A microburst is a small, concentrated downburst whose outflow covers less than about 2.5 miles across at the surface, typically peaking in a few minutes and often producing damaging straight-line winds.
How a microburst forms
A parcel high in a storm becomes denser than its surroundings through some combination of evaporative cooling, melting of ice and hail, and the drag of falling precipitation. Once negatively buoyant, it accelerates downward, and if the acceleration is concentrated in a narrow shaft, the result is a small, intense downdraft. When that shaft hits the ground, it spreads outward radially, producing a compact but violent burst of surface wind.
Fujita's fieldwork in the late 1970s and early 1980s, including Project NIMROD in Illinois and the JAWS project in Colorado, identified the microburst as a distinct phenomenon separate from tornadoes. Doppler radar analysis in JAWS showed the descending core, the surface divergence, and the short lifetime that define the modern definition.
Wet and dry versions
A wet microburst falls from a storm with heavy rain and often shows a rain foot, a bowed-out precipitation shaft near the ground. Precipitation loading and evaporation both matter, but the dominant driver in humid environments is often downward transport of high-momentum midlevel air combined with the weight of water and melting hail.
A dry microburst forms from a high-based storm in a deep, dry subcloud layer. Most of the precipitation evaporates before reaching the surface, and the outflow is chilled and dense by that evaporation. Visually, a dry microburst can look like a curtain of virga followed by an expanding dust ring at the ground. The National Weather Service glossary notes that dry microbursts are most common in semi-arid regions.
How forecasters and chasers identify them
In pre-storm forecasting, the key ingredients are steep midlevel lapse rates, a dry layer above the surface moisture (or a deep dry boundary layer for dry events), and high DCAPE. GOES-derived nowcasting products such as the Microburst Windspeed Potential Index and the Wet Microburst Severity Index use satellite proxies for these ingredients.
On radar, a rapidly descending high-reflectivity core, a midlevel radial convergence signature, and a strong divergent velocity couplet near the surface are the classic microburst signatures. Visually, spotters watch for a rain foot in wet events and for a dust foot or a collapsing virga curtain in dry events, and for a shelf cloud that suddenly races outward from the parent cell.
Aviation and public safety
Microbursts are one of the most dangerous weather threats to aircraft on approach or departure. An airplane flying through a microburst first meets an unexpected headwind that lifts it above the glide path, then a downdraft, then an equally strong tailwind, all within seconds. Several fatal airline crashes in the 1970s and 1980s prompted the deployment of Terminal Doppler Weather Radars, the Low-Level Wind Shear Alert System, and revised pilot training that persists today.
For people on the ground, the hazard is straight-line wind damage: trees down, structural failure of weaker buildings, and airborne debris. A single microburst can devastate a small area while leaving neighboring streets untouched.
Where microburst reasoning goes wrong
A quiet-looking storm can produce a strong microburst. Radar and satellite give clues, but small microbursts sometimes fall between radar volume scans, and dry events may be nearly invisible in reflectivity. Environmental ingredients matter more than the visual size of the parent cloud.
Microburst damage is also easy to misclassify. Because the outflow spreads outward from a small point, damage close to the touchdown can appear tornadic. The definitive diagnostic in a survey is the direction pattern of fallen trees and debris, which points radially outward for a microburst and shows convergent or twisted signatures for a tornado.