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By Climatologist Cliff Harris

Article published on July 12, 2012

I promised several CDA Press subscribers including Ken Royal of Athol, Dennis Williams of Hayden and Betty Larsen of Post Falls, that I would attempt in this article to give an accurate description of the meteorological phenomenon known as a ‘derecho’ (deh-REY-cho).

According to the National Oceanic and Atmospheric Administration (NOAA), a derecho is a widespread violent windstorm that is usually associated with a strong band of thunderstorms accompanied by torrential downpours, large-sized hail and, at times, even deadly tornadoes.

The term "straight-line wind damage" is given to the disastrous consequences of a derecho like the one at the very end of June that left millions without power from Ohio eastward through the Mid-Atlantic states, in some cases, for more than 10 days.

Unlike a hurricane, which can be tracked for days or even weeks prior to its impact, the recent derecho gave no warnings of its arrival. It came ‘like a thief in the night.’

Randy Mann’s sister, Susan Ayers, who lives in Annapolis, Maryland, was effected by the extremely damaging derecho that moved into the Mid-Atlantic region at the very end of June.

Susan told Randy what happened at that time. "We were 5 days without power, which means, in addition to no electricity, and no air conditioning in 97 degree plus heat, that those of us with wells and septic tanks had no water. Therefore, no showers! Smelly!

The power generated by our portable generator was used for keeping the refrigerator and freezer going, and plugging in and out small appliances, such as a microwave, a coffee pot and a couple of hairdryers.

After 5 full days, I gave up and went to our marina for a shower. Then we went boating, and Mark (my husband) washed in the bay, with a bar of soap!"

By definition, if the wind-damage swath extends for more than 240 miles and includes gusts of at least 58 miles per hour or more, then the event may be classified as a derecho.

Following the recent derecho, power crews from as far away as Florida and Oklahoma assisted local power crews helping to get the power back on after massive outages like the one in Maryland.

Derechos in the U.S. are most common in the late spring and summer seasons from May through August. In the Midwest Corn and Soybean Belt, hit this summer by the worst crop damage since at least 1988 due to extreme heat and drought, a damaging derecho usually strikes at least once or twice a year.

Elsewhere, east of the Rockies, a derecho hits other areas approximately every three or four years, including the East Coast. West of the Continental Divide, derechos are very rare, occurring less than once a decade.

The term ‘derecho’ was coined in 1888 by Dr. Gustavus Hinrichs, a physics professor at the University of Iowa. Dr. Hinrichs decided to use the word derecho to define various non-tornadic events. In Spanish, derecho means ‘direct or straight ahead.’ The definition was published later in 1888 in the American Meteorological Journal, so it’s been around for nearly 125 years. Despite this fact, most people had never heard the term ‘derecho’ until this past destructive event. Thank you, Dr. Hinrich. (Better late, than never.)