Dust devils, are small, brief whirlwinds occurring most frequently in the early afternoon when a land surface is heating rapidly. Dust devils are occasionally made visible by the lofting of dust, leaves, or other loose matter from the ground. Dust devils form when a pocket of hot air near the ground rises quickly through cooler air above it, forming an updraft. If conditions are just right, the updraft may begin to rotate. As the air rapidly rises, the column of hot air is stretched vertically, which causes intensification of the spinning effect. Most dust devils are usually small and weak, often less than 3 feet in diameter with strongest winds averaging about 45 miles per hour, and they often dissipate less than a minute after forming. On rare occasions, a dust devil can grow very large and intense, sometimes reaching a diameter of up to 300 feet with winds in excess of 60 mph and can last for upwards of 20 minutes before dissipating. Dust devils typically do not cause injuries, but rare, severe dust devils have caused damage they typically occur in dry, arid areas with the most common being Arizona, New Mexico, Eastern California. But on April 21, 1963, the combination of prolonged dry weather along the eastern seaboard and strong southerly winds transporting hot weather from the southern United states resulted in a dust devil in Reading, Pennsylvania. The giant dust devil extended up to half a mile high, tore bricks from the side of a school, uprooted trees and downed power lines.
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