In July, a fierce thunderstorm raged through Washington, D.C., damaging homes, felling trees, producing blinding rain and leaving thousands without power. The storm was the District’s worst since a derecho swept through the Midwest and Mid-Atlantic regions in 2012 (SN: 8/27/20). Was the new damage the result of another derecho or a tornado? No, said area meteorologists. This time the culprit was a down burst.
These lesser-known products of severe storms first sparked U.S. public attention in the 1980s when one caused an airplane to crash near Dallas-Fort Worth International Airport, killing 137 people (SN: 3/21/87). Threats to aircraft have diminished thanks to more research on the phenomenon and greater monitoring of wind speeds at airports. But these fierce winds still pose a danger, evidenced by the damage left in the wake of severe storms that have battered the United States and parts of Europe this summer.
What is down bursts?
A down burst is an area of powerful winds produced by a storm with strong downward-moving air, known as a downdraft. When the downdraft reaches the ground, it slams into the surface and spreads out, sending winds outward in all directions like a water balloon dropped from the sky.
Every storm has a downdraft which sends cold air toward the ground and along the surface, says Charles Kuster, a research meteorologist at the National Severe Storms Laboratory in Norman, Okla. But for that downdraft to create a down burst, it needs to reach a certain velocity. There are a few different thresholds in use, but a common indicator is wind speeds above 93 kilometers per hour (58 miles per hour), which is also the wind speed that merits a severe thunderstorm warning from the National Weather Service.
Down bursts can come in two varieties: microbursts, which affect an area smaller than 4 kilometers wide, and macro bursts, which affect a larger area.
How do down bursts form?
Down bursts need two things to form: a favorable storm environment and a strong cooling mechanism. Humidity that increases with altitude and a large temperature difference between the storm and the surrounding environment both create favorable conditions for a down burst, as does high precipitation loading — the amount of rain or hail being held aloft by upward winds feeding into the storm.
The cooling mechanism can take a few different forms. Melting hail or evaporating rain in the atmosphere can cool the surrounding air because those processes require energy. Djordje Romanic, an atmospheric scientist at McGill University in Montreal, likens this cooling effect to stepping out of a hot shower. “You run across the room to get your towel, and you’re cold, but you just took a warm shower,” he says. “It’s possible because that water is evaporating, and evaporation takes energy” in the form of heat. The mass of dense, cooled air eventually becomes too heavy for the upward winds to keep it aloft and it falls to the earth, creating a down burst.
Wild wind
Down bursts are an area of powerful winds produced by a storm with strong downward-moving air, known as a downdraft. Down bursts smash into the ground and radiate outward. Wind speeds when a down burst strikes can rival a weak tornado.
Anatomy of a down burst
Diagram showing wind flowing down and out from the cloud base during a down burst.
Potentially giving people in the affected area a few extra moments to prepare.
Will down bursts get worse with climate change?
It’s hard to say. Reports of down bursts have increased over the years, but Romanic cautions that much of that increase could be tied to improvements in detection via radar, and to the greater potential for damage as cities have grown. After accounting for those factors, there hasn’t been a significant increase in down bursts since the 1990s, he says.
But that doesn’t mean extra heat has no effect on down bursts. Anything that increases a storm’s energy increases the likelihood of severe winds, Kuster says. That includes higher temperatures and more humidity. As summers get hotter, the planet could see more storms with the potential to produce down bursts, researchers suspect.
For now, Kuster emphasizes that while down bursts may not be as common as a typical thunderstorm or as well-known as a tornado, they are still hazardous. “Down bursts are serious. Severe thunderstorm warnings are serious,” he says. “So when you go under a severe thunderstorm warning, make sure that you take shelter.”
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