Thunderous line-dancing


Every so often a new word appears in the lexicon of meteorology. Until about a decade ago, for example, few meteorologists had ever heard of a derecho, and even today, many outside the United States would be unfamiliar with the term. But the word is now widely accepted as describing an American mid-west phenomenon which surpasses even the tornado in its sheer destructive power.

If the term is unfamiliar, there is nothing new about the derecho itself. As 19thcentury settlers trekked westward across the plains, the "great blow of the prairie", and the praeces et lacrimae that it induced, was legendary. Then, during the 1880s, one Gustavus Hinrichs, director of the Iowa Weather Service, coined the name by which it is still known.

Derecho, pronounced "day-RAY-cho", comes from a Spanish word which seems to mean both "right" and "straight ahead". In its former guise it has been borrowed as a name by several groups of human rights enthusiasts in South America.

In meteorology, however, it describes a "straight-line windstorm", a long ribbon of storm force winds which occurs when dozens of thunderstorms arrange themselves in a line - sometimes several hundred miles long - and then performs a tempestuous, destructive linedance. Thunderstorms arranged like this combine to acquire a very awesome synergy. The strong winds have their origin in the vicious "downbursts" that are characteristic of a thundercloud - strong jets of air that surge earthwards through the cloud's core, and which, when they hit the ground, spread radially in all directions like a jet of water from the tap when it encounters the bottom of the kitchen sink. It is not uncommon for a derecho to produce winds in excess of 100 mph; it cuts a swathe of damage across a landscape perhaps 800 miles long and more than 150 miles in width, during a lifetime of some 16 hours or more. To make matters worse, most derechoes also spawn tornadoes - destructive as always, even if their existence is difficult to detect in the surrounding mayhem.

Statistics suggest that some 15 to 20 derechoes surge through the mid-west of the United States every year during late spring and summer. One of the most destructive of recent years was that which hit Wichita in Kansas on June 19th, 1990, during which winds of 116 mph were recorded - as strong as many a tornado; it felled trees, ripped many buildings apart, toppled more than 1,000 electricity poles, and caused, in total, more than $50 million worth of damage.