Airplanes leave behind white grooves in the sky, also known as contrails or defractory trails. These grooves are the result of a complex polynomial that involves several factors. The first factor is cloud formation. Clouds form when air condenses, which occurs when its humidity reaches 100 percent and the temperature is extremely low. Commercial airplanes fly in the highest layer of the troposphere, where the temperature is around -56°C.
The second factor is the engines. Airplanes use their engines to generate thrust force, which involves burning fuel and oxygen to produce combustion gases and water vapor. The water vapor is much hotter than the ambient air, so it condenses and creates the snowy groove that airplanes are famous for. The third factor is the expansion of gas when leaving the plane. Inside an engine, molecules are compressed, but they expand when they exit the plane, contributing to contrail formation.
Contrails are called “contrail” by Anglo-Saxons because they are a contraction of “condensation” and “trail.” The next question raised by this physical phenomenon is why not all airplanes leave a trail behind them. The efficiency of a turbojet depends on several factors, including its coefficient between work done by the engine and chemical energy produced. An interesting aspect related to contrails is that their nature and persistence can be used to predict weather conditions.
During air shows, you may see colored contrails called ‘polychrome grooves.’ These colors are achieved by mixing dyes and releasing them at specific times. Finally, there’s a striking type of contrail called Prandtl-Glauert condensation clouds that form when airplanes exceed the speed of sound and result in sudden drops in air pressure that create these unique cloud shapes.