When it comes to windstorms, tornadoes receive the most media attention for their destructive nature, spinning in cyclonic fashion across the flatlands of the Midwest, lapping up gravel and detritus — even homes and vehicles — and spitting them out into scatter shots of debris. Yet, straight-line windstorms cause more injuries, deaths and destruction annually. Chief among these storms are derechos.
Derechos are fast-moving, linear winds that can race across hundreds of miles, tearing down nearly everything in their path. Tornadoes typically spin in a range of just a few miles.
The word derecho comes from Spanish, meaning “straight.” These winds are produced by thunderstorm squall lines and can whip up into speeds of 58 miles per hour or more. A tornado by comparison can have rotating winds of more than 250 miles per hour, but their average forward-moving speed is just 30 miles per hour. Both tornadoes and derechos are born from thunderstorms.
When a downburst of wind from a thunderstorm hits the ground it spills out laterally and into straight lines. These are the makings of a derecho. The wind speed builds as the squall line moves ahead pushing winds along. This continuous path is how derechos come to barrel their way for 240 miles or more.
In June 2012 a super derecho pushed its way along an 800-mile path from the Upper Midwest through the Mid-Atlantic States and caused 28 deaths, as well as some $3 billion worth of damage.
Meteorologist can usually spot a derecho before or as it forms, but typically there isn’t enough time to warn people in its path because derechos take shape so quickly.
On radar, a squall of thunderstorms appears in the shape of an archer’s bow. This is the first piece of evidence that a derecho may be forming. These bow echo storms concentrate dangerous winds at the center of the curved line formation. If the right type of conditions persist, such as high temperatures, derechos march forward, picking up speed as they go.
Take the 2012 Super Derecho. It began as a small thunderstorm in central Iowa. Record-setting heat that month, however, began to fuel the storm. Thunderstorms muscle up heat into updrafts and downdrafts.
Chugging along into Illinois, the would-be derecho began to strengthen. It narrowly missed hitting Chicago, but lapped up even more heat surrounding the city’s famous “urban heat island,” where temperatures downtown soar on account of blacktops and dark roofs locking in the sun’s rays.
Next, the flat terrain of Indiana gave the derecho the leeway it needed to shift into overdrive, and it began to take its bow shape. By the time the storm reached Ohio it had scaled to Super Derecho status, with wind gusts exceeding 80 miles per hour. From there it zoomed through West Virginia, knocking down trees, and taking out power in Virginia before crashing through Washington, DC and Maryland, where it caused more death and destruction, until heading out to sea.
Derechos die when dry air in the upper atmosphere quash their power, or when the winds pushing it along calm. The cool air of the Atlantic Ocean calmed that particular Super Derecho’s winds.
NOAA calls derechos severe and potentially deadly. If you hear their seemingly strange name, pay close attention to warnings and advisors. Treat them as you might a tornado, and head quickly for sturdy structures and basements, or storm shelters, if they are an option.
Thomas M. Kostigen is the founder of The Climate Survivalist.com and a New York Times bestselling author and journalist. He is the National Geographic author of "The Extreme Weather Survival Guide: Understand, Prepare, Survive, Recover" and the NG Kids book, "Extreme Weather: Surviving Tornadoes, Tsunamis, Hailstorms, Thundersnow, Hurricanes and More!" Follow him @weathersurvival, or email firstname.lastname@example.org
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