The white grooves left by airplanes in the sky, also known as contrails or defractory trails, are the result of a complex polynomial. On the one hand, clouds form when a mass of air condenses, that is, when its humidity reaches one hundred percent, and for this to occur the temperature has to be extremely low. Commercial airplanes fly in the highest layer of the troposphere, where the temperature is around -56ºC.
The second aspect to take into account is the engines. We know that airplanes use them to generate a thrust force and that in the process fuel and oxygen are burned, generating a series of combustion gases (carbon dioxide, sulfur and nitrogen oxides, metal particles and soot) and water vapor. The water vapor is much hotter than the ambient air, so it condenses and creates the snowy groove that airplanes are used to. In some ways this phenomenon resembles the small cloud that is formed when we exhale air on a very cold morning.
The last component of the polynomial would be the expansion of the gas when leaving the plane, given that inside the engine the molecules are much more compressed. And to all this we should also add that in order to see them we need the wind to be uniform and not too intense and that there be no clouds in the lower layers.
The Anglo-Saxons call the wake of airplanes with the term “contrail”, which is a contraction of condensation (condensation) and trail (wake). The next question raised by this physical phenomenon is why not all airplanes leave a wake. Surely on more than one occasion we have seen two planes that supposedly fly at the same altitude but only one of them generates condensation trails. This is basically due to the efficiency of its motors.
The efficiency of a turbojet is measured by the coefficient between the work done by the engine and the chemical energy it produces. The more efficient the engine, the lower the altitude it will start to generate the wake.
At the moment, aviation is using the most efficient engines that exist, so airplanes draw contrails at increasingly lower altitudes, but since airplanes from different generations still coexist, it is possible to observe how some leave contrails and others do not.
An interesting aspect in relation to contrails is that their nature and persistence can be used to predict weather conditions. For example, a groove that is thin and of short duration translates the existence of air with low humidity and high altitude, which predicts good weather; while a thick, long-lasting track is a sign of moist air at high altitudes, which can be an early indicator of storms. If, on the other hand, what the plane leaves is a trail that gets larger, it may indicate the proximity of rain.
Sometimes, during air shows, we can see that the contrails are colored. It must be kept in mind that these types of flights are made at low altitude, so that they can be enjoyed, and usually during the summer months, when the weather is good and temperatures are high. These ‘polychrome grooves’ are achieved by mixing dyes and releasing them at the ideal moment, therefore, they are not true condensation trails.
Finally, there is a very striking type of contrail, the ones left by airplanes when they exceed the speed of sound: a cloud that takes on the shape of a disk or cone. They are called Prandtl-Glauert condensation clouds and are formed as a result of a sudden drop in air pressure.