Strange winter warmth . . .
These unusually warm, winter winds happen at mid-latitude locations throughout the world and they are known by other names. In the west and northwest USA they are called Chinooks, and they can happen at any location that has high mountains to the west, such as the Helena Valley (15 miles east of the Continental Divide). They are most common along the eastern slopes of the Rockies from northern New Mexico all the way up through Alberta (Canada). The area of Montana outlined by the yellow line (see map) is one place where Chinooks are especially common, and sometimes extreme.
Recipe for a Chinook . . .
1. For one, a strong westerly (or southwesterly) flow of air is needed. West to east is the prevailing wind direction in Montana. However, if there is going to be a dramatic Chinook effect, the winds need to be especially strong.Cloud formation releases the latent heat . . .
As the moist Pacific air blows into the mountains of Glacier Park, the mountains force the air to rise, causing the air to cool by expansion. As a result the vapor changes into ice crystals that grow and then fall as snow on the west slopes of the mountains. The key to the Chinook effect is that when the molecules of water vapor freeze, they release heat to their surroundings. This is the same latent heat that was absorbed as the molecules evaporated from the ocean. It is this heat that is released as clouds form on the west slopes that makes the Chinook winds so warm. As this air flows down the east side of the park into Browning, it will also be warmed by compression . . . But this warming by compression happens whether or not there is a Chinook wind.
Below: The top diagram illustrates a situation in which air that is completely dry moves into Montana. Although air without even a trace of vapor is not realistic, the diagram shows the temperature changes associated with rising (expanding) and sinking (compressing) air. The bottom diagram shows the role that moisture-laden air plays in the development of Chinooks. With the heat released by water molecules as they become ice crystals, the temperature of the air as it reaches the peak is not nearly as cold. Then the air is compressed as it flows down slope toward Browning, reaching Chinook-like temperatures. NOTE: The diagram is highly simplified. The mountains between Washington and Montana are much more complex and the elevation of Browning is actually quite a bit higher than that of locations in Central Washington.
Term: latent heat
|*More about the Chinook Effect|
|Chinook Winds in Spearfish, South Dakota|
|NWS Report on Loma Event of 1972 (pdf file)|
Earth Science Teacher at Helena High School