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.

Persistent condensation trails can last for many hours, gradually spreading out to form large, sometimes dense areas of cirriform cloud; they can have dimensions typically several kilometres wide and several hundred metres in depth (thus they can be seen in visible satellite imagery). They spread because of turbulence at the 'trailing' level ( enhanced by the aircraft passage), differences in wind speed along the flight-path and there is also thought to be a contribution from solar heating. Because trails can last so long and come to dominate the upper troposphere in any particular synoptic situation, the production of such form part of the debate on the overall global radiation balance.