MICROBURST

pattern of intense winds that descends from rain clouds, hits the ground, and fans out horizontally. Microbursts are short-lived, usually lasting from about 5 to 15 minutes, and they are relatively contained, usually affecting an area of 0.6 to 1.9 miles (1 to 3 km) in diameter. They are often but not always associated with thunderstorms or strong rains. By causing a sudden divergence in wind direction or speeda condition known as wind shearmicrobursts create a particular hazard for airplanes at takeoff and landing because the pilot is confronted with a rapid and unexpected shift from headwind to tailwind. In arid places the rain that is often associated with microbursts has evaporated before the downdraft reaches the ground; dry microbursts thus afford virtually no visible clue to their presence. Wet microbursts, typical of more humid areas, generally produce a visible rain shaft. The mechanics of the microburst phenomenon are not yet completely understood. Their existence was first observed in 1974 by Tetsuya Theodore Fujita, a Japanese-American meteorologist working at the University of Chicago.