The contrails, or white grooves left by airplanes in the sky, are a result of a complex equation. Firstly, clouds form when air condenses, which occurs when its humidity reaches 100 percent and the temperature is extremely low. Commercial airplanes operate in the highest layer of the troposphere, where temperatures are around -56°C.
The second factor to consider is the engines. Airplanes use them to generate thrust and burn fuel and oxygen to create combustion gases and water vapor. The water vapor is much hotter than the surrounding air, so it condenses and creates the snowy trail that planes leave behind. The final component of this equation is the expansion of gas as it leaves the plane; inside engines, molecules are compressed but expand rapidly when released.
Contrails are called “contrail” by Anglo-Saxons because they combine “condensation” and “trail”. One question raised by this physical phenomenon is why not all airplanes leave a trail. The efficiency of a turbojet is determined by the ratio between work done by engine and chemical energy produced. Interestingly, weather conditions can be predicted from studying contrails’ nature and persistence.
During air shows, we may see colored contrails known as “polychrome grooves.” These are achieved by mixing dyes and releasing them at just the right moment; therefore, they are not true condensation trails. Lastly, there’s an impressive type of contrail: those left behind by planes flying faster than sound – a disk or cone-shaped cloud called Prandtl-Glauert condensation clouds – which form due to a sudden drop in air pressure when planes exceed Mach speed (the speed of sound).