Nib asked
Why do airplanes leave white trails?
The white trail is a thin cloud of ice: the jet's hot, wet exhaust freezes in air colder than minus forty degrees Celsius.
6 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1a cloud made by a plane
A contrail, short for condensation trail, is a thin line of cloud. It is made of tiny ice crystals, just like a high natural cloud, and it forms because of what comes out of the engines meeting the air at cruising height.
A contrail is a thin cloud of ice crystals made by the plane.
a common mix-upContrails are not smoke and not a chemical spray. They are made of water, frozen into ice crystals, from the plane's own exhaust and the cold air.
2how cold up there
Airliners usually cruise between about 9 and 12 kilometers up. There the air is typically between minus 40 and minus 60 degrees Celsius, far colder than any freezer.
At cruise the air is colder than about minus 40 °C.
3wet exhaust
Jet fuel is made of carbon and hydrogen. Burning it releases carbon dioxide and water. Burning one kilogram of kerosene produces roughly 1.2 kilograms of water vapor. The exhaust leaves hot and very humid. Tiny soot particles in it give the water something to cling to.
Burning jet fuel releases water vapor: about 1.2 kg per kg of fuel.
What does burning kerosene produce besides carbon dioxide?
Show the answer
Water vapor, about 1.2 kg for every kg of fuel.
4vapor becomes ice
The hot exhaust mixes with the surrounding air within a second or two and cools very fast. The water vapor condenses and freezes onto the soot, forming millions of tiny ice crystals. Together they make the white line. You see the same thing when your warm breath turns to a puff of mist on a cold day.
Hot vapor mixes with air below minus 40 and freezes into ice crystals.
5short or long
If the surrounding air is dry, the ice crystals evaporate again within seconds to minutes and the trail disappears. If the air is already humid enough to hold ice, the crystals survive and grow. Persistent contrails can spread for hours into sheets of thin cirrus-like cloud, which can affect the climate.
Dry air: the trail vanishes. Humid air: it lingers and spreads.
a common mix-upNot every plane leaves a trail, and not every trail lasts. Short trails mean dry air up there; long, spreading ones mean humid air.
What does a long, spreading trail tell you about the air?
Show the answer
That the high air is humid enough for the ice to survive.
6the whole idea
So the whole idea: hot, wet exhaust meets air colder than minus forty and freezes into ice. Dry air erases it, humid air lets it spread.
A contrail is just your plane's breath freezing in the cold sky.