A microburst is a small, intense thunderstorm downdraft that slams into the ground and spreads outward in a violent burst of straight-line wind, sometimes topping 100 mph over an area just a mile or two wide. It is not a tornado. There is no rotation, only a sudden column of sinking air that hits the surface and blasts outward in every direction. In Southern California, microbursts show up most often with summer thunderstorms over the deserts and mountains, and occasionally with stronger cool-season storms crossing the LA Basin, and they are a genuine hazard for aircraft, drivers, and anyone caught outdoors near a collapsing rain shaft.
What Is a Microburst?
A microburst is a concentrated downdraft from a thunderstorm or shower that reaches the ground and fans out horizontally, producing a short-lived but powerful burst of straight-line wind. The term was coined by meteorologist Ted Fujita, the same scientist behind the tornado damage scale, after he investigated a string of unexplained aircraft accidents near thunderstorms in the 1970s and found that sudden, localized downdrafts, not wind shear from a distant system, were slamming planes into the ground during approach and takeoff. By definition, a microburst covers an outflow area less than 2.5 miles across; a larger downdraft with the same mechanism is called a macroburst.
A microburst is a different phenomenon from generic thunderstorm outflow, the broad gust front that spreads ahead of ordinary storms and produces a brief cooling breeze. Outflow winds are usually 20 to 40 mph and spread over a wide area as a storm's cold pool advances. A microburst is narrower, more sudden, and considerably stronger, capable of producing wind speeds that rival a weak tornado, but doing it through straight-line divergence rather than rotation. Confirming that a specific gust or damage pattern was a microburst, rather than ordinary outflow or something else, requires a National Weather Service damage survey, not a guess based on video alone.
| Feature | Wet microburst | Dry microburst |
|---|---|---|
| Visible precipitation | Heavy rain reaches the ground with the wind | Little or no rain reaches the ground; often just virga and dust |
| Driving mechanism | Weight of falling rain drags air downward | Evaporative cooling of rain in dry sub-cloud air makes the air denser and it sinks |
| Typical Southern California setting | Monsoon thunderstorms over the mountains and basin with deep moisture | Desert and high-desert convection over the Mojave and Antelope Valley |
| Visual warning sign | A collapsing, curtain-like rain shaft | A dust ring or blowing dust erupting under a cloud with little rain visible |
How Does a Thunderstorm Create One?
A microburst forms when air inside or beneath a thunderstorm becomes heavier than the air around it and accelerates downward, then spreads out on impact with the ground. In a wet microburst, the weight of heavy rain and hail dragging through the cloud is enough on its own to drag a pocket of air down with it, and that downdraft arrives at the surface still carrying visible rain.
A dry microburst works differently and is the pathway most relevant to Southern California's deserts: rain falls from the cloud base into a deep layer of dry air near the ground and evaporates before reaching the surface. Evaporation cools the air around it rapidly, and that cooled air becomes denser than its surroundings and sinks fast, accelerating as it falls through thousands of feet of dry sub-cloud air.
When that fast-sinking column of air reaches the ground, it has nowhere to go but outward, spreading in a radial burst that can produce a ring-shaped damage pattern, tree limbs and light debris pushed away from a central point in multiple directions rather than swirled together as a tornado would leave them. Because dry microbursts often produce little or no rain at the surface, the giveaway is usually virga, streaks of precipitation visible under the cloud that evaporate before reaching the ground, followed moments later by a sudden wall of dust and wind with the sky still relatively clear overhead.
The NOAA JetStream online weather school covers this evaporative-cooling process in more technical detail for readers who want the full thermodynamics. For the related but distinct question of why rain evaporates before reaching the ground in the first place, see what is virga.
Where and When Do Microbursts Occur Locally?
Southern California's best chance for microbursts comes with summer monsoon moisture, typically July through September, when Gulf of California moisture pushes thunderstorms over the mountains and deserts covered by NWS Los Angeles/Oxnard and NWS San Diego. The Antelope Valley, the Mojave Desert, and the high desert communities east of the San Gabriel and San Bernardino Mountains see the driest, dustiest storm bases and are the classic setting for dry microbursts, since the air beneath the storm cloud stays deep and dry enough to evaporate falling rain well before it reaches the ground.
Mountain and foothill areas that catch monsoon thunderstorms with more available moisture, including parts of the San Gabriels above Pasadena, can see the wetter version, where the downdraft arrives with the rain still falling rather than having evaporated away.
A secondary, less frequent window opens during the cool season, when a strong Pacific storm brings enough instability to spark thunderstorms over the LA Basin itself, including areas near Downtown Los Angeles and the San Fernando Valley communities like Woodland Hills. These events are less common than the summer desert pattern and tend to occur alongside broader severe-thunderstorm ingredients rather than isolated single-cell storms. In every case, confirming that a specific wind-damage report was actually a microburst, rather than ordinary gusty outflow, straight-line thunderstorm wind, or a tornado, requires a National Weather Service field survey of the damage pattern. Treat any specific past event you read about online as unconfirmed unless it cites an official NWS survey or storm report.
