Explanation:

 In the cold winter months of regions east of the Rocky Mountains, a strong, dry, warm wind sometimes blows from the mountains across the land. These winds, known as Chinook winds, can bring quick temperature changes.

Chinook winds develop when warm, moist air blows from the Pacific Ocean in the northwest region of North America toward the Rocky Mountain range.  The air mass cools as it climbs the mountains, bringing rain or snow to the peaks. The air mass, now dry after releasing its moisture in the mountains, warms as it moves down the eastern side of the mountains. The air mass warms rapidly, eventually becoming warmer and drier than the original air mass coming from over the Pacific Ocean. When the winds suddenly change direction toward the west or southwest, the Chinook winds begin with rapidly increasing speeds.