Montana Earth Science Picture of the Week

Precipitation in Montana

Precipitation Map Courtesy of Oregon Climate Service

A Correlation . . .
This map shows average annual precipitation amounts for Montana from 1961-1990. As you can see, precipitation amounts vary greatly throughout the state. If you were to compare the map shown above with one that shows the locations of mountains, you would see a strong correlation between mountains and areas of heavier precipitation. As air moves across Montana, mountains make the air rise. As it rises, the air cools by expansion. If there is enough humidity (vapor) in the air, this cooling will cause cloud formation and precipitation. When mountains provide the lift needed to cause precipitation, it is referred to as "orographic" precipitation. On the other hand, as air moves down into valleys, it becomes warmer by compression. As a result, valleys like the Helena Valley may get less than 15 inches of precipitation per year, whereas the surrounding mountains receive much more.

Mountains make air rise. . .
The prevailing winds influence which side of the mountains will have the most precipitation. In Montana, winds usually blow from west to east (the "westerlies"). For this reason, the western slopes of the mountains where air is rising can expect more precipitation than the the eastern slopes where air is sinking. A dramatic example of this can be seen in the Glacier Park area. There the westerlies encounter the high mountains of the park, causing tremendous amounts of precipitation to fall in places like Logan Pass (over 80 inches of precip.). Then as the air continues eastward it descends into the Browning area where precipitation is much less likely (less than 20 inches of precipitation).

The image below shows the shape of the land. Compare it to the precipitation map above. Do you see a correlation?

Term: correlation


*How mountains affect precipitation
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By Rod Benson
Earth Science Teacher at Helena High School