Of all the scary bad weather that can befall your community, hurricanes and tornadoes fill people with dread, but what about derechos? They might not trigger the same feelings of panic, but these rare, powerful storms are worth paying attention to.
Weather is complicated, but one important thing to know about storms is most of the time they are the product of interactions between areas of warm, moist air and cool, dry air. Depending on how these two types of air collide, they can create a variety of different, outrageous storms. One of those is a derecho.
It's a rare type of thunderstorm complex that consists of linear bands of bow-shaped thunderstorms at least 240 miles (386 kilometers) long and producing winds of at least 58 miles per hour (93 kilometers per hour), but with gusts that can reach over 100 miles per hour (161 kilometers per hour). The bands of storms hit a particular area within no more than three hours of each other. Derechos are uncommon, but the United States sees about one derecho a year, on average — usually in the Midwest. They have also been observed in Europe, Asia and South America.
Unlike normal thunderstorms, in which warm, moist air on the ground rises, creating an "updraft" that creates those big cumulonimbus clouds we associate with thunderstorms, a derecho creates a "downburst." This happens when a thunderstorm, already doing its thing, moves into an area of dry air. This dry air evaporates the moisture in the storm and rapidly cools it down, driving this denser, cooler air toward the ground and creating the powerful winds derechos are famous for.
The storm itself becomes a wind engine, pulling in more and more dry air, creating long bands of strong, destructive winds that shoot out ahead of the system, creating a sort of phalanx of bow-shaped storms that can travel fast over hundreds of miles.
Some of the highest wind gusts ever recorded in a derecho occurred in June 2012. Beginning in the Midwest, it traveled more than 700 miles (1,126 kilometers) through the Ohio Valley in just 12 hours, fizzling out when it hit the Atlantic Ocean around Washington, D.C. This storm was responsible for the deaths of 22 people and more than 5 million power outages from the Chicago area to the mid-Atlantic Coast.