A chinook is a warm westerly wind that sometimes flows over Canada's Rocky Mountains in the winter. It can raise temperatures in southern Alberta by twenty or thirty degrees in an hour. A chinook is caused when air flowing off the Pacific Ocean is raised by the mountains and heated as the water vapor in the air condenses. Then it is warmed more as it flows down the other side of the mountains and is compressed. It flows out over the prairie, and may travel as far as the eastern border of Alberta if it is especially strong. The chinook is also called "snow eater", because it can melt snow so quickly.