Bob Henson | July 2, 2012 • With a ferocity to match the record
heat it displaced, a thunderstorm complex raced from Illinois to
the Delaware coast in a mere 12 hours on Friday evening, June
29. It knocked down countless trees and power lines, with wind
gusts topping 80 miles per hour in many spots. It threw millions
of people into turmoil, with air conditioners, computers, and
phones out for days. And it brought to light a weather word du
jour with an obscure but intriguing history. This storm complex
was a derecho (pronounced deh-REY-cho). It’s a phenomenon too
infrequent to be familiar, but too dangerous to be ignored. A
derecho is akin to the gust fronts we commonly experience when a
thunderstorm arrives, except it plays out in far more
spectacular fashion. While the high winds of a tornado or
hurricane spin around powerful, circular updrafts, a derecho’s
wind consists of rain-cooled air that descends and plows into
very warm, unstable air. Most such downbursts only span a few
miles and last a few minutes