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, but they could also be called clouds. 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 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. Besides water vapor, the exhaust also contains carbon dioxide, oxides of sulfur and nitrogen, unburned fuel, metal particles and soot, which provides condensation sites for the water vapor.  

Since during flight at high altitude the temperature is about minus 40 degrees, the vapor condenses, turning into fog or small ice crystals.

Crystals evaporate more slowly than regular water. For this reason, the white plane trails remain in the sky for a very long time. At the same time, the higher the humidity of the climate and the colder it is outside, the longer, thicker and brighter the white stripes are.

The mechanism of contrails’ formation is essentially the same one which happens when a cold bottle is left in a hot room. If the temperature difference is high enough, plastic fogs up and water droplets form on the bottle.  The same thing happens if you walk through the frost with glasses and then enter a warm room.

Image by Liz West

Will the white trails appear in the air depends on the humidity. If a plane flies over a region with dry air, there are no small particles of water in the air and nothing to freeze outside. There will either be no condensation trails from the aircraft in such areas at all or they will be very pale and disappear quickly. Besides, white trails do not form after airplanes and helicopters that fly at low altitudes. The air temperature is not low enough and water particles simply do not have time to turn into crystals.

On the other hand, in some northern regions where the air temperature reaches minus 50 degrees and below, a condensation (or contrail) trail from an aircraft can form even during takeoff or landing.