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The Derecho

© Keith C. Heidorn

On May Day, solar summer begins — that quarter of the year when solar heating reaches its maximum potential in the Northern Hemisphere — and therefore, April's showers become bold thunderstorms.

We associate many storm elements with severe thunderstorms. Lighting and thunder, gusty winds, hail, flash floods, and tornados are the most well-known features, but we cannot forget downbursts — and their little brothers, the microbursts — gustnadoes, heatbursts and derechos.

The derecho (pronounced Deh-RAY-cho) is a violent, widespread windstorm emanating from long-lived thunderstorm complexes. Derechos produce damaging, straight-line winds of 100 to 160 km/h (60 to 100 mph) that are strong enough to down trees and power lines. They rush across a region along paths tens of kilometres (miles) wide and hundreds of kilometres (miles) long. Derecho damage is continuous and non-random over a large area.

Gustavus Hinrichs coined the term in the 1880s when he served as Director of the Iowa Weather Service. Derechos are straight-line winds in contrast to the circular winds of tornadoes. For this reason, Hinrichs named them from a Spanish word meaning "straight ahead" as a contrast to "tornado," which derives from the Spanish for "turning."

To be classified as a derecho, the storm must produce winds in excess of 93 km/h (58 mph) over a path at least 450 km (280 miles) long. When a derecho strikes, its damaging winds, often gusting to 250 km/h (155 mph), may blow for several hours. Thus, the dangers associated with derechos arise from both the strength and duration of the wind. Derecho widths vary from 80-480 km (50-300 miles), thus, they are also large spatial events, affecting areas as great as 2000 km² (780 mi²), rivalling the area covered by a hurricane.

Much of our current understanding of the nature of derechos and their formation has only been gained in the past couple of decades. Recent advances in weather science and technology, such as doppler radar and detailed satellite imagery, have enabled meteorologists to peer into the core of severe storms and unravel many of their secrets.

Derechos are members of a group of weather phenomena known as mesoscale-convective systems (MCSs) — large, organized weather systems comprised of a number of individual thunderstorms. MCSs evolve from isolated thunderstorm cells or from a small cluster of thunderstorm cells, but at times, they simply burst forth fully formed from the outset. MCSs typically arise in the late afternoon or evening, lasting from two to twenty hours. They are associated with regions of very heavy rains and often spawn squall lines, bow echos and derechos. MCSs are most common in the Plains, Midwest and Ohio Valley States. About 80 percent of

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