Aircraft contrail. Composite image of a dissipating aircraft contrail, or vapour trail. The top image shows the same contrail 5-10 seconds later than the bottom image. 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. Sometimes these 'chromosome-shaped' patterns form as the contrail dissipates. Photographed over Finland.