A preliminary examination of trends in the annual frequency of dense fog in the central Midwestern United States has been made employing hourly surface observations for the period 1948-1996. During this period, fog has been found to decrease at many sites in the region around the Great Lakes and to increase in a spatially contiguous region of Iowa and Missouri. Further during this period, factors related to changes in cooling rates (cloud cover, snow extent, minimum temperature), moisture availability (humidity, dew point temperature), and possibly to changes in fog nuclei concentrations (visibility, SO2 emissions) have been observed to change. Whether changes in some or all of these factors can be associated with changes in fog frequency is under investigation.

The temporal trends in the frequency of dense fogs will be presented, as well as, some of the meteorological conditions related to fog, to assess the validity of the changes in fog frequency. Trends in relative humidity, minimum temperature, and cloud cover will be emphasized. Shorter duration fogs are often found in the warm season (April-September) characterized by longer nights that are important for radiational cooling. During the cold season (October-March) long-lasting fogs have been found to be more common, probably associated with the advection of warm moist air from the passage of frontal systems. Thus, the data will be examined on an annual basis and by warm and cold season.