Montana Earth Science Picture of the Week

At Browning on Feb. 10, 1988 the temperature rose from 13 F to 46 F between 8 am and noon. The National Weather Service has officially recorded a temperature rise of 43 degrees in 15 minutes in Havre. In both cases the sudden warming was due to a weather phenomenon known as a Chinook wind. Although warm winter winds happen in many parts of the world, the area of Montana outlined by the yellow line is one place where “Chinooks” commonly form.

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 on the map above.

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.

Term: latent heat