From SKYbrary Wiki
A trail of cloud which forms behind an aircraft flying, usually at high level, in moist and very cold air.
How Contrails are Formed
One of the main constituents of exhaust gasses emitted by a jet engine is water vapour. This has the effect of increasing the humidity of the layer of air that an aircraft flies through. In certain conditions, if the humidity is raised beyond saturation, a trail of cloud forms behind the aircraft. This condensation trail is known as a CONTRAIL. The critical temperature, below which Contrails may form (MINTRA, minimum trail), is approximately -24°C at Sea Level and -45°C at 50,000 feet. However, the heat generated by the engine raises the temperature in the wake of the aircraft and consequently lowers the relative humidity so jet aircraft do not necessarily generate contrails unless the temperature is approximately 15°C below the MINTRA temperature. In broad terms, Contrails are more likely in a warm Tropical Maritime air-masses with a high but cold tropopause, whereas Contrails are less likely to be seen in cold, polar air-masses with a low, relatively warm tropopause.
Since a Contrail indicates where an aircraft has been, their existence may be an indication of potential Wake Vortex Turbulence.