The Great Plains are a land of rapidly changing and often extreme weather, where thunderstorms, blizzards, and other inclement conditions can arrive with little warning. In summer, the sun heats the ground and drives powerful convection that builds cumulonimbus clouds rising to 50,000 feet or more, producing baseball-size hail, tornadoes, and winds typically reaching 40 mph or greater. Wind is a constant presence across the region, with calm days rare and winds of 20 mph or more common. The northern Great Plains have a semiarid climate shaped by their inland position far from coastlines, bringing highly variable temperatures. Polar air masses can hold the area in weeks of subzero cold, and the record low was -47°F at Hettinger, North Dakota, on January 13, 1916. In summer, dry air can warm by 30° to 40°F in a single day, and the record high was 120°F at Usta, South Dakota, on July 15, 2006. The Grand River National Grassland averages 15-18 inches of precipitation each year, mostly in spring and early summer, but totals vary sharply, from wet years such as 1982, when 24 inches fell at Lemmon, South Dakota, and 27 inches at Hettinger, to the locally driest recorded year, 1936, when only 4.17 inches fell at Hettinger. A large share of annual precipitation may come in one storm or a few strong summer thunderstorms, and the highest single-day rainfall recorded was 5.04 inches at Lemmon on August 12, 1999. Drought is a recurring and powerful force on the Great Plains, with severe multiyear droughts occurring roughly every 20 years; the most severe on record was the Dust Bowl of the mid-1930s, when drought and high winds produced dust storms in 1934 unmatched in U.S. history.