• Fri. Dec 1st, 2023

Uncovering the Mystery of Airplane’s White Exhaust: What Causes It?


Nov 20, 2023

The white lines left behind by airplanes in the sky, commonly referred to as contrails or defractory trails, are the result of a complex interplay of factors. On one hand, clouds form when air condenses, which happens when its humidity reaches 100%, and this can only occur at extremely low temperatures. Commercial airplanes fly in the highest layer of the troposphere, where temperatures are around -56°C.

The engines used by airplanes generate thrust by burning fuel and oxygen, producing a series of combustion gases and water vapor. The hot water vapor condenses and creates the snowy trail that is visible in the sky. The last factor contributing to the formation of contrails is the expansion of gas when it leaves the plane, as the molecules inside the engine are much more compressed than those outside.

The Anglo-Saxons call these wakes “contrails,” which is a shortened version of “condensation trail.” One question that arises from this physical phenomenon is why not all airplanes leave a wake. The efficiency of a turbojet engine is measured by its ability to convert chemical energy into work output. An interesting aspect about contrails is that their nature and persistence can be used to predict weather conditions.

During air shows, we may see contrails that are colored. These “polychrome grooves” are achieved by mixing dyes and releasing them at just the right moment, so they are not true condensation trails. There is also a striking type of contrail that is left behind by airplanes when they exceed the speed of sound: a cloud that takes on the shape of a disk or cone. These clouds are called Prandtl-Glauert condensation clouds and are formed as a result of a sudden drop in air pressure.

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