Microburst
Illustration of a microburst. Note the downward motion of the air until it hits ground
level, then spreads outward in all directions. The wind regime in a microburst is
completely opposite to a tornado.
A microburst is a very localized column of sinking air or downburst, producing
damaging divergent and straight-line winds at the surface that are similar to but
distinguishable from tornadoes which generally have convergent damage. Microbursts
can generate wind speeds higher than 75 mph that can knock over full grown trees. They
are completely opposite to a tornado.
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 precipitationcooled 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).
Dry microburst schematic from NWS.
Dry microbursts
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 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 schematic from NWS.
Wet microbursts
Wet microbursts are downbursts accompanied by significant precipitation at the surface
which are warmer than their environment. 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, especially in the lowest one
kilometer above ground level. These factors, among others, make forecasting wet
microbursts a difficult task.
Characteristic
Dry Microburst
Location of Highest
Midwest/West
Probability
Wet Microburst
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
Deep dry layer/low relative
Environment below
humidity/dry adiabatic lapse
Cloud Base
rate
Shallow dry layer/high relative
humidity/moist adiabatic lapse
rate
Surface Outflow
Pattern
Gusts of the direction of the midlevel wind
Omni-directional
Development stages of microbursts
The University of Illinois breaks the evolution of downbursts into three stages, the
contact stage, the outburst stage and the cushion stage.
A downburst initially develops as
the downdraft begins its descent
from cloud base. The downdraft
accelerates and within minutes,
reaches the ground (contact stage).
It is during the contact stage that
the highest winds are observed.
During the cushion stage,
During the outburst stage,
winds about the curl
the wind "curls" as the cold
continue to accelerate,
air of the downburst moves
while the winds at the
away from the point of
surface slow due to
impact with the ground.
friction.
Simple explanation
In the case of a wet microburst, the atmosphere is warm and humid in the lower levels
and dry aloft. As a result, when thunderstorms develop, heavy rain is produced but some
of the rain evaporates in the drier air aloft. As a result the air aloft is cooled thereby
causing it to sink and spread out rapidly as it hits the ground. The result can be both
strong damaging winds and heavy rainfall occurring in the same area. Wet downbursts
can be identified visually by such features as a shelf cloud, while on radar they
sometimes produce bow echoes.
In the case of a dry microburst, the atmosphere is warm but dry in the lower levels and
moist aloft. Thus when showers and thunderstorms develop, most of the rain evaporates
before reaching the ground.
A photograph of the surface curl soon after a microburst impacted the surface
Danger to aircraft
Further information: Downburst
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:
 Eastern Airlines Flight 66, John F. Kennedy International Airport - June 24, 1975
 Pan Am Flight 759, Miami International Airport - July 9, 1982
 Delta Airlines Flight 191, Dallas-Fort Worth International Airport - August 2,
1985
 Martinair Flight 495, Faro Airport - December 21, 1992
 USAir Flight 1016, Charlotte/Douglas International Airport - July 2, 1994
 Goodyear Blimp, Coral Springs, Florida - June 16, 2005


Air France Flight 358, Toronto Pearson International Airport - August 2, 2005
One-Two-GO Airlines Flight 269, Phuket International Airport - September 16,
2007
A microburst often causes aircraft to crash when they are attempting to land (except Pan
Am Flight). 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, although it is possible that for light aircraft, the descent rate induced by the
microburst will exceed their maximum climb rate, leading to an unavoidable crash.)
For more information, see "http://en.wikipedia.org/wiki/Microburst"