Wednesday night’s storm, which stretched more than 700 miles from eastern Iowa to northern Virginia, produced more than 1 million lightning events over 18 hours — more than 236,000 of them occurring in Illinois and over Lake Michigan, and is blamed for a house fire in Evanston.
Since 1989, every stroke and flash of lightning that happens in the continental U.S. has been recorded in real time by the Vaisala-owned National Lightning Detection Network, based in Tucson, Ariz., which monitors lightning activity 24 hours a day, 365 days a year. This includes the time, location and polarity of cloud-to-ground lightning and cloud pulses, which can stay in a cloud or be connected to cloud-to-ground lightning.
Vaisala estimates the Chicago area had 12-20 flashes of lightning per square mile each year from 2005-14. During the same time period, Illinois had the eighth-highest cloud-to-ground lightning flash density among U.S. states at 14.2 per square mile. Florida had the most — 21 per square mile.
Ron Holle, a lightning expert and Vaisala meteorologist, said Wednesday night’s storm was a derecho. That’s a widespread, thunderstorm-induced event that can cover up to thousands of miles, have wind speeds of up to 100 mph and last for 24 hours or longer. Derecho is the Spanish word for “straight ahead,” which refers to its wind direction (as opposed to rotary winds). It was first used as a weather term in 1878 by Iowa meteorologist Gustavus Detlef Hinrichs and appeared in a March 30, 1890 Chicago Tribune article on tornado prediction.