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 precipitation-cooled 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).
Microbursts are recognized as capable of generating wind speeds higher than 75 m/s (168 mph; 270 km/h).
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. (Wakimoto, 1985) developed a conceptual model (over the High Plains of the United States) 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.
|Characteristic||Dry Microburst||Wet Microburst|
|Location of Highest Probability within the United States||Midwest/West||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|
|'''Environment below Cloud Base||Deep dry layer/low relative humidity/dry adiabatic lapse rate||Shallow dry layer/high relative humidity/moist adiabatic lapse rate|
|Surface Outflow Pattern||Omni-directional||Gusts of the direction of the mid-level wind|
The evolution of downbursts is broken down into three stages: the contact stage, the outburst stage and the cushion stage.
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.
Start by using the vertical momentum equation
By decomposing the variables into a basic state and a perturbation, defining the basic states, and using the Ideal Gas Law (), then the equation can be written in the form
where B is used to denote buoyancy. Note that the virtual temperature correction usually is rather small and to a good approximation, it can be ignored when computing buoyancy. Finally, the effects of precipitation loading on the vertical motion are parameterized by including a term that decreases buoyancy as the liquid water mixing ratio () increases, leading to the final form of the parcel's momentum equation:
The first term is the effect of perturbation pressure gradients on vertical motion. In some storms this term has a large effect on updrafts (Rotunno and Klemp, 1982) but there is not much reason to believe it has much of an impact on downdrafts (at least to a first approximation) and therefore will be ignored.
The second term is the effect of buoyancy on vertical motion. Cleary, in the case of microbursts, one expects to find that B is negative meaning the parcel is cooler than its environment. This cooling typically takes place as a result of phase changes (evaporation, melting, and sublimation). Precipitation particles that are small, but are in great quantity, promote a maximum contribution to cooling and, hence, to creation of negative buoyancy. The major contribution to this process is from evaporation.
The last term is the effect of water loading. Whereas evaporation is promoted by large numbers of small droplets, it only takes a few large drops to contribute substantially to the downward acceleration of air parcels. This term is associated with storms having high precipitation rates. Comparing the effects of water loading to those associated with buoyance, if a parcel has a liguid water mixing ration of 1.0 gkg-1, this is roughly equivalent to about 0.3 K of negative buoyancy; the latter is a large (but not extreme) value. Therefore, in general terms, negative buoyancy is typically the major contributor to downdrafts.
Using pure "parcel theory" results in a prediction of the maximum downdraft of
where NAPE is the Negative Available Potential Energy,
and where LFS denotes the Level of Free Sink for a descending parcel and SFC denotes the surface. This means that the maximum downward motion is associated with the integrated negative buoyancy. Even a relatively modest negative buoyancy can result in a substantial downdraft if it is maintained over a relatively large depth. A downward speed of 25 m/s results from the relatively modest NAPE value of 312.5 m²s-2. To a first approximation, the maximum gust is roughly equal to the maximum downdraft speed.
A microburst often causes aircraft to crash when they are attempting to land (the above mentioned Pan Am flight is a notable exception). 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 decrease in airflow over the wings of the aircraft causes a drop in the amount of lift produced. This decrease in lift combined with a strong downward flow of air can cause the thrust required to remain at altitude to exceed what is available.