meters, or about 30,000 to 40,000 feet, allowing pilots to avoid turbulence found in the lower troposphere.