The white line behind a plane: smoke or cloud?

When jet fuel burns, it gives off heat along with water in the form of vapor. Airliners fly around 10 kilometers above the ground, where the air is often colder than minus 40 degrees Celsius. When the hot, moist exhaust mixes with this cold air, the vapor clings to tiny particles from the engine, turns into droplets and freezes right away into ice crystals.

It is a bit like seeing your breath turn white on a winter day, only on a much bigger scale and far up in the sky.

Even behind the same plane, the line looks different from day to day. When the air high up is dry, the ice crystals quickly turn back into vapor, so the line is short and disappears within a few minutes.

When the air high up is humid, the ice crystals instead grow by soaking up the vapor around them. The line can then last for hours, get pushed and spread out by the wind, and turn into a thin cloud that is hard to tell apart from wispy cirrus.

Many people think the white line is soot or smoke from the engines. But most of what you see is ice, so even the same plane leaves almost no visible line in a dry sky. The air high up can be humid even when the weather near the ground is clear and dry, so the humidity you feel on the ground tells you little about whether a line will stay.

That makes a contrail more like a sign showing whether the air at that height is humid right now.

A day when contrails stay long and spread out means there is a lot of water vapor high in the sky. This often happens as a rain-bringing low pressure system approaches, so some take it as a sign the sky may cloud over within a day or two. It is not always right, so check it against the forecast.

The short white lines you sometimes see at the wingtips of planes taking off or landing near an airport are a little different. Air passing over the wing drops in pressure and cools, so droplets form; they appear low in the sky on humid days and vanish just a few meters behind the plane.