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)
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