banner cloud (ger: Wolkenfahne; fr: nuage en banderole)
Stationary orographic cloud which forms downwind of an mountain crest or isolated peak and takes the shape of a banner or a flag streaming from the mountain peak. This type of cloud must not be confused with snow which is blown off a mountain summit and carried downwind.
Two mechanisms are possibly involved, the principle one being the presence of a lee eddy, which raises air behind the peak above the condensation level. A reduction in pressure behind the obstacle would also facilitate condensation, similar to the way condensation trails form at the tip of an aircraft wing.
The most famous banner cloud is that associated with the Matterhorn, Switzerland (4474m a.s.l.).
