What Is a Microburst?

A microburst is a small damaging windstorm in which a column of sinking air (downdraft) descends from the core of a thunderstorm and plummets earthward, creating an outflow of high-speed winds. If the descending air is accompanied by heavy rain or hail, the storm is referred to as a “wet” microburst. If the precipitation evaporates before reaching the ground, it’s referred to as a “dry” microburst.

As their name suggests, microbursts are fairly small in size and span an area less than 2.5 miles wide. They’re also short-lived, generally lasting less than 5 minutes. But don’t let any of that fool you — according to the National Weather Service, their wind speeds can reach up to 100 miles per hour, which is similar to that of EF0 and EF1 tornadoes. As a result, microbursts pose a significant danger to life, property, and aviation. 

Microbursts and Other Types of Winds

Microbursts don’t occur in all thunderstorms — only in those whose updrafts (rising columns of air that feed a storm’s growth by funneling warm, moist air into it) are strong enough to suspend raindrops and hailstones in the middle and upper parts of a storm cloud. (Normally, gravity overtakes an updraft’s strength, causing rain and hail to fall out of a storm cloud.) Eventually, drier air outside of the thunderstorm enters into it, and when this drier air meets the storm’s moist air, evaporational cooling occurs. Because cooler air is denser, it sinks downward, creating a downdraft that, in turn, weakens the updraft. As the updraft weakens, it can no longer hold large amounts of precipitation, so the rain and hail plummets earthward and drags a lot of downdraft air along with it. When the downdraft hits the ground, it becomes a “downburst,” and its air rushes outward in all directions (much like a stream of water running out of a faucet and hitting the sink basin). 

Microbursts are only one type of damaging thunderstorm-related wind. 

Macrobursts are nearly identical to microbursts, except they span a slightly larger area of more than 2.5 miles. Their damaging winds also last longer, from 5 to 20 minutes.

Like microbursts, derechos are another type of downburst. However, these windstorms occur when downbursts suck additional dry air into a thunderstorm, triggering clusters of additional downbursts that measure over 200 miles wide.  

Microbursts are often compared to tornadoes, but while both events are linked to severe thunderstorms, there’s a major difference between the two: The motion of their winds. Microbursts are straight-line winds — winds that travel horizontally along the ground — whereas tornadic winds rotate, or move in many directions.

How Microbursts Are Predicted

Microbursts affect such small areas and occur so suddenly that it’s tricky to predict them in advance. Meteorologists can, however, predict when conditions might be favorable for microburst formation by monitoring the upper atmosphere on severe weather days. If there is instability, dry air at mid-levels, and strong winds aloft, it can signal the potential for microbursts.

Microbursts can also be detected on weather radar. At their onset, the downbursts appear as converging air streams (air that comes together) located in the middle section of a thunderstorm, whereas, after reaching the ground, they appear as diverging air streams (air that moves apart). If forecasters notice these patterns early enough, they’ll issue a severe thunderstorm warning; however, there isn’t always sufficient time to do so since microbursts can form and dissipate within a span of 5 minutes. 

Where Do Microbursts Occur?

Microbursts can occur anywhere in the United States. That being said, dry microbursts are generally more common in drier climates, like those found in the western United States and in the High Plains region. Wet microbursts are more common east of the Rockies, especially in the southeastern United States, which is prone to thunderstorms. 

Similarly, microbursts can occur at any time of year and time of day, but they're common in summer and spring, and during the afternoon and evening hours, since this is when thunderstorms most frequently occur. (One notable exception is a rare, pre-dawn dry microburst reported near Denver, Colorado in August 2020.)