Contrails over the English Channel, satellite image. Contrails (condensation trails) are artificial clouds formed from frozen water droplets from the exhaust of the engines of an aircraft. They are formed when the aircraft is flying at an altitude of over 5000 metres and when the upper atmosphere is nearly saturated with moisture. In dry air, the contrails will evaporate quickly. These contrails show how common air travel is in this region. Image data obtained on 9 December 2003 by the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on NASA's Aqua and Terra satellites.