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A downburst is a strong ground-level wind system that emanates from a point source above and blows radially, that is, in a straight line in all directions, from the point of contact at ground level. Often producing damaging winds, it may be confused with a tornado, but is different in that a tornado, high velocity winds circle a central area, moves inward and upward whereas in a downburst, winds are directed downward and then outward from the surface landing point. Downbursts are created by an area of significantly rain-cooled air that, after reaching ground level, spreads out in all directions producing strong winds. Dry downbursts are associated with thunderstorms with very little rain, while wet downbursts are created by thunderstorms with high amounts of rainfall. Microbursts and macrobursts are downbursts at very small and larger scales respectively. Another variety, the heat burst, is created by vertical currents on the backside of old outflow boundaries and squall lines where rainfall is lacking. Heat bursts generate significantly higher temperatures due to the lack of rain-cooled air in their formation. Downbursts create vertical wind shear or microbursts, which is dangerous to aviation. ==Definition== A downburst is created by a column of sinking air that, after hitting ground level, spreads out in all directions and is capable of producing damaging straight-line winds of over , often producing damage similar to, but distinguishable from, that caused by tornadoes. This is because the physical properties of a downburst are completely different from those of a tornado. Downburst damage will radiate from a central point as the descending column spreads out when having an impact on the surface, whereas tornado damage tends towards convergent damage consistent with rotating winds. To differentiate between tornado damage and damage from a downburst, the term straight-line winds is applied to damage from microbursts. Downbursts are particularly strong downdrafts from thunderstorms. Downbursts in air that is precipitation free or contains virga are known as dry downbursts;〔Fernando Caracena, Ronald L. Holle, and Charles A. Doswell III. (Microbursts: A Handbook for Visual Identification. ) Retrieved on 9 July 2008.〕 those accompanied with precipitation are known as wet downbursts. Most downbursts are less than in extent: these are called microbursts.〔Glossary of Meteorology. (Macroburst. ) Retrieved on 30 July 2008.〕 Downbursts larger than in extent are sometimes called macrobursts.〔 Downbursts can occur over large areas. In the extreme case, a derecho can cover a huge area more than wide and over long, lasting up to 12 hours or more, and is associated with some of the most intense straight-line winds,〔 Peter S. Parke and Norvan J. Larson.(Boundary Waters Windstorm. ) Retrieved on 30 July 2008.〕 but the generative process is somewhat different from that of most downbursts. ===Straight-line winds=== Straight-line winds (also known as thundergusts and hurricanes of the prairie) are very strong winds that can produce damage, demonstrating a lack of a rotational damage pattern.〔Glossary of Meteorology. (Straight-line wind. ) Retrieved on 1 August 2008.〕 Such rotational damage patterns are associated with cyclonic storms including tornadoes and tropical cyclones. Straight-line winds are common with the gust front of a thunderstorm or originate with a downburst from a thunderstorm. These events can cause considerable damage, even in the absence of a tornado. The winds can reach and can last for periods of twenty minutes. Such straight-line wind events are most common during the spring when instability is highest and weather fronts routinely cross the country. Straight-line wind events in the form of derechos can take place in areas outside of the traditional tornado alley (such as in the northeastern United States/Great Lakes Region and across southern Canada). Straight-line winds may be damaging to marine interests. Small ships, cutters and sailboats are at risk from this meteorological phenomenon. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「downburst」の詳細全文を読む スポンサード リンク
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