condensation trail / contrail

A narrow, elongated cloud left in its wake by a jet aircraft, as a consequence of the expulsion into the air of gases at very high temperature, mainly water vapor. At the cruising altitude at which commercial aircraft usually fly - usually above 8 kilometers of altitude -, pressure and temperature are so low (on the order of 20 hPa and –40°C, respectively) that the expelled water vapor freezes almost immediately, undergoing sublimation. Aircraft also expel minuscule particles from combustion, which act as condensation nuclei and whose presence favors the formation of the ice crystals forming the trails. The length of time a trail remains in the atmosphere mainly depends on the temperature and humidity conditions in the flight zone. Up there, the air is fairly dry, so it is most common for trails to disappear quickly after the aircraft passes; however, when the moisture content of the air in the mid and upper levels of the troposphere increases, the trails remain marked in the sky, widening over time. Contrails - the term by which they are internationally known - were incorporated into the Atlas Internacional de Nubes of the WMO in its 2017 edition, being one of the most representative examples of a cloud of the species homogenitus.