A microburst is a strong, localized column of air that sinks within a thunderstorm with a diameter of 2.5 miles or less. A microburst can occur in two different ways:
- Wet Microbursts are accompanied by heavy precipitation.
- Dry Microbursts do not contain precipitation as most of it evaporates before reaching the ground.
Microbursts form from strong thunderstorms that suspend rain/ice high up in the cloud from the strong updraft. As dry air works in from the mid levels, it causes evaporational cooling making the air “heavier”. The “heavier” air becomes more dense than the surrounding air and sinks to the surface. Other factors outside of evaporation include, melting of hail in the cloud and downward transport of higher momentum from the drag of the precipitation.
The stronger more significant tornadoes form from supercell thunderstorms. Supercell thunderstorms have a rotating updraft that allows the thunderstorm to sustain its intensity for long periods of time.
Supercells are favored when the vertical shear is strong (greater than 30Kts) over the lower 6km of the atmosphere. This produces a separation from the warm inflow air and the cooler downdraft air. This process leads to strong upward motion in the updraft if the instability is strong enough.
Supercells gain rotation from the environmental winds in the lower levels increasing and changing direction with height. Directional shear refers to the winds turning with height, ie. from the south at the surface and westerly aloft. Vertical shear refers to the winds increasing speeds with height. The combination of these two variables produces the rotating updraft.
Not all supercells produce tornadoes, the number of supercells that spawn tornadoes is estimated at 20%. From case to case, it is very difficult to distinguish why some produce tornadoes and others do not. The latest research from Paul Markowski states that the temperature of the rain cooled air from the downdraft may be a key player. The intersection of the warm inflow and the downdraft on the forward flank produces baroclinic vorticity near the surface as the warm air overrides the cooler air. This vorticity generated parallel to the storm inflow is then wrapped around the supercell and stretched by the strong instability and vertical pressure gradients in the supercell. IF the outflow is too cold it becomes much less likely to be stretched as the air is not as buoyant. If it is cool but not cold it can be rapidly stretched despite being less buoyant. Each supercell is different in terms of how strong the “suction” is and how cool/cold the downdraft air is.
Tornadoes can also occur in ordinary thunderstorms through stretching of pre existing vertical vorticity. This is known as a “landspout”.