by Nayab Naseer

Created on: March 15, 2009   Last Updated: March 24, 2009

WHAT IS A DERECHO

"Derecho" is a Spanish word that means "direct" or "straight ahead". True to its etymology, a derecho is a straight-line wind associated with a band of rapidly moving showers or thunderstorms. Dr. Gustavus Hinrichs, a physics professor at the University of Iowa, in a paper published in the American Meteorological Journal in 1888 first used the name "Derecho" for such winds, to distinguish it from the circular winds of the tornadoes. Derechoes take different names outside North America, like "nor'wester" in Bangladesh and eastern parts of India.

To qualify as a derecho, the wind gust should be at least 58 miles per hour (92 kilometers per hour) at most points along the derecho path, with a length of at least 250 miles (460 kilometers). Strong derechoes however have a wind speeds that exceed 100 miles per hour (136 kilometers per hour).

HOW THEY FORM

Derechoes usually form during times of weather changes, when there is high atmospheric instability.

Hot air that rises from the oceans undergoes convection and forms cumulus clouds. When these clouds precipitate, some heat releases into the upper atmosphere. This warms the surrounding air, and produces an updraft. The updraft caused by the cooling water particles creates a low-pressure zone directly beneath the precipitation, pulling up more air from the surrounding regions. This pattern of rising and cooling continues until the thunderstorm reaches the mature stage.

Derechoes are members of Mesoscale-convective systems (MCSs), a large, organized weather system comprised of a number of individual thunderstorms. Derechoes arise from these extra-tropical MCSs when squall-line thunderstorms create a sustained series of downbursts as they advance. These cold downbursts blast outward from the squall line striking, then hugging the surface as the storm advances. These bands of showers or thunderstorms are often "curved" in shape, earning such storms the nickname "bow echoes". The derecho typically moves eastward along the front, veering toward the warm air mass to the south or right.

Derecho winds are at times further enhanced by embedded super-cells within the storm system that produced the derecho. Because of this, derechoes often occur in the same storm systems that produce tornadoes.

The winds associated with derechoes are not constant and may vary considerably along the derecho path, sometimes below the 57 miles per hour mark and sometimes being very strong, above 100 miles per hour. The name given to patches

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