The Rocky Mountains certainly aren't known for their mild winters. But in contrast to the upper Midwest, which seems to exist in a perpetually frozen state from November through March, the plains just east of the Rockies do get an occasional reprieve from the bitter cold: the Chinook winds. When these strong, warm winds blow, they can raise temperatures as much as 50 degrees Fahrenheit and melt away the snow -- a welcome respite for residents sick of sweeping slick sidewalks and wearing puffy parkas.