Download Microbursts

By: Pam Avilez & Shelby Frazer
("Highway 72 Microburst - September 28, 2006")
•A microburst is a sudden, powerful, localized air current.
•It’s smaller than a macroburst but deadlier!
•Microburst’s effect areas less than 3km, but can have winds exceeding 250
km/h. Luckily, these only last fewer than 10 minutes on average.
•It’s small size makes it very difficult to predict and detect.
•Also, Microburst’s can be as destructive as a tornado!
("Microburst")
A microburst has a downdraft that hits the ground and spreads out with a burst
of wind. The strong downdraft causing the microburst is formed by cooling. The
cooling is cause by evaporation in a cloud. Once the strong downdraft has
formed, it is trying to push the cool air out of the downdraft to create a balance
with the warm temps of the surrounding air.
A microburst produces “straight-line winds.” These winds can be
greater then 104mph to as much as 168 mph. the wind speeds can be equal to
the winds of a tornado. The difference between a microburst is that the wind
from a microburst is pushed out of the storm, wind from a tornado flows into
the storm. (Pearce 1)
("Physical processes of dry and wet microburst's")
When rain falls below a cloud base or is mixed with dry air, it evaporates.
This evaporation 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. The difference in the wind is the signature
of the microburst. Dry microburst's, produced by high based
thunderstorms that generate not much rainfall. A dry Microburst must
include three important items:
Mid-level moisture.
A deep and dry drop rate in the sub-cloud layer
Low surface relative humidity
(Razor 1)
("Dry Microburst")
Wet microburst’s are downburst’s with major precipitation at the surface.
These downburst’s rely more on the drag of the precipitation for
downward acceleration. As a result, higher mixing ratios are necessary for
these burst’s to form. Melting of ice, usually hail, appears to be important
in the downburst formation. These factors make forecasting wet
microburst’s very difficult. (Razor 1)
("Wet Microburst")
("Hail Storms")
Microburst’s are dangerous to aircraft at low levels, especially on takeoff or landing. An aircraft coming at a downburst will first experience a
strong wind, which will lead to an increase in air speed. When trying to
fly a set airspeed, a pilot might be tempted to reduce power. This
would be very dangerous because as an aircraft passes through a
downburst, wind becomes a tailwind. The airspeed and lift will drop. The
downward force of a microburst may be enough force to bring the
aircraft to the ground. (Sky 1)
("Crash from Microburst")
("Microburst from Plane")
A derecho is a widespread tornado event which causes new thunderstorms
along an overflow boundary. The boundary helps produce more
thunderstorms. Occur mostly in the summer months. Derecho’s are usually
in northern plain states. The winds from Derecho's can last a long time and
cover large areas.
Since Derecho's are common in the warm season, people
involved in outdoor activity are most at risk. Campers or hikers in forested
areas are most likely to be injured or killed by falling trees. People in boats
risk injury or drowning, high winds and high waves can overturn boats.
People in cars and trucks can be hit by falling trees and utility poles. Also,
Semi-trucks can be blow over and crush other cars. (Corfidi 1)
("Damage from Derecho")
("Rare ‘derecho’ leaves swath of damage across Iowa ")
ADD VIDEO!
•If you are in an airplane you have to use full throttle
IMMDEDIATLY and climb altitude. You may have to fly on the
edge of the microburst.
•If you are in a jet, you should pitch up to the point of a stickshaker activation to escape. It’s best to think of this
maneuver as an escape maneuver so you don’t confuse it
with a normal go-around.
•You must be calm during this process. (Pendleton 1)
•DO get a accurate weather report, especially if there could be convective
activity.
•DO watch temp. and dew point spread. Microburst’s are more common with
large temp. and dew points spreads.
•DO maintain visual conditions any time thunderstorms are in the area.
•DO perform an escape maneuver at any time you think there might be a
microburst.
•DON’T fly under ANY thunderstorm for any reason!
•DON’T fly under the overhang of any thunderstorm.
•DON’T fly into or near a rain shaft.
•DON’T fly under Virga ( built in microburst.)
•DON’T stand outside.
(Pendleton 1)