Microburst

	A microburst is a sudden, short-lived, localized wind that
	often appears to radiate outward from a central point.
	Microbursts rarely occur in their classic form, but portions
	or variations of them are frequently seen. Clasiic
	microbursts are visible as a descending lpume of rain or
	dust that spreads horizontally when it reaches the groun.
	in extreme cases, dust or rain may curl from the ground
	back up toward the cloud as downdrafts blast the ground
	at speeds up to 150mph (240kph). Microbursts can spread
	outward in a circle, but winds usually produce fan-shaped
	patterns in one direction. A microburst last 1 to 5 minutes
	and covers an area less than 2.5 miles (4 km) in diameter. A
	downburst is a strong downdraft-induced wind over a larger 
	area. Both types come from a single3 swelling cumulus or 
	cumulonimbus. Microburst winds descend and spread
	outward, unlike tornado winds, which converge and rise.

Effects

	Microbursts were first identified because of the major
	aircraft disasters they caused. Special types of radar are 
	being installed at many airports for identifying these
	dangerous phenomena. Training programs for commercial
	pilots describe the impact of microbursts on aircraft
	performance and explain avoidance procedures, but 
	private aviation continues to be highly vulnerable.
	Microburst are also a hazard to boating, and they can
	fan forest fires in unexpected directions.

Dry Microbursts

	Dry Microbursts, which occur over the semiarid western
	Great Plains and the mountain regions of western North
	America, typically emerge from a swelling cumulus or
	cumulonimbus cloud with a high base (10,000'/3,000 m or
	more) that has developed in moist air middle levels.
	Most of the precipitation from dry microbursts evaporates
	before reaching the ground, and the evaporative cooling
	intensifies the downdraft in the dry low-level air.

Wet Microbursts

	Wet microbursts ocurr in extremely wet environments
	east of the Rocky Mountains. These wet downdrafts have
	nearly saturated lower levels, but there is drier air at
	middle levels outside the storm; the dry air fuels the
	evaporative cooling that is necessary to accelerate the
	downdraft. Wet microbursts are often embedded in heavy
	rain; they are sometimes associated with tornadoes and
	larger-scale squall line gust fronts.