The state as a whole has hot summers and cold winters. The Black Hills are generally cooler and wetter than the surrounding plains area.
Average January temperatures are everywhere less than -4° C (24° F) and decrease to less than -12° C (10° F) in some northern sections. The January temperatures in Sioux Falls range from -16° to -4° C (3° to 24° F). Nightime lows of -29° C (-20° F) occur during most winters. July averages are in the low and middle 20°s C (low and middle 70°s F) throughout most of the state, and are somewhat lower in the Black Hills. The average temperature range in Sioux Falls in July is 17° to 30° C (62° to 86° F).
Average annual precipitation (rainfall and snowfall) increases from roughly west to east, ranging from about 330 mm (about 13 in) in the northwest to about 630 mm (about 25 in) in the southeast. The Black Hills receive an average of between 360 and 610 mm (14 and 24 in) of precipitation a year. However, precipitation varies considerably from year to year, and prolonged droughts can occur, especially in the western and central areas. Fortunately, three-fourths of all precipitation falls as rain during the crop-growing season. The growing season, or period between the last killing frost in spring and the first killing frost in fall, increases from under 120 days in the northwest to 150 days in the southeast.
In the valleys of the Black Hills the growing season totals as little as 101 days. The last killing frost occurs in early May in the south, about three weeks later in the north, and often as late as June in the Black Hills. The first killing frost in fall occurs in August or early September in the Black Hills and at the end of September or in early October elsewhere. "South Dakota" © Emmanuel BUCHOT, Encarta, Wikipedia
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