Montana Earth Science Picture of the Week

Montana's Northwest Chinook Zone

Strange winter warmth . . .
In Great Falls on January 11, 1980 the temperature rose from -32 F to 15 F in 15 minutes. During the night of January 14-15, 1972 an official weather observer in Loma (50 miles NE of Great Falls) recorded a 103 degree change within 24 hours! The mercury went from -54 F to 49 F as calm arctic air was replaced by winds from the west blowing 30-40 mph (see hot link below for official report). In both cases the dramatic warming was due to a weather phenomenon known as a Chinook Wind.

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 . . .
In order for these warm winds to happen, three things are needed.

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.

2. Moist air from the Pacific is another key ingredient. This air contains an abundance of water vapor (humidity) that entered the air as water evaporated from the ocean. In order to make the change from liquid to vapor these water molecules had to absorb heat from their surroundings. So, especially moist air also contains lots of energy, referred to as "latent heat".

3. The final ingredient is mountains. Since the Rockies are somewhat narrower and higher in the area of Glacier Park and the Bob Marshall Wilderness, Chinook winds are especially frequent in the zone shown within the yellow line on the map. To see an elevation map of Montana that shows how the mountains west of the Chinook Zone stand above mountains to their west and the prairie to their east CLICK HERE

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.

To view a short video about latent heat, CLICK HERE

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.

To watch an 11-minute pencast that explains Chinook Winds, CLICK HERE.

Term: latent heat


*More about the Chinook Effect
Chinook Winds in Spearfish, South Dakota
NWS Report on Loma Event of 1972 (pdf file)
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By Rod Benson
Earth Science Teacher at Helena High School

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