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downburst

 
Dictionary: down·burst   (doun'bûrst') pronunciation
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n.
An extremely powerful downward air current from a cumulonimbus cloud, typically associated with thunderstorm activity.


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Boating Encyclopedia: Downburst
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Beware of dangerous jets of wind from thunderstorms

Section through a thunderstorm cell. This is a young storm. As it matures, the downdrafts could turn into more dangerous downbursts.
A downburst, or microburst, is a dangerous mass of cool air that shoots downward out of a thunderstorm. Downbursts with the energy of small tornadoes achieve speeds of 100 knots or more, and shift direction rapidly.Some are bigger than others, but all are capable of capsizing sailboats whose crews are unprepared. There is no way of forecasting downbursts; therefore, if you find yourself stuck in the path of an approaching thunderstorm, the safest action is to douse the sails and make everything fast on deck.The telltale sign of a thunderstorm large enough to create downbursts is a cumulus cloud that appears to be growing larger as you watch it. Cumulonimbus is tall and imposing, with a cauliflower-shaped top and a black, menacing base. You’ll often see the top being blown away into the shape of an anvil by high-speed upper-atmosphere winds.As the cloud approaches, you might notice another characteristic of the thunderstorm: an area along the leading edge of the base where the clouds seem to roll vertically.The winds ahead of the thunderstorm will weaken as the roll cloud draws near and, as it passes overhead, you can expect violent winds from many different directions, including downdrafts and more violent downbursts. Even in a normal summer afternoon storm, these winds can exceed 60 knots. Behind the roll cloud, heavy rain (and possibly hail) begins to fall as the dark area of the cloud passes overhead.On inland and coastal waters, thunderstorms are more common in summer, and usually develop in the late afternoon when the ambient air has been heated for many hours. In midocean, away from the influence of land, they mostly occur between midnight and sunrise.Always take thunderstorms seriously—most are accompanied by dangerous lightning, if not downbursts. An inexpensive AM radio receiver will provide advance warning of approaching thunderstorms, announcing their arrival with loud crashes of static noise. If possible, plot the course of any thunderstorm you spot, and take early action to get out of its direct path.See also Clouds.
Wikipedia: Downburst
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The curl phase soon after an intense microburst impacted the surface
Downburst damages in a straight line. (Source NOAA)

A downburst is created by an area of significantly rain-cooled air that, after hitting ground level, spreads out in all directions producing strong winds. Unlike winds in a tornado, winds in a downburst are directed outwards from the point where it hits land or water. 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 microburst which is dangerous to aviation.

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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 150 mph (240 km/h), 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 impacting 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;[1] 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.[2] Downbursts larger than 2.5 miles (4 km) in extent are sometimes called macrobursts.[2] Downbursts can occur over large areas. In the extreme case, a derecho can cover a huge area more than 200 miles (320 km) wide and over 1000 miles (1600 km) long, lasting up to 12 hours or more, and is associated with some of the most intense straight-line winds,[3] 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 produce damage, demonstrating a lack of a rotational damage pattern.[4] 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.

Straight-line winds may be damaging to marine interests. Small ships, cutters and sailboats are at risk from this meteorological phenomenon.

Formation

The formation of a downburst starts with hail or large raindrops falling through drier air. Hailstones melt and raindrops evaporate—this is an endothermic process that demands a lot of energy (in the 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 a 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.

Heat Bursts

Main article: Heat burst

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, usually on the backside of a dying squall line or outflow boundary.[5] 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.

Danger to aviation

Further information: Microburst#Danger to aircraft

Downbursts, particularly microbursts, are exceedingly dangerous to aircraft which are taking off or landing due to the strong vertical wind shear caused by these events. A number of fatal crashes have been attributed to downbursts.[6]

See also

References

  1. ^ Fernando Caracena, Ronald L. Holle, and Charles A. Doswell III. Microbursts: A Handbook for Visual Identification. Retrieved on 9 July 2008.
  2. ^ a b Glossary of Meteorology. Macroburst. Retrieved on 30 July 2008.
  3. ^ Peter S. Parke and Norvan J. Larson. Boundary Waters Windstorm. Retrieved on 30 July 2008.
  4. ^ Glossary of Meteorology. Straight-line wind. Retrieved on 1 August 2008.
  5. ^ "Oklahoma "heat burst" sends temperatures soaring". USA Today. http://www.usatoday.com/weather/wheatbst.htm. Retrieved 2007-05-09. 
  6. ^ NASA Langley Air Force Base. Making the Skies Safer From Windshear. Retrieved on 2006-10-22.

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Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.  Read more
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