Distrail passing through iridescent cloud

Distrail passing through iridescent cloud · Photo: Claudia Hinz

At 6.35 A.M. on June 25, 2015, I noticed a plane passing through a clear part of the sky without leaving any trace (contrail) behind. Then I observed a beautifully irisating foehn cloud, when suddenly a distrail moved into the cloud dissipating it within two minutes.

Distrail is a short word for dissipation trail. It describes streaky cloud holes caused by airplanes. When a plane flies through or directly above a thin cloud layer, the wake vortices mix the dry air around the cloud into it and the cloud droplets evaporate. This effect is even strengthened by the hot exhausts of the plane, and a clear trail forms behind the plane. Often dust particles in the exhausts act as condensation nuclei making the cloud droplets freeze and form ice crystals. As the saturation vapour pressure above ice is lower than it is above water, the adjacent droplets evaporate. The result is then a white streak of ice clouds between two clear streaks.

Amateur pilots report that the dissipation of clouds also works at small airplanes without jet engines. In this case the propellers stir the air making the cloud dissipate.

Author: Claudia Hinz, Fichtelberg (1215m), Erz mountains, Saxony