A downburst is a strong thunderstorm downdraft that reaches the ground and spreads outward as a burst of damaging straight-line winds, sometimes rivaling the damage of a tornado.
How a downburst forms
The physics is simpler than a tornado's. A parcel in the mid-troposphere becomes negatively buoyant when it is chilled by evaporating rain, melting hail or graupel, and weighted down by the mass of precipitation it carries. Once the parcel is denser than its surroundings, gravity accelerates it toward the surface. If mid-level environmental winds are strong, downward transport of that horizontal momentum adds to the surface impact.
When the downdraft strikes the ground, it cannot keep sinking, so it flares outward radially in every direction. That expansion produces the characteristic straight-line wind burst. Fujita's 1970s and 1980s field studies, including Project NIMROD and JAWS, first documented this pattern and named the phenomenon.
Downburst sizes and flavors
The National Weather Service and NSSL classify downbursts by the size of their outflow footprint. A macroburst covers at least four kilometers across and lasts several minutes. A microburst is smaller than four kilometers and typically lasts only three to seven minutes, but peak winds can be more intense per unit area.
Downbursts are also grouped by moisture content. A wet downburst is accompanied by heavy rain and often shows a distinct rain foot, a bowed-out precipitation shaft at the surface. A dry downburst reaches the ground with little visible rain, usually because the precipitation has already evaporated to virga on the way down, and is often marked by a dust plume or dust foot instead.
How forecasters and chasers identify them
In pre-storm assessment, forecasters look for the environmental fingerprints of a downdraft-favoring atmosphere: steep midlevel lapse rates, a dry layer between the surface moisture and the anvil, high DCAPE, and a well-mixed subcloud layer. The specific mix of dry air aloft, midlevel wind speed, and precipitation type determines whether wet or dry downbursts are more likely.
On radar, a descending reflectivity core, a midlevel radial convergence signature, and divergent outbound and inbound velocities at low levels are the classic clues. Visually, chasers and spotters look for a rain foot in a wet event, a rapidly expanding dust ring or virga curtain in a dry event, and a shelf cloud that suddenly races outward from the storm.
Damage patterns and hazards
Downburst damage is straight-line: trees fall away from the impact point in a spreading fan, buildings show pressure and impact damage along a single dominant direction, and swaths can run for miles when a series of downbursts strings together into a derecho track. Peak surface winds can reach or exceed hurricane force and can match EF2 to EF3 tornado damage in localized spots.
For aircraft, downbursts near the ground are one of the most serious weather threats. They can push an airplane into the runway or into terrain on approach, which is why terminal Doppler weather radars, low-level wind shear alert systems, and cockpit training exist to detect and avoid them.
What downbursts do not do
A downburst is not a failed tornado. It is a distinct dynamical event, and treating it as an also-ran of the tornadic process leads to under-warning. A storm with weak mid-level rotation and no evidence of tornado potential can still produce a wind swath that flattens neighborhoods.
It is also a mistake to assume the strongest downbursts always come from the strongest-looking storms. Dry downbursts in particular can drop from a modest cumulus congestus with a small radar signature. The pre-storm environment, especially the depth of the subcloud dry layer and the amount of DCAPE, matters more than the visual size of the parent cloud.