What is a derecho, and why is it so destructive?

In the right conditions, walls of wind made up of several thunderstorms can blow across hundreds of miles in just hours.

Thursday, August 13, 2020,
By Amy McKeever
Cars fleeing a derecho on May 30, 2012. Primarily seen in the central and eastern United ...

Cars fleeing a derecho on May 30, 2012. Primarily seen in the central and eastern United States, these unusual storms create walls of wind that streak for hundreds of miles at high speeds.

Photograph by Dave Chapman, Alamy

Derechos may not be as well known as hurricanes or tornadoes, but these rare storms can be just as powerful and destructive. Primarily seen in late spring and summer in the central and eastern United States, derechos produce walls of strong wind that streak across the landscape, leaving hundreds of miles of damage in their wake. On August 10, 2020, a derecho swept across the Midwest from South Dakota to Ohio, travelling 770 miles in 14 hours and knocking out power for more than a million people.

The term derecho—which means “straight ahead” in Spanish—was coined in 1888 by Gustavus Hinrichs, a physics professor at the University of Iowa who sought to distinguish these straight-moving winds from the swirling gusts of a tornado. Though the term disappeared from use shortly afterward, meteorologists at the National Oceanic and Atmospheric Administration (NOAA) resurrected it a hundred years later. It entered the public lexicon in 2012, when one of the most destructive derechos in history swept across roughly 700 miles from Ohio to the mid-Atlantic coast, killing 22 people and causing serious damage in metropolitan areas, including Chicago and Washington, D.C.

NOAA officially defines a derecho as “a widespread, long-lived windstorm that is associated with a band of rapidly moving showers or thunderstorms.” For a swath of storms to be classified as a derecho, it must travel at least 240 miles and move at speeds of at least 58 miles an hour, though the winds are often more powerful. The August 2020 Midwest derecho had winds up to 112 miles an hour.

How derechos form

Normal thunderstorms occur when warm air rises from the surface of Earth into colder air in the upper atmosphere. This cools the air to its dew point, the temperature at which water vapour condenses into droplets, which causes clouds to form. The cooled air drops back to the surface, where it warms up again and starts the process over, generating further convection and ultimately causing a thunderstorm.

The steeple at College Church in Wheaton, Illinois, was toppled during a derecho on Monday, August 10, 2020, which also left several trees in the nearby park heavily damaged.

Photograph by Mark Welsh, Daily Herald/AP

A storm’s downdraft—when the cooled air drops back down to the surface—is key to producing the powerful winds that create derecho conditions. When the cool air hits the ground, it spreads out in all directions, pushing warm air near the surface into a front of gusty wind. Stretching from four to six miles across, downbursts also suck more air into a storm, causing it to strengthen.

When a cluster of strong downburst winds in the center of a storm races ahead of the rest of the storm, it creates what’s known as a bow echo. This bowing of the storm front forces even more warm air into the atmosphere, intensifying the thunderstorm. When a bow echo—or a series of bow echoes—moves across more than 250 miles with wind gusts of more than 58 miles an hour, it can officially be classified as a derecho.

Where derechos occur

Derechos have been documented in other parts of the world, including Eastern Europe and South Asia. In 2002, a derecho over eastern Germany killed eight people and injured 39, hitting Berlin the hardest.

But while derechos are a global phenomenon, they primarily occur across the central and eastern United States, which see an average of one to two of these storms per year, compared to more than a thousand tornadoes that churn across the country each year. These straight-ahead storms most commonly form in the late spring and summer, when high pressure weather systems—whirling masses of descending air—move north from the tropics into the U.S. Some derechos, however, occur during cooler weather and are most likely to form in the region stretching from Texas across the Southeast.

In May 2009, a “Super Derecho” pushed gusts of wind up to 106 miles an hour from the plains of Kansas to eastern Kentucky. Though many described it at the time as an inland hurricane, this storm system was in fact a derecho with several small tornados embedded within its winds.

Derecho damage

A derecho can be as destructive as a tornado, but it is destructive in a decidedly different way. The strong, swirling winds of a tornado will cause debris to fall every which way, while a derecho’s straight-line winds are similar to a regular thunderstorm—but stronger.

Downburst clusters of wind caused by derechos can range from four to six miles long, containing smaller pockets of extreme wind called microbursts and burst swaths. The latter are only about 150 to 450 feet long, but they are severe and concentrated like the winds of a tornado and can exceed 100 miles an hour. After a derecho, pockets of massive destruction can sit next to areas that escape relatively unscathed.

Derechos can cause protracted power outages, such as the June 2012 storm that knocked out power for five million people from Chicago to the mid-Atlantic. But these rapidly moving winds pose a particular risk to anyone who happens to be outdoors, such as campers and hikers. Derechos move quickly and offer little notice of their arrival, giving people hours or minutes to seek safety. Thunderstorms can also rapidly evolve into derechos, making it all the more important to take storm warnings seriously.

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