SCALE

620 miles
or
1000 km

62 miles
or
100 km

6.2 miles
or
10 km

0.6 miles
or
1000 meters

Drs. T. Theodore Fujita and Roger Wakimoto developed terminology for the varying scales of strong outflow winds produced by convective storms (typically thunderstorms). The wind scale terminology was linked to the damage patterns asso- ciated with a bow echo system that moved across the southern Great Lakes region on the morning of July 16, 1980. The figure on the left shows the terms assigned to four scales of outflow winds and the associated damage patterns (Modified from Fujita and Wakimoto 1981).

In 1987, when Robert Johns and William Hirt revived the 19th century term "derecