A Microburst is a form of wind shear that is usually associated with convective activity. It is a severe downdraft that it critical to every type of aircraft flying through it. They are most dangerous during the landing or take-off phase of flight because of the aircrafts close proximity of the aircraft to the ground. A microburst is a basically a localized column of sinking air, producing damaging divergent winds. This means that the wind shear coming out from a microburst can scatter in every direction without warning. They can generate wind speeds higher then 170mph and up to about 15 minutes. They can cover and area of up to 2.5 square miles, usually less, and can have downdrafts of about 6000 feet per minute. In laymen’s terms, it is a rapidly descending column of air that can make an airliner loose up to 3000ft in about 15 seconds.
There are 3 main stages of a microburst:
downburst: highest wind velocity stage; a column of moving area descends from the could base and reaches the ground in under minutes
outburst: the air hitting the ground curls and starts moving away from the gorund
cushion: the winds that have already hit the ground slow due to friction and the wind curling accelerates.
There are two main types of downbursts. They occur in different areas depending on the current weather conditions and have different effects. They are wet and dry microbursts:
Wet Microburst: generally occur in the Southeast. The have moderate to heavy precipitation and clouds that occur at or below the 850 MB level. They give off strong shafts or precipitation that reach the ground and have a higher momentum downdraft. The cloud base environment tends to have a shallow dry layer of air with a high relative humidity and a moist adiabatic lapse rate. The wind shear gusts associated with wet microbursts have a surface outflow pattern in the direction of the mid-level wind.
Dry Microbursts: generally occur in the Midwest/West. There is little to no precipitation associated with them but Virga is a common meteorological phenomenon that occurs at and around the cloud base. The clouds tend to form as high as the 500 MB level and have a base atmosphere with a deep dry layer or air, low relative humidity, and a dry adiabatic lapse rate. The wind shear gusts associated with dry microbursts tend to be omni-directional.