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Why planes leave white trails in the air

contrails

Lufthansa Boeing 747 overflying forming engine exhaust contrails behind in high altitude. The iconic double-decker Jumbo Jet is flying at 34.000 feet over the Netherlands as it is flying a trans-Atlantic route from Frankfurt FRA airport in Germany to Washington IAD, USA the flight LH418 / DLH418. The aircraft is a Boeing 747-830, B748 with registration D-ABYS and speed 380 kts (knots). Lufthansa is a Star Alliance member. (Photo by Nicolas Economou/NurPhoto via Getty Images)

On a sunny day, when the sky is blue and clear, you can usually spot white trails left in the air by passing planes. But why do some planes leave white trails and others do not?
White trails from airplanes are often called condensation trails or contrails.
The main reason behind their appearance is the temperature difference between hot humid air around a plane’s engine and low temperatures outside the aircraft.
Whether or not condensation trails will form mainly depends on the height and composition of the surrounding atmosphere.
The atmosphere at high altitude is of much lower vapor pressure and temperature than the exhaust gas from a plane’s working engine.
The interaction between these two gives rise to contrails.
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