A contrail, also known as a condensation trail, is a cirrus-like trail of condensed vapor (often resembling the tail of a kite) that is produced by jet aircraft flying at high altitudes. Contrails are produced at altitudes high enough for water droplets to freeze in a matter of seconds before they evaporate. Temperatures at such altitudes are typically below -38 degrees Celsius.

Contrails form through the injection of water vapor into the atmosphere by exhaust fumes from a jet engine. If there is sufficient mixing between the cold upper tropospheric air and the hot exhaust gases to produce a state of saturation, ice crystals will develop. Even tiny nuclei released in the exhaust fumes may be sufficient enough to generate ice crystals.

Contrails spread apart and evaporate with time. If the air in which the cloud develops has a low relative humidity, the cloud particles will quickly evaporate. However, even in the presence of higher relative humidities, upper level winds can spread contrails apart, forming a horizontal sheet-like cloud. For a contrail to remain in tact for a long period of time, the air must have high a relative humidity and light winds.