Summer in the Midwest often brings a variety of weather events, but few are as destructive as a phenomenon known as a “derecho.”
This mass of straight-line winds can cause extensive damage, tearing down buildings, flattening farm fields, and leaving thousands of people without power.
According to the National Weather Service (NWS), a derecho is a widespread, long-lasting wind storm linked to fast-moving showers and thunderstorms.
A derecho can produce wind speeds similar to those seen in tornadoes, but it does so in a single direction along a “straight swath,” according to the NWS.
To be classified as a “derecho,” the wind damage swath must extend more than 240 miles and must include wind gusts of at least 58 miles per hour along its length.
NWS data indicates that derechos are most common from May to July, when nearly two-thirds of such events have historically occurred.
Derechos form with what is known as a “bow echo.” Thunderstorms undergo “updrafts” early in their formation, where warm surface air rises until condensation starts, eventually turning into rain.
This forces cool air toward the ground at the back end of a storm, which generates strong winds near the ground, according to researchers at South Dakota State University.
As the line of storms intensifies, updrafts continue at the storm’s edge, and the mass of rain-cooled air at the surface expands the storm horizontally, pushing it forward faster and creating higher wind speeds. In certain cases, these winds can sustain for hundreds of miles, becoming known as derechos.
According to the NWS, Illinois experiences about one derecho each year, with slightly higher occurrences in areas that include parts of Kansas, Missouri, Oklahoma, and Arkansas.
It is not yet clear if Monday’s severe weather in the Chicago area was part of a derecho, but the National Weather Service will release its findings in the coming days.