Northern Prairie Wildlife Research Center
Fog is most apt to reduce visibility in February and March when it is present 8 to 9 percent of the total hours during the month. Fog is most common in these months because of warmer air overriding the snow cover which is usually present at that time of the year.
Dust, smoke and haze are usually present during only a few tenths of one percent of the hours in any particular month. In April, however, dust may limit visibility during nearly 3 percent of the hours. Dust occurrence is highest in April as the normally strong winds sweep the barren soils which have not yet been cultivated nor a crop cover established. The extremely low percentage of hours in which smoke or haze limits visibility attests to the fine quality of North Dakota air.
The number of days annually in which fog reduces visibility to one-fourth mile or less ranges from nine days at Devils Lake and Williston to 13 days at Fargo (Table 11). Fog reducing visibility to 1/4-mile or less is usually of short duration, and a fog which restricts visibility in the afternoon hours is also unusual.
Table 11. Average number of days by month and for year when fog reduces visibilities to 1/4-mile or less.
|1Number of years of record.
*Less than 1/2 day.