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Derechos
Fri May 14, 2004
Listen in RealAudio 
We associate many high wind events with severe thunderstorms such as microbursts
and tornados. Another less familiar result is the derecho. Hi, I'm Bryan Yeaton for The
Weather Notebook.
Derechos emerge from long-lived thunderstorm complexes as violent, widespread
windstorms. They produce damaging, straight-line winds as they rush along paths
hundreds of miles long and tens of miles wide.
By definition, a derecho produces winds in excess of 58 mph over a trajectory
exceeding 280 miles. Its damaging winds, often gusting to 155 mph, may blow for
several hours. Thus, the dangers associated with derechos arise from both the
strength and duration of the wind. A derecho's real extent rivals that of a
hurricane.
Although you may not have heard the word before, Gustavus Hinrichs coined the term
back in the 1880s. Since derechos are straight-line winds, he named them using a
Spanish word meaning "straight ahead" to contrast with a tornado's circular winds.
Derechos are weather phenomena known collectively as mesoscale-convective
systems -- large, organized weather systems comprised of a number of individual
thunderstorms that typically arise in the late afternoon or evening.
Meteorologists have distinguished two derecho types, progressive and serial, that
differ in their formation process and season of occurrence. Progressive derechos
generally emerge during spring and summer, spawned by strong solar heating, and
account for about three quarters of all derechos. Serial derechos are most often
associated with intense low-pressure systems forming in the cold seasons.
Derechos most commonly sweep down the Central and Northern Plains across the
Midwest into the Ohio Valley, but can occur in the Northeastern and Mid-Atlantic states
as well as Southeastern Canada. Roughly a dozen derechos are observed across the
United States every year.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook
is generously funded by the National Science Foundation and Subaru of America.
Today's Links
Syracuse Labor Day Storm of 1998
http://www.syracuse.com/features/labordaystorm/stories/19980913_cfweath.html
The Derecho
http://www.suite101.com/article.cfm/science_sky/99905
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