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The following data on climate and weather have been summarized from Visher (1966), Black (1971) and Jensen (n.d.).

Temperature

Temperatures form roughly south-to-north gradients in the Prairie Pothole Region of the Dakotas. Normal annual temperature ranges from about 3°C in northern North Dakota to about 8°C in southern South Dakota. In January, the coldest month, temperatures average -19°C in northeastern North Dakota and -8°C in southeastern South Dakota. During July, the warmest month, the range is 16°C to 23°C for the two areas, respectively. Rapid plant growth begins when mean daily temperatures rise to 6.1°C. The date for this event varies from about 26 April in northeastern North Dakota to about 3 April in southeastern South Dakota, a difference of over three weeks. Conversely, vegetative growth largely ceases when mean daily temperatures fall to 1.7°C. This event occurs about 20 October in northern North Dakota and about 10 November in southeastern South Dakota. The normal length of the frost-free season varies from about 110 days in northern North Dakota to about 150 days in southeastern South Dakota. The frost-free season may be slightly shorter in basin wetlands, as cold air flows downhill and can accumulate in topographic depressions. Crops planted in and around wetlands often are the first to be injured by freezing temperatures, giving rise to what some farmers call "frost pockets."

The highest summer temperatures officially recorded in North and South Dakota to 1945 were 49.4°C and 48.9°C, respectively, whereas the lowest official winter temperatures prior to 1952 (the last year for which summarized data are available) were -51.1°C and -50°C, respectively. Soils usually freeze to depths of 0.9-1.8 m in northern North Dakota and 0.5-0.9 m in southeastern South Dakota.

Winds and Storms

Surface winds average 21 km/hr in northeastern North Dakota and 17.7 km/hr in southeastern South Dakota. Meteorologists plot winds as net excesses of wind in a given direction during various time periods. For the region, there is a relatively large excess of wind from the northwest in January, and a relatively small excess of wind from the northeast in July. Many low-pressure cells track from west-northwest to east-southeast across the region during the year. The number of spring and summer thunderstorms ranges from 25 per year in northern North Dakota to 35 per year in southeastern South Dakota. This combination of temperature and wind differences cause the normal annual evaporation from pans to vary from 1.02 m in northeastern North Dakota to 1.65 m in southeastern South Dakota.

Precipitation

The relatively small amount of precipitation in the Canadian prairies has been attributed by Hare and Hay (1974) to the weakness of atmospheric disturbances and their associated uplift. Air masses move eastward from the Rocky Mountains and fall steadily toward lower elevations in the northern prairies. The rate of fall is sufficient to reduce cyclonic action appreciably, thus reducing the effectiveness of the mechanism that causes precipitation. This phenomenon also reduces precipitation in the Prairie Pothole Region of the Dakotas. The southern part of the region has more precipitation because it receives more moisture-laden airmasses from Gulf of Mexico.

Annual precipitation in the region ranges from 33 cm in northwestern North Dakota to 58 cm in southeastern South Dakota. Larger amounts of spring and summer precipitation in the southeastern part of the region account for most of this difference. About 70% of the annual precipitation falls as rain during spring and summer, with June the wettest month. Distinctly dry years, having <75% of normal precipitation, occur with 10% frequency in northwestern North Dakota, but only 4% in southeastern South Dakota. There is a large gradient across the Prairie Pothole Region of the Dakotas in the length of the relatively dry season, that is, when weekly normals of <1.27 cm of precipitation can be expected. In northwestern North Dakota, this season averages 8-10 months, whereas in southeastern South Dakota, this season lasts only 2-5 months.

Snowfall in the region averages <1 m/yr, much less than areas directly to the east or west. In the dry northwestern corner of the region, 25% of normal annual precipitation falls as snow, whereas in southeastern South Dakota only 12% falls in this form. Appreciable amounts of snow fall on >60 days/yr in northeastern North Dakota, whereas significant amounts fall on only about 28 days/yr in southeastern South Dakota. Dry winters, when <7.6 cm of precipitation falls, occur one year in five.

Blizzards, as defined by the National Weather Service, are storms with wind speeds >51.5 km/hr, temperatures <-6.67°C, and visibility not >152 m. Reduced visibility is caused by snow, blowing snow, or mixtures of snow and soil. North and South Dakota have a far greater frequency of blizzards than any of the adjacent states. North Dakota has 2.2 blizzards per year, with 54% of them originating as Alberta lows. South Dakota has 2.1 blizzards per year, with 52% of them originating as Colorado lows. Snow from February and March blizzards and snowstorms is often an important source of spring runoff for prairie wetlands.

Hail falls in moderate amounts compared to the rest of the nation, on an average of 2-4 days/yr. Brief heavy rains occur more frequently than in the arid west, but are only rare or occasional compared to the southeastern United States and the gulf coast. Normal annual water loss by runoff and evaporation is 0.36 m in northwestern North Dakota and 0.56 m in southeastern South Dakota; the rest enters the ground.