Why do some high flying aircraft leave white trails in their wake?
The white trails are ribbons of ice crystals. As a by-product of the exhaust of aircraft engines, water vapour is trailed from the engine exhaust which adds to the local humidity of the air the aircraft is flying through, and which tends to super-saturation of the air. However, the exhaust gases are of course hot, and so these hot gases help to raise the temperature of the air and thus is can hold more vapour before saturation is reached. There are therefore two opposing mechanisms at work: the water vapour in the exhaust trying to saturate the air; the hot gases of the exhaust trying to decrease relative humidity. When the balance between outside air temperature (OAT) and local humidity is just right, then condensation trails will occur: usually abbreviated to CONTRAILS, and sometimes referred to, from old coding conventions, as COTRA. Sometimes, such trails persist for a considerable time, gradually spreading out to form large, sometimes dense areas of cirriform cloud - for this reason, the production of aircraft condensation trails form part of the debate on the overall global radiation balance.