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Table 9. Percent of possible sunshine by months and for year at four locations and average solar radiation (langleys) at Bismarck.

Sunshine is not only measured by the duration of actual sunshine, but also by an evaluation of the intensity of the sun's rays. This evaluation is called solar radiation and is an energy measurement which is expressed in langleys (a langley is one gram-calorie per square centimeter). Annually, North Dakota averages 350 to 370 langleys of solar radiation per day, which like the percentage of possible sunshine is higher than any state on the Canadian border with the exception of eastern Montana which receives about the same amount. As would be expected, the greatest amount of solar radiational energy is received in June and July and least in December. In June and July, more solar energy impinges on the earth's surface in North Dakota than in any state east of North Dakota. The high energy input is another important factor contributing to the successful agriculture carried on in North Dakota despite its relatively short growing season.

More days average out to be cloudy than clear or partly cloudy over the course of a year (Table 10). The most cloudy days occur from late fall to early spring. The number of clear days is about the same for all months from November through June. In July the number of clear days sharply increases and remains at about the same level through August, before becoming somewhat fewer in September and October. As a generality, it can be said that July and August will record about twice as many clear days as during any other months of the year, while the number of cloudy days in July and August will only be half as many as for other months of the year.

Table 10. Average number of clear days (C), partly cloudy days (PC), and cloudy days (CY) between sunrise and sunset at four locations. Sky cover is expressed in a range of 0 for no clouds to 10 for complete sky cover. A clear day is based on a daytime average cloudiness of 0 to 3, partly cloudy day from 4 to 7, and cloudy days from 8 to 10.