A microburst is an intense, localized column of sinking air that develops within a thunderstorm. This phenomenon is relatively small, typically less than 2.5 miles in diameter, but it produces highly damaging winds upon impact with the ground.
Microbursts are particularly hazardous to aviation due to the sudden shift in wind speed and direction, called wind shear, which has caused accidents during takeoff and landing. The event is short-lived, often lasting only five to fifteen minutes, making its localized destructive power difficult to predict and avoid.
Atmospheric Conditions Required
The development of a microburst requires a specific atmospheric setup, often beginning with high instability within the storm cloud. This instability is represented by a steep lapse rate, meaning the air temperature drops quickly with altitude, allowing rising air within the storm’s updraft to accelerate. For the downward motion to become exceptionally strong, the air must become much colder and heavier than the surrounding air.
A common ingredient is the entrainment of mid-level dry air into the thunderstorm cloud structure. This drier air enhances the cooling of the air mass as precipitation falls through it, making the air dense enough to overcome the storm’s natural upward motion and accelerate its plunge toward the surface.
The Mechanics of Dry Microburst Formation
Dry microbursts form in environments where the air below the cloud base is significantly dry, often characterized by a deep atmospheric boundary layer. The formation process is dominated by evaporational cooling, which rapidly chills the air mass.
As precipitation (rain or hail) falls from the cloud, it encounters this dry layer and begins to evaporate before reaching the ground; this visible precipitation is known as virga. Evaporation requires latent heat to be drawn from the surrounding air, causing the temperature to drop sharply. This rapid cooling makes the air much denser, creating negatively buoyant air that accelerates downward. Since the downdraft is not slowed by precipitation drag, it can reach speeds exceeding 100 miles per hour, creating powerful outflow winds despite the lack of surface rain.
The Mechanics of Wet Microburst Formation
Wet microbursts occur in more humid environments where substantial precipitation reaches the ground, driven by two primary factors. The first is precipitation loading, which refers to the weight of water droplets and hailstones suspended within the storm’s updraft. When the updraft can no longer support this column of hydrometeors, the weight acts as a downward force, dragging the air with it.
The second factor is the cooling effect, which is less dominant than in the dry type. Cooling is primarily caused by the melting of hail and the sublimation or evaporation of precipitation below the cloud base; this combined effect creates a powerful downdraft that slams into the ground, generating destructive straight-line winds accompanied by heavy rainfall.
Distinguishing Microburst Damage from Tornado Damage
Once a microburst’s downdraft strikes the surface, it immediately spreads out horizontally, creating a distinctive damage pattern known as straight-line winds. The damage is divergent, meaning debris, such as fallen trees, will be aligned in a starburst pattern radiating outward from the central impact point. Winds can exceed 100 miles per hour, capable of causing destruction similar to a weak tornado.
This damage pattern is the differentiator from a tornado, which is characterized by rotational winds and a convergent damage pattern. Tornadoes cause debris to be twisted and thrown inward toward the center of the vortex. Surveying the fall direction of trees and the alignment of structural damage is the most reliable method for meteorologists to determine if the event was a microburst or a tornado.