How long do contrails last?

A contrail forms when hot, humid exhaust from a jet engine meets air that is typically colder than −40 °C. Water vapour from burning kerosene, plus the vapour already in the air, condenses on soot and sulphate particles in the exhaust and freezes almost at once into ice crystals. The visible line usually begins a few aircraft lengths behind the engines, the gap being the time it takes the plume to cool enough for condensation to begin. In the first second the crystals are tiny and dense; what happens next depends entirely on the surrounding air.

If the ambient air is dry, the ice crystals sublimate as the plume mixes with its surroundings, and the contrail disappears within seconds or a few minutes. These are the short, stubby trails that seem to chase the aircraft across the sky. If the air is supersaturated with respect to ice, a condition common in the upper troposphere, the crystals do not evaporate; they grow by capturing ambient vapour. Such persistent contrails can last for hours. Wind shear stretches and spreads them, and over a few hours a straight line can become a diffuse sheet tens of kilometres wide that is indistinguishable from natural cirrus. Satellite studies have tracked contrail-cirrus clusters drifting for hundreds of kilometres downwind of busy air corridors.

Persistent contrails matter because they are clouds, and high, thin ice clouds trap outgoing infrared radiation more than they reflect sunlight. Research summarised by EASA and NASA suggests that contrail cirrus contributes a warming effect comparable to, and possibly larger than, aviation's CO2 emissions, although with large uncertainty. The effect is strongly uneven: a small fraction of flights, those passing through ice-supersaturated regions, especially at night, account for most of the warming. That has prompted trials in which airlines adjust cruise altitude by a few hundred metres to avoid the humid layers, often at a fuel cost of well under one percent.

The rule of thumb for an observer is simple. Trails that vanish quickly indicate dry air aloft and usually settled weather. Trails that linger and spread signal moist air at altitude, often ahead of a warm front, and were used as a folk forecasting sign long before their radiative effects were studied.