The Rocky Mountain Front is known for its chinook winds, or snow-eaters as the Indians called them. When they occur, the temperature may rise from 30 degrees below zero to 30 degrees above within a couple of hour or less. There have been reports of a 26-degree within 45 seconds, and 43 degrees in 15 minutes.

The chinooks result from a steady, warm and moist westerly or southwesterly flow of air across Montana. This pattern also shows a high-pressure system to the south of the state over Nevada and Utah, and a low-pressure system to the north in Canada. As the moisture-laden winds begin to rise to cross the mountains of the Bob, they release their precipitation, usually in the form of heavy wet snow, on the uphill side. As they come rapidly down the mountain front, they become warmer and drier. At times their approach is heralded by a freight-train-like sound through the down-slope canyons.

The country east of the Chinese Wall also witnesses another extreme � that of severe cold, a result of arctic outbreaks. If the arctic air is deep enough, and comes on strong north winds, the cold air spills over to the valleys west of the Divide, putting the entire wilderness complex in a dome of very cold air. Temperatures may reach 40 to 60 degrees below zero. The record low temperature for the contiguous 48 states was set at the southern edge of the Bob Marshall country at Rogers Pass when the mercury dipped to 70 degrees below zero on January 20, 1954. Actually it was probably colder, the thermometers minimum mark was minus 70 and that reading was observed at 2:00 a.m. � the coldest temperature usually occurs just before sunrise.

-from page 75, "Weather"