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Gap Winds
Fri Jul 30, 2004
Listen in RealAudio 
When surface winds encounter large topographical features that block their progress,
they can push over the obstacle or find passage around it. When winds blow through
terrain gaps such as valleys, canyons and passes, we call them, appropriately, "gap
winds." Hi, I’m Bryan Yeaton for The Weather Notebook.
Gap winds develop when the pressure gradient lies across breaks in the terrain
ranging from yards to miles wide. The terrain steers the wind flow to more or less
follow the gap axis and thus increases its speed. Typically, the stronger the pressure
gradient and narrower the gap, the stronger the resulting wind speed.
For years, gap winds have been explained by the Venturi Effect, in which a fluid’s
speed increases as it flows through a tapered tube or nozzle, like the end of a garden
hose, peaking at the point of tightest constriction.
Meteorologists, however, now understand this is not the complete picture. Unlike the
nozzle, the terrain gap is not a fully enclosed tunnel but an open-topped channel. Thus,
the fluid build-up at the terrain gap not only pushes the air through but also forces
some to rise over it. As a result, the strongest winds generally do not blow at the
narrowest width of the gap but near its downwind "re-opening."
Gap winds typically blow over shallow depths several hundred to a few thousand feet
deep, and they can reach velocities of over 50 knots near the surface, particularly
hazardous to boaters along the coastlines or among islands, and to motorists winding
through rugged mountain terrain.
Urban dwellers can experience a special type of gap wind: the street canyon wind,
caused by wind squeezing through gaps between the tall buildings.
Thanks to our contributing writer, meteorologist Keith Heidorn. Our show is generously
supported by the National Science Foundation and Subaru of America.
Today's Links
Comet Module- Gap Winds:
http://meted.ucar.edu/mesoprim/gapwinds/frameset.htm
Navy Mesoscale Primer- Gap Winds:
http://www.atmos.washington.edu/~cliff/Navygap4.html
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