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The trails in the sky left by planes are called contrails, which is short for condensation trails. Photo: Shutterstock
The trails in the sky left by planes are called contrails, which is short for condensation trails. Photo: Shutterstock

Aviation

Ask the pilot: Why do planes leave trails in the sky?

They´re called contrails which is short for condensation trails and there are mainly two ways for these trails to be created – either from the engines or the wings.

The contrails formed by engines occur at high altitudes when the small amount of water vapor from the engine’s exhaust combines with the low pressure and low temperatures outside. The temperature at high altitudes can be as low as -40C.

Water vapor is one of the main products of airline fuel combustion. When it leaves the engine, it’s very hot, but it freezes instantly when it hits the extremely cold air outside, creating millions of small ice crystals and forming a trail behind the airplane. These contrails are usually created at altitudes higher than 26,000 feet and can sometimes persist for several hours before dispersing into the clouds.

On humid days, contrails can also be formed at lower altitudes. This happens when the wings generate lift and cause a vortex to form at each wingtip or wing flap. The vortices remain in the atmosphere after the plane has passed and the reduction in pressure and ­temperature across the vortices can cause water to condense or freeze and therefore result in the visible trails.

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