It’s the water vapour. They create the white trails left by the aeroplanes we see in the sky. However, when and why aircrafts leave vapour trails is fascinating.

When an aeroplane is in the air, the engines produce and release a sizable amount of water in the form of water vapour out the back. In the cool, high-altitude air, this hot, saturated water vapour from the plane's engine(s) condenses on other exhausting particles and freezes around them. The frigid temperatures at a high altitude causes water vapour to freeze as it condenses in the atmosphere behind the aircraft, leaving lengthy, white clouds in the sky.

Depending on the aircraft and, the air conditions the plane is flying through, some flights will create vapour trails that are different from another planes. The duration of the contrails (condensation trails) until they disappear can be as short as a few minutes, depending on the humidity and temperature of the surrounding air. But not all aircraft produce vapour trails since it depends on the surrounding atmosphere. The air must be sufficiently chilly and humid for vapour trails to be left in the aircraft's wake. Insufficient moisture in the air prevents condensation from forming on engine exhaust particles. Additionally, the water vapour particles won't freeze and form the “clouds” if the air is not cold enough (i.e., the plane is not flying at a high enough altitude).

The main reason that all planes don't produce vapour trails, aside from the differing engine types, is the state of the air. Mostly, a personal single-engine aircraft won't produce vapour trails since it isn't high enough in the atmosphere for the air to be sufficiently cold and humid to create the tiny particles of frozen water vapour.

Source: Joe Haygood, skytough.com

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