Sunset vapour trail. A vapour trail or condensation trail, contrail, is a line-shaped cloud of water vapour left in the sky by an aeroplane, a rocket, or a missile. Contrails are produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruising altitudes several miles above the Earth's surface. Contrails are composed primarily of water, in the form of ice crystals. Engine exhaust is predominantly made up of water and carbon dioxide, the combustion products of hydrocarbon fuels. In general, aircraft contrails are believed to trap outgoing longwave radiation emitted by the Earth and atmosphere more than they reflect incoming solar radiation, resulting in a net increase in what is called radiative forcing, what happens when the amount of energy that enters the Earth's atmosphere is different from the amount of energy that leaves it.