Chinook is a native Indian word used for foehn winds on the lee side of mountain ranges and particularly used in the northwest Pacific region. It is characterized by strong and unusually warm winds, often with a sudden and drastic temperature increase at onset. Another common feature is a Chinook arch which is a stationary band of clouds caused by a wave generated by orographic lifting of air passing the mountains. In front of the Chinook arch is a region of sinking air, giving a strip of clear sky which allows for outstanding sunrise/sunset displays under the arch. That's where Fairbanks sits when southern winds are forced across the Alaska Range. Before the Chinook set in, interior Alaska had been in a deep freeze for two weeks with temperatures averaging about -40F/C. The warming came gradually over a few days, topping at about +50F/+10C later this day.