On a sunny day, when the sky is blue and clear, you can usually spot white trails left in the air by passing planes. But why do some planes leave white trails and others do not?
White trails from airplanes are often called condensation trails or contrails.
The main reason behind their appearance is the temperature difference between hot humid air around a plane’s engine and low temperatures outside the aircraft.
Whether or not condensation trails will form mainly depends on the height and composition of the surrounding atmosphere.
The atmosphere at high altitude is of much lower vapor pressure and temperature than the exhaust gas from a plane’s working engine.
The interaction between these two gives rise to contrails.