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Downburst

[doun-burst]
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.

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; 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. 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, 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. 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 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. 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

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.

References

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