Banner clouds

B. Geerts


Banner clouds are found in the lee of isolated mountains, especially sharp mountain peaks. It is as if the mountain carries a flag or banner (Fig 1).

Fig 1. A banner cloud associated with the Matterhorn in Switzerland.

Banner clouds are believed to form when air flows around a mountain (rather than over it). Vortices on both sides of the mountain are shed to the rear and produce upslope flow, opposite in direction to the prevailing wind, on the lee side of the mountain. This upslope flow is drawn to a non-hydrostatic pressure minimum just below the mountain crest (Fig 2). This low is a relative vacuum due to the flow reduction behind the obstacle (according to the Bernoulli theorem). The same flow pattern can also be explained through conservation of potential vorticity (1) (Note 12.K). The upslope flow may reach the lifting condensation level, causing cloudiness. The cloudy air in the lee of the mountain is continuously mixed with ambient air and shed to the rear, explaining the banner-like appearance.



Fig 2 (left). Schematic explanation for the formation of banner clouds.



(1) Geerts B., 1992: The Origin of Banner Clouds: A Potential Vorticity Perspective. Preprint Volume, Sixth Conference on Mountain Meteorology, AMS, Portland, Oregon, Sept. 29 -Oct 2, 1992, 97-98.