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Airplane Contrails

Why do airplanes sometimes leave long white contrails behind?

Contrails from airplanes like these over Bergen, Germany are formed when water vapor condenses around exhaust particles

Imagine two or three long white lines made by airplanes smeared across a perfect blue sky. They look like smudges on a pane of glass, and are known as contrails. Why do airplanes sometimes leave these behind.

Contrails, short for condensation trails, are clouds. They are no different from the cirrus clouds that naturally form in that region of the atmosphere.

That high in the atmosphere, water can exist in one of two basic forms.  It can either be invisible water vapor, a gas, or it can condense around microscopic dust particles into tiny droplets or ice crystals.  Every visible cloud is made of tiny droplets or crystals like this, and so is a contrail.

[pullquote]The size of a contrail may gave an indication on what the weather will be like the next day.[/pullquote]

A recipe for airplane contrails is water and microscopic particles for the water to condense around. When jet fuel burns, it releases water vapor and microscopic particles of exhaust.  A contrail occurs when the jet’s water vapor condenses around its own exhaust particles.

When a plane leaves no contrail or a very thin one, this means the upper atmosphere is relatively dry.  Dry air reabsorbs the water as invisible vapor almost as soon as it condenses.

If a contrail lasts a long time and looks thick, this means the upper atmosphere is already saturated with as much water vapor as it can hold.

Because of this, it’s possible to use contrails to predict the weather.  Quickly fading ones suggest a dry upper atmosphere and fair weather. If there are many long contrails in the sunset, it could mean storms the next day.

Sources And Further Reading:

  • Dennis, Jerry. It’s Raining Frogs and Fishes. New York: Harper Perennial, 1992.
  • Rossmann, Jen Stroud. “Why do jets leave a white trail in the sky?” Scientific American. February 18, 2002. Accessed January 11, 2017.

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