Chinook Arch over the Rocky Mountain Front
Image courtesy of
Where the mountains meet the prairie . . .
G.O.E.S. West satellite image shows a classic Chinook arch that
formed over the Rocky Mountain Front area of Montana on
January 5, 2006. It is called an "arch" because an observer
standing on the prairie, looking west (toward the mountains),
sees a curved patch of clear sky between the band of clouds and
the mountains below. The "arch" is the edge of the clouds just east of
the long clear area that extends from southwestern Montana
into southern Alberta. The edge of the clouds appears to
converge with the mountains to the north and south due to the
curvature of the Earth. The photo below shows a Chinook arch
as seen looking west from the prairie toward the mountains in Alberta (Canada). CLICK HERE to see an infrared image of an arch over Alberta.
Photo of a Chinook
Arch By Donovan Watson
A Mountain Wave . . .
The clear area between the arch and the mountains exists
because air in this area is down sloping. As air flows over the
Rockies it develops an up and down motion similar to the
motion of water as it flows up and down the surfaces of rocks in
the rapids of a river. Although the air flows downward once it
gets over the mountains, it can continue to oscillate up and
down as it flows away from the mountains (like a boat on a lake
as waves pass underneath) for several hundred miles. The
upward flowing part of this "mountain wave" is what forms the
long arch of clouds.
Here's how it works . . .
As the air flows down along the east slopes of the Rockies it is
warmed by compression. This is part of the reason why the
Chinook winds are warm (see Hot Link for complete
explanation). Then, as the wave action continues and the air
begins to rise again, the air is cooled by expansion. If there is
enough vapor in the air, the arch of clouds will form as vapor
condenses to form cloud droplets (or cloud crystals). Typically,
the long area of clouds will form near the crest (top) of the first
wave and then get blown eastward by higher level winds. If the
mountain wave continues, and another downward turn is taken,
the arch (cloud) will evaporate farther downstream
Source: Jason Schaumann, Forecaster at the
National Weather Service Forecast Office in Great Falls