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Mount Everest's flag cloud

Q:  What is the plume often seen trailing from the downwind side of Mount Everest? Is it a cloud or is it blowing snow lifted from the upwind side of the mountain? If it is a cloud, exactly how does it form? And why is it only on the downwind side?  Walter, Barboursville, West Virginia, USA

Mount Everest with its flag cloud blowing east. Photo courtesy of René and Gail Hochreiter.

A: The plume in this picture taken by mountain climber René Hochreiter, is a cloud.  Moisture-laden air buoys up the south side of the mountain from the Nepalese lowlands, and condenses to form a cloud about the summit.  Actually, "Everest clouds form the same way as the more familiar clouds we see all the time," says meteorologist Craig Bohren, professor at Penn State University.  Prevailing winds from the west blow the cloud east like a billowing flag attached to the mountain.  

"When the wind reaches 80 km/h (50 mph), the flag cloud is at a right angle to the peak," says geographer John Ford Shroder, professor at the University of Nebraska, Omaha. Everest often protrudes into the high-speed, world-girdling jet stream, and, thereby, produces such flag clouds, Shroder emails.

Ascending air flow caused by mountains (called orographic lifting) creates the cloud.  "You may have heard the saying, 'Mountains make their own weather.'  This is orographic lifting stated in a more charming way," Bohren says. 

Blowing snow could also create the plume.

Further Reading:

Why clouds are puffy, WeatherQuesting

Everest:  South African 2006 expedition, South Col

Mount Everest, MSN Encarta

Clouds in a glass of beer by Craig Bohren

Himalaya to the sea by John Shroder

Geographic Information Science and Mountain Geomorphology by Michael Bishop and John Shroder

(Answered Feb. 12, 2007; updated Oct. 16, 2007)

Find this article at: http://weatherquesting.com/everest-flag-cloud.htm