Why Are Microbursts Dangerous to Aircraft?
A microburst is one of the most serious hazards in aviation because it flips the wind an aircraft experiences within seconds during the most vulnerable phase of flight. An aircraft flying into the outflow first meets a strong headwind, which briefly increases lift and can cause the aircraft to rise above its intended flight path. Moments later, as the aircraft passes through the core of the downdraft and out the other side, that same wind becomes a tailwind, and lift drops sharply just as the pilot may have already reduced power to compensate for the earlier headwind surge.
During takeoff or landing, when an aircraft is close to the ground with little room to recover, that rapid headwind-to- tailwind reversal, combined with the downdraft itself pushing the aircraft toward the runway, has caused fatal accidents.
Ted Fujita's research into this exact problem led directly to the airborne and ground-based wind shear detection systems now standard at major airports, including Doppler weather radar and Low-Level Wind Shear Alert Systems that warn pilots and air traffic controllers of sudden wind shifts near the runway. Los Angeles International Airport and other major Southern California airports operate under FAA and NWS wind shear protocols, and any detailed operational guidance on how a specific airport handles a microburst alert should come from those official sources rather than general explainers. The broader category of sudden wind change with height, of which a microburst is one cause, is covered in more depth in what is wind shear.
How Is a Microburst Different From a Tornado?
The core distinction is motion. A tornado is a rotating column of air, with wind converging inward and spiraling upward around a central axis. A microburst has no rotation at all: it is a column of sinking air that diverges outward from a central point once it hits the ground, more like water from a hose hitting a floor and spreading in every direction than like a spinning funnel. That difference shows up clearly in the damage pattern a survey team finds afterward. Tornado damage tends to show debris thrown in curving, converging paths consistent with rotation, sometimes with damage severity increasing toward a narrow central track. Microburst damage typically shows debris and fallen trees pushed outward in straight, diverging lines from a central point, occasionally called a starburst pattern.
Despite that clear mechanical difference, telling the two apart from a single photo or a homeowner's account is genuinely difficult, and wind speeds can overlap enough that eyewitnesses reasonably confuse one for the other. That is why the National Weather Service sends survey teams to examine the actual fall pattern of trees and structural damage before classifying an event, rather than relying on how fast or scary the wind felt to someone on the ground. For a fuller comparison of tornadoes, waterspouts, microbursts, and dust devils, including how each forms and where each is most likely in the region, see can tornadoes hit Southern California.
What Warning Signs and Products Matter?
The clearest visual warning is a collapsing rain shaft, a curtain of rain visibly falling out of the bottom of a thunderstorm cloud and appearing to accelerate toward the ground, sometimes fanning outward just before or as it hits. Under drier storm bases common in the Mojave and Antelope Valley, watch instead for virga trailing beneath a cloud followed within moments by a sudden wall of blowing dust racing outward from a point on the horizon, often with little or no rain visible at all. On radar, meteorologists look for a rapid outward bulge in the reflectivity or velocity data beneath a storm cell, though radar detection of a small, brief microburst can lag behind what is already happening at the surface.
The most important official product to watch is a Severe Thunderstorm Warning from NWS Los Angeles/Oxnard or NWS San Diego, since microburst-strength wind gusts are one of the criteria that trigger that warning. If you are outdoors, at an event, or hiking in open desert or mountain terrain and see a rain shaft collapsing or a dust wall approaching, move to sturdy shelter immediately, stay away from trees and freestanding structures that can topple in a sudden gust, and avoid open ridgelines or exposed areas until the storm passes.
Drivers should be aware that a microburst crossing a highway can produce a sudden, blinding wall of dust or rain combined with a severe crosswind gust, a genuine hazard on desert highways during monsoon season. The NWS lightning safety guidance is also directly relevant, since any storm capable of producing a microburst is also producing lightning, and the same shelter-seeking response applies to both hazards. For a broader rundown of how to read gusts and warnings across the region beyond thunderstorm downdrafts, see wind hazards in Los Angeles, and for why Southern California sees relatively few thunderstorms to begin with, see how do thunderstorms form and why LA sees so few.
Microbursts are brief, usually lasting just a few minutes at full intensity and dissipating within 5 to 15 minutes of hitting the ground, but a few minutes is enough to snap trees, damage roofs, and put an aircraft or a driver in real danger. The mechanism is simple once you see it: sinking air with nowhere to go but out. Southern California's desert thunderstorm season and its occasional strong cool-season storms both produce the ingredients, so before heading into open desert, mountain trails, or a summer outdoor event during monsoon season, check radar, lightning activity, and any active Severe Thunderstorm Warning on WeatherEscape's Los Angeles forecast pages, and treat a collapsing rain shaft or a sudden wall of dust as your cue to get to shelter right away.