A microburst is a form of wind shear that is usually associated with convective activity. It is a severe downdraft that is 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 aircraft's proximity to the ground. A microburst is basically a localized column of sinking air, producing damaging divergent winds. This means that the wind shear coming out of a microburst can scatter in every direction without warning. They can generate wind speeds of more than 170 mph for up to about 15 minutes. They can cover an area of up to 2.5 square miles, typically less, and can have downdrafts of about 6,000 feet per minute. In laymen’s terms, it is a rapidly descending column of air that can make an airliner loose up to 3,000 ft 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 cloud base and reaches the ground in under minutes.
Outburst: the air hitting the ground curls and starts moving away from the ground.
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 microbursts generally occur in the Southeast. They have moderate to heavy precipitation and clouds that occur at or below the 850 MB level. They give off strong shafts of 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 mostly 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 at altitudes as high as the 500 MB level and have a base atmosphere with a deep dry layer of air, low relative humidity, and a dry adiabatic lapse rate. The wind shear gusts associated with dry microbursts tend to be omni-directional.