During a helicopter operation at Mt. Wendelstein on September 9, 2011 in very moist air, Harald Jauernig observed that the rotor blades of the helicopter caused condensation of the moisture.
This phenomenon is called wing tip trails and is caused by the difference of pressure between the lower and the upper part of plane wings. The air passing above the wing moves faster and so has less pressure than the air passing below the wing, causing an updraft which lifts the plane. This is what makes airplanes fly.
When pressure drops, the air cools down, so that moisture can condensate and form droplets or even ice crystals when it is cold enough. These become visible as a thin stripe of cloud extending backwards from the wing tips (wing tip trails).
In this case the wing tip trails are forming circles because the rotor blades of the helicopter rotate.
Authors: Claudia Hinz and Peter Krämer