A microburst is a severe, localized weather event characterized by an intense downdraft of air that strikes the ground and spreads out rapidly. This phenomenon is a type of downburst, but it is smaller and more concentrated, making it exceptionally destructive. While brief, microbursts generate straight-line winds strong enough to cause damage similar to a weak tornado, posing a threat to both ground structures and air traffic. Due to the speed and localized nature of this event, people often have little warning before its intense winds impact an area.
Defining the Microburst Phenomenon
A microburst is defined as a small-scale, intense column of sinking air, or a downdraft, which originates within a thunderstorm or rain cloud. When this column of air hits the surface, it spreads out in all directions, creating an outward burst of damaging, straight-line winds. The distinguishing characteristic separating a microburst from a larger downburst, sometimes called a macroburst, is its size constraint. A microburst’s damaging wind path at the surface must be 2.5 miles (4 kilometers) or less in diameter.
This small diameter, combined with extreme wind speeds, makes the microburst difficult to detect. The event is short-lived, with a life cycle that lasts only a few minutes from initial impact to dissipation. Although brief, the surface winds generated can exceed 100 miles per hour (160 kph). This localized outflow creates extreme wind shear—a rapid change in wind speed or direction over a short distance.
Physical Mechanics of Microburst Formation
The air within a thunderstorm accelerates downward through two primary physical processes that create a column of cold, dense air. One mechanism is known as precipitation loading, where the weight of rain, hail, or ice suspended high in the cloud drags the surrounding air downward. Strong updrafts within a developing storm can suspend this precipitation far aloft, but when the updraft weakens or the water mass becomes too great, the core plummets toward the ground.
The second, and often dominant, mechanism involves rapid cooling of the air mass through evaporation and sublimation. As precipitation falls through a layer of dry air beneath the cloud base, water droplets evaporate, or ice crystals sublimate (change directly from solid to gas). Both processes require heat energy from the surrounding air, rapidly cooling it and making it denser than the air around it. This colder, heavier air sinks violently, accelerating the downdraft to high speeds.
Distinguishing Between Wet and Dry Microbursts
Microbursts are categorized into two main types based on whether precipitation reaches the ground. Wet microbursts occur in humid environments and are defined by precipitation accompanying the downdraft. The falling rain often makes the downdraft visible, appearing as a sudden, localized curtain of heavy rain. These are common in warm, moist climates, such as the Southeastern United States during the summer months.
In contrast, dry microbursts form when the precipitation evaporates completely before reaching the surface. This process, where rain falls but evaporates mid-air, is called virga, and it is the primary visual signature. Because the air mass is cooled entirely by evaporation in the dry layer beneath the cloud, there is little to no visible rain at ground level. The only observable evidence of a dry microburst is often a ring of dust and debris kicked up when the powerful outflow hits the dry surface.
Hazards to Aviation and Ground Structures
The concentrated wind energy of a microburst poses hazards on the ground and in the air. For ground structures, the outflow creates straight-line winds that can reach speeds up to 150 mph, comparable to an EF-2 tornado. This powerful horizontal wind can uproot trees, flatten homes, and severely damage infrastructure in a localized area. Because the wind spreads radially outward from a central point, the resulting damage pattern is distinct from the twisting path of a tornado.
Microbursts are particularly dangerous to aviation, especially during takeoff and landing, due to the extreme wind shear they generate. As an aircraft enters the microburst zone, it first encounters a strong headwind, which briefly increases lift and airspeed. This sudden performance gain can tempt a pilot to reduce engine power. Immediately afterward, the aircraft enters the core of the downdraft, causing a rapid loss of altitude, followed by a sudden switch to a tailwind as it exits. This transition causes a severe loss of airspeed and lift, which can force the aircraft toward the ground with little time for recovery.