Contrail, << KON trayl, >> is a thin line of cloud that forms behind an aircraft. Contrails consist of tiny water droplets or ice crystals. They form when water vapor in the air condenses (becomes liquid) and then freezes. Contrails are also called condensation trails, exhaust trails, or vapor trails. Contrails last longest in extremely cold air that can hold little additional moisture. They are thus often seen at high altitudes.
A contrail may form in two ways. (1) The exhaust from an aircraft engine contains water vapor. This vapor may condense when it mixes with cold air around the plane. This type of contrail is called a combustion contrail. (2) When a plane moves through the air, water vapor may condense in thin clouds over the wings and behind the wing tips and the tips of the propellers. The condensation occurs because the air temperature drops as the air flows over these surfaces. The resulting contrails are referred to as aerodynamic contrails. They are often seen by airplane passengers as white filaments of condensation during take-off or landing.
Contrails may have a small effect on the weather. For example, ice crystals from a contrail may cause rain or snow to fall from certain clouds. The crystals act like the chemicals used to “seed” clouds in rainmaking operations. See Cloud seeding .
