The white trails that airplanes leave in the sky, also known as contrails or deflectory trails, are a complex phenomenon caused by a combination of factors. Firstly, clouds form when air condenses and its humidity reaches 100%, which can only occur at extremely low temperatures. Commercial airplanes fly at an altitude of around -56°C in the highest layer of the troposphere.
Secondly, the engines on airplanes generate thrust force by burning fuel and oxygen, producing a series of combustion gases and water vapor. When this hot water vapor is expelled from the plane, it condenses and forms the snowy trail that is visible in the sky.
The term “contrail” was coined by the Anglo-Saxons to describe the wake left by airplanes, which is short for “condensation trail.” The persistence and nature of contrails can be used to predict weather conditions.
However, not all airplanes leave a wake behind them. The efficiency of a turbojet is measured by its coefficient between work done by engine and chemical energy produced. Interestingly, polychrome grooves are formed when dyes are mixed and released at just the right time during air shows, but they are not true condensation trails.
Finally, there is another fascinating type of contrail that occurs when an airplane exceeds the speed of sound: a cloud that takes on the shape of a disk or cone. These Prandtl-Glauert condensation clouds are formed as a result of sudden drop in air pressure caused by high speeds.
In summary, contrails left behind by airplanes are formed due to condensation caused by extremely low temperatures at high altitudes combined with combustion gases from engines expelled from planes. Not all planes leave a wake behind them and their efficiency determines whether they do so or not. Additionally, colorful contrails are formed through mixing dyes while those left behind when planes exceed speed of sound take on unique shapes due to sudden drops in pressure caused by high speeds.