Contrails (condensation trails) are artificial linear clouds that form behind aircraft flying at high altitude, typically above 8,000 metres. They are produced when hot, moist engine exhaust gases mix with the extremely cold outside air (below -40 °C) and low humidity, causing instantaneous condensation and freezing of water vapour into tiny ice crystals. The persistence of contrails depends on upper tropospheric humidity: in dry air, contrails dissipate within seconds or minutes; in humid air (near ice saturation), they persist for hours and can spread laterally to cover extensive areas of sky, forming what are called artificial cirrus or contrail cirrus. These persistent clouds are the subject of climate research because they reflect solar radiation during the day (cooling effect) but trap infrared radiation at night (warming effect), with a net balance that contributes to the greenhouse effect. Contrails serve as useful informal weather indicators: if aircraft contrails persist and spread, they indicate high humidity in the upper troposphere, frequently associated with the approach of a warm front with its precursor cirrus clouds. If they dissipate rapidly, the air aloft is dry, which usually indicates fair weather.