Montana Earth Science Picture of the Week

Chinook Arch over the Rocky Mountain Front

Image courtesy of N.O.O.A.

Where the mountains meet the prairie . . .
This 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 (east).

Source: Jason Schaumann, Forecaster at the National Weather Service Forecast Office in Great Falls

Term: oscillate


*A more technical explanation of the Chinook Arch
Montana Earth Science Pictures
Photo of Chinook Arch over Calgary
Complete explanation of the Chinook Winds
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By Rod Benson
Earth Science Teacher at Helena High School

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