WRAL WeatherCenter Blog

Do you know what type of clouds these are? http://youtube.com/watch?v=MSepRaFfK6o

Posted September 16, 2007

MIKE MOSS SAYS:        Tim,      Those are nice examples of lenticular clouds, which in the past have occasionally been reported as UFOs or other strange phenomena. They are quite common in some locations and are generally a form of orographic cloud, meaning the formation and maintenance of the cloud is influenced or modulated by the interaction of the atmosphere with underlying terrain. In brief, these clouds occur when a stable layer of air flows across a significant topographic feature such as a mountain or ridgeline, so that the air is forced upward and then allowed to sink back to its original level as it continues downwind of the terrain feature.

If the air is quite dry, then no one notices that it is flowing over a aterrain obstacle because it remain free of any cloud cover. On the other hand, if the air contains some layers of more humid air, those layers may become cooled sufficiently by the upward motion forced by the topography to become saturated and form cloud droplets on the way up - then, as the air descends farther downwind, the reverse occurs and the cloud droplets evaporate. In this way the air is moving continuously, but the cloud appears stationary due to constant formation of new cloud droplets on the upwind edge of the cloud and evaporation of droplets at the downwind edge.

Depending on how water vapor is distributed vertically through the airmass passing across a mountain or ridge system, and on the stability of the airmass, this process can lead to some stunningly beautiful, almost otherworldly,  cloud formations. In some cases, the air that is lifted above a ridgeline or mountaintop is set into a standing wave pattern in which it oscillates above and below its level of equilibrium, so that there can be several bands of orographic cloud located along the peak of the upward displacement, separated by clear bands along the base of the downward displacement, for many miles downwind of the terrain feature that got it all started.

For a good variety of photos of these cloud types (and a few other interesting cloud-related images) see:

http://pic1.piczo.com/valuca/?g=25544746

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