Aeroplane contrail against the Moon. Aeroplane, at a height of 10 kilometres, producing a contrail. Contrails are artificial clouds formed from frozen water droplets from the exhaust of the engines of an aircraft. They form when aircraft are flying at altitudes exceeding 5 kilometres and when the upper atmosphere is nearly saturated with moisture. It is thought that contrails at this height could possibly destroy parts of the ozone layer with which they come into contact.