Microburst

A photograph of the surface curl soon after an microburst impacted the surface

A microburst is a very localized column of sinking air, producing damaging divergent and straight-line winds at the surface that are similar to but distinguishable from tornadoes which generally have convergent damage.

History of term

The term was defined by severe weather expert Tetsuya Theodore Fujita as affecting an area 4 km (2.5 mi) in diameter or less, distinguishing them as a type of downbursts and apart from common wind shear which can encompass greater areas. Dr. Fujita also coined the term macroburst for downbursts larger than 4 km (2.5 mi).

A distinction can be made between a wet microburst which consists of precipitation and a dry microburst which consists of virga. They generally are formed by precipitation-cooled air rushing to the surface, but they perhaps also could be powered from the high speed winds of the jet stream deflected to the surface in a thunderstorm (see downburst).

Microbursts are recognized as capable of generating wind speeds higher than 75 m/s (168 mph; 270 km/h).

Dry Microburst
When rain falls below cloud base or is mixed with dry air, it begins to evaporate and this evaporation process cools the air. The cool air descends and accelerates as it approaches the ground. When the cool air approaches the ground, it spreads out in all directions and this divergence of the wind is the signature of the microburst.
Dry microbursts, produced by high based thunderstorms that generate little surface rainfall, occur in environments characterized by a thermodynamic profile exhibiting an inverted-V at thermal and moisture profile, as viewed on a Skew-T log-P thermodynamic diagram. (Wakimoto, 1985) developed a conceptual model (over the High Plains) of a dry microburst environment that comprised of three important variables: mid-level moisture, a deep and dry adiabatic lapse rate in the sub-cloud layer, and low surface relative humidity.

Wet Microburst
Wet microbursts are downbursts accompanied by significant precipitation at the surface (Fujita, 1985) which are warmer than their environment (Wakimoto, 1998). These downbursts rely more on the drag of precipitation for downward acceleration of parcels than negative buoyancy which tend to drive "dry" microbursts. As a result, higher mixing ratios are necessary for these downbursts to form (hence the name "wet" microbursts). Melting of ice, particularly hail, appears to play an important role in downburst formation (Wakimoto and Bringi, 1988), especially in the lowest one kilometer above ground level (Proctor, 1989). These factors, among others, make forecasting wet microbursts a difficult task.

Characteristic	Dry Microburst	Wet Microburst
Location of Highest Probability	Midwest/West	Southeast
Precipitation	Little or none	Moderate or heavy
Cloud Bases	As high as 500 mb	Usually below 850 mb
Features below Cloud Base	Virga	Shafts of strong precipitation reaching the ground
Primary Catalyst	Evaporative cooling	Downward transport of higher momentum
Environment below Cloud Base	Deep dry layer/low relative humidity/dry adiabiatic lapse rate	Shallow dry layer/high relative humidity/moist adiabatic lapse rate
Surface Outflow Pattern	Omni-directional	Gusts of the direction of the mid-level wind

Danger to aircraft

The scale and suddenness of a microburst makes it a great danger to aircraft, particularly those at low altitude which are taking off and landing. The following are some fatal crashes that have been attributed to microbursts in the vicinity of airports:

A microburst often causes aircrafts to crash when they are attempting to land. The microburst is an extremely powerful gust of air that, once hitting the ground, spreads in all directions. As the aircraft is coming in to land, the pilots try to slow the plane to an appropriate speed. When the microburst hits, the pilots will see a large spike in their airspeed, caused by the force of the headwind created by the microburst. A pilot inexperienced with microbursts would try to decrease the speed. The plane would then travel through the microburst, and fly into the tailwind, causing a sudden decrease in the amount of air flowing across the wings. The sudden loss of air moving across the wings causes the aircraft to literally drop out of the air. The best way to deal with a microburst in an aircraft would be to increase speed as soon as the spike in airspeed is noticed. This will allow the aircraft to remain in the air when traveling through the tailwind portion of the microburst and also pass through the microburst with less difficulty.

List of notable microbursts

A microburst squall with windspeeds of 80 miles per hour is responsible for capsizing and sinking the Pride of Baltimore in May 1986 in the Caribbean, about 250 miles north of Puerto Rico. The ship took the lives of her captain and three of her other 11 crew members.
A particularly violent microburst is a possible alternative explanation to the 1961 sinking of the American school brigantine Albatross. The ships captain Dr. Christopher Sheldon claimed that the ship was hit by a white squall on the voyage from Progreso, Mexico to Nassau in the Bahamas.

References

Printed Media

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).

External links and sources

http://youtube.com/watch?v=dIRQHXCqodc http://youtube.com/watch?v=gUtIsVCsRTo