Fog can be prevalent in winter, both in mountain valleys and on the prairie. For ground fog to occur, the air just above it must be moist from either a snowmelt or rain. This time of year fog can occur in two ways. Advection fog, common to mountain valleys, results from warm, wet air flowing over cold ground. The cool surface causes the moist air temperature to drop and reach its dew or condensation point forming fog. Radiation fog happens on calm, clear nights when the air near the ground is chilled by radiation heat loss and the low level air is moist enough. Once again the dew point is reached and a cloud literally develops on the ground. As a rising sun warms the air, the mist dissipates. however, dense fog often takes days to go away.

UM Geography Department