Because Boulder is a dry, high-elevation locale next to the Great Plains, large temperature swings are common. Arctic cold fronts often slip down the Front Range before they move east. A cold front is the leading edge of a relatively cold air mass moving in to replace warmer air.

Behind such a front, the temperature may drop as much as 40 degrees F (22 degrees C) in a few hours. In the severe cold wave of February 1989, temperatures in Boulder stayed below 0 degrees F (-18 degrees C) for three full days, reaching a low of -24 degrees F (-31 degrees C).

As a cold front forces air upward, it usually becomes visible as a mass of low, ragged clouds or fog moving from the north or northeast toward Boulder. If the cold air mass is shallow, temperatures may be warmer in the mountains than they are in town.