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What are the white streaks that planes leave behind?

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Have you noticed the white trails that planes draw across the sky?
When you peer at the sky on a bright day, you can often see criss-crossing white lines left behind by aircraft. But have you ever wondered how they are formed and why? We looked into the phenomenon.
1. Contrails are formed when vapor from the engines freezes
The trails left behind by planes are officially called contrails, short for concentration trails. They form a bit similarly to how the breath you exhale can condense into vapor on a cold day.
In short, contrails are formed when the water vapor in the exhaust from the plane’s engines condenses into water droplets, which then freeze into ice particles composing a line-shaped cloud.
The impurities in the jet exhaust form some of the particles on which water droplet grow, before freezing.
2. Contrails commonly occur at high altitudes
The most important factors for contrail formation are temperature and humidity.
Most typically, the trails are formed by jet planes that fly at the right altitude, around 25,000 to 40,000 feet, where the air is cold enough. The trails form most often at a temperature below minus 40 degrees Celsius.
3. Concentration trails can stay in the sky for hours
If the air is not very humid, contrails disappear from the sky quickly, in mere minutes.
In the right humid atmosphere they can, however, stay in the air for hours and even form wispy Cirrus clouds.
Unlike what the abundant internet “chemtrails” conspiracy theories claim, these long-lasting contrails have nothing to do with chemicals being deliberately sprayed into the environment or to the ground.
4. Military aviation has studied contrails in order to avoid them
Contrails and how they form has been of special interest to military aviation, as many operations require the plane to remain as undetected as possible. For instance, surveillance aircraft may want to stay hidden and therefore avoid leaving a visible trail.
The first scientific studies on the formation of contrails were made during World War II in Germany, Great Britain and the United States.
5. The effect on climate remains controversial
The effect of contrails on the Earth’s climate remains somewhat unclear.
As contrails sometimes form clouds in the atmosphere, they can affect the Earth’s radiation balance. On the other hand, as contrails can have both a warming and a cooling effect, a scientific consensus on what the overall net effect is has yet to be reached.

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