Wakes of War: Contrails and the Rise of Air Power, 1918-1945 Part Ii--the Air War over Europe, 1939-1945‪.‬

Behind the engine-carrying body (fuselage or nacelle) a turbulent region or wake is formed as the airplane flies. The exhaust moisture and some of the engine heat are discharged into this wake and become diffused throughout the wake as a result of the mixing action of the turbulence. The moisture and heat do not, however, mix with the air outside the wake because there the air is "smooth.'... It is easy to see that, if the air is so cold that it cannot hold much water as vapor, the water in the exhaust may be sufficient, when added to the moisture already in the atmosphere, to raise the humidity in the turbulent wake to or beyond the saturation value. If this condition exists, some of the water vapor will condense and a visible trail will form.