GREAT FALLS — The Chinook Arch is a weather phenomena unique to our area due to our proximity to the Rocky Mountain Front. To understand how this phenomena works, it is important to understand mountain waves.
As warm, Pacific air flows in and reaches the mountains, they act as a roadblock and force the air up and over the mountain. Air cools and condenses as it rises creating clouds and precipitation on the windward side of the mountain. However, during its descent, the air dries out and warms up about 5.5 degrees Fahrenheit for every 1 thousand feet.
Air also begins to speed up during descent. It speeds up to the point that a vertical circulation, known as a rotor, forms on the leeward side of the mountain. This forces the flow of air to move up and over the rotor and once again forms clouds.
The atmosphere is a liquid, so you can envision the flow of air similar to that of a stream or river. A rock forces water to move over it and you often see ripples downstream from the location of the rock. The same thing happens when Pacific air reaches the Rockies. There is an extended stretch of rising motion and this creates what is known as the Chinook Arch cloud.
The cloud can be identified by its abrupt, western edge typically in the shape of an arch. The Chinook Arch can span hundreds of miles and most commonly precedes a strong, Chinook wind so it can act as a predictor of things to come.
