Recent episodes of severe weather both here in southern Wisconsin and around the country have included reference to “derechos.” Forecasters worried a storm that passed through Wisconsin on June 11-12 could become a derecho, although it never became that severe.
This term, which is a Spanish word for straight, refers to a widespread, long-lived wind storm that is associated with a rapidly moving band of thunderstorms. Though a derecho can produce destruction similar to what is associated with tornadoes, the damage is produced by straight-line winds, not rotary winds.
Despite the fact that “derecho” seems to be a new word, it was introduced by Gustavus Hinrichs in 1888 in his description of a storm that crossed Iowa on July 31, 1877. Hinrichs was convinced not all damaging convective storms in the Great Plains were tornadoes, and he set out to prove it. Lt. John Finley, of the Army Signal Corps (which later became the National Weather Service), was a leading tornado expert at the time and was not convinced of Hinrichs’ theory.
Hinrichs proved to be right: These straight-line wind storms were early summer phenomena (late May, June and July), and could affect huge areas in a relatively short time.
He chose a Spanish word to name them, hoping this choice would provide a complementary term to “tornado.”
Around 1890, the U.S. Weather Bureau instituted a ban on use of the word “tornado” on the grounds that it incited panic within the populace. This ban was not removed until the early 1940s when “tornado” reappeared in Weather Bureau warnings and forecasts.
“Derecho” seems to have disappeared during that period and was resurrected in 1987.
Last year, on June 29, a major derecho event roared east from Chicago to Washington, D.C., leaving 5 million people without power and substantially raising the profile of derechos in the United States.