Crossing aircraft contrails. 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 5000 metres and when the upper atmosphere is nearly saturated with moisture. In dry air the contrail quickly dissipates. These contrails were photographed to the west of Heathrow, London, UK, on the flight path to America. There is increasing concern about the environmental effects of air travel and its contribution to global warming.