Downburst
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Image:Microburst (NSSL).jpg A downburst is a column of sinking air that is capable of producing damaging straight-line winds of over 150 mph (240 km/h), similar to, but distinguishable from tornadoes. Downburst damage will radiate from a central point as the descending column spreads out when impacting the surface, whereas tornado damage tends towards convergent damage consistent with rotating winds.
Downbursts are particularly strong downdrafts from thunderstorms. Downbursts in air that is precipitation free or contains virga are known as dry downbursts; those accompanied with precipitation are known as wet downbursts. Most downbursts are less than 2.5 miles (4 km) in extent, these are called microbursts. Downbursts larger than 2.5 miles (4 km) in extent are sometimes called macrobursts. Sometimes downbursts are larger, in the extreme case, the derecho can cover a huge area of more than 200 miles wide (320 km) and over 1000 miles (1600 km) long, lasting upto 12 hours or more, and are associated with some of the most intense straight-line winds, but the generative process is somewhat different than for most downbursts.
The formation of a downburst starts with hail or large raindrops falling through dryer air. Hailstones melt and raindrops evaporate -- this is an endothermic process that demands a lot of energy (in form of latent heat) so the air is cooled. Cooler air has a higher density than the warmer air around it, so it falls as a "cold air balloon" (compare to hot air balloon, which rises because hot air has a lower density than the surrounding air). As the cold air balloon hits the ground, it spreads out, and a mesoscale front can be observed as a gust front.
A special, and much rarer, kind of downburst is a heat burst, which results from precipitation-evaporated air compressionally heating as it descends from very high altitude. Heat bursts are chiefly a nocturnal occurrence, can produce winds of up to 100 mph (160 km/h), are characterized by exceptionally dry air, and can suddenly raise the surface temperature up to 120 degrees Fahrenheit (49 degrees Celsius), sometimes persisting for several hours.
Downbursts, particularly microbursts, are exceedingly dangerous to aircraft which are taking-off or landing. A number of fatal crashes have been attributed to downbursts.
References
- Fujita, T.T. (1981). "Tornadoes and Downbursts in the Context of Generalized Planetary Scales". Journal of the Atmospheric Sciences, 38 (8).
- Fujita, T.T. (1985). "The Downburst, microburst and macroburst". SMRP Research Paper 210, 122 pp.
- Wilson, James W. and Roger M. Wakimoto (2001). "The Discovery of the Downburst - TT Fujita's Contribution". Bulletin of the American Meteorological Society, 82 (1).