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Climate of North Dakota

Obstructions to Visibility

Whenever the visibility is less than seven miles and precipitation is not occurring, an obstruction to vision must be entered in the standard weather observations taken by National Weather Service personnel and others who make such observations. In Figure 51, the percentage of hours in the month is graphed when visibility is limited to less than seven miles by fog, smoke or haze, blowing snow, and dust. Blowing snow is snow lifted from the surface of the earth by the wind to a height of six feet or more and blown about in such quantities that horizontal visibility is restricted at that level. Drifting snow is distinguished from blowing snow because it does not rise into the air in such a quantity that visibility is restricted at the six-foot height. Blowing snow restricts visibility to the greatest extent in January when it is recorded in nearly 17 percent of the weather observations . In the other winter months, blowing snow restricts visibility less than 5 percent of the time except in February when it reduces visibility about 7 percent of the time. The high frequency of hours that blowing snow limits visibility in January is characteristic of the extreme eastern portion of the state in the flat Red River Valley, but is not characteristic of other portions of the state. Elsewhere, the frequency of blowing snow in January is generally less than 10 percent.

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.

Station	P¹	J	F	M	A	M	J	J	A	S	O	N	D	Yr.
Bismarck	30	1	2	2	1	*	1	1	1	1	1	1	1	12
Devils Lake	57	1	1	1	*	*	1	*	1	1	1	1	1	9
Fargo	28	1	2	2	1	*	*	1	1	1	1	1	2	13
Williston	5	*	2	1	1	*	1	*	*	*	1	1	1	9
¹Number of years of record. *Less than 1/2 day.

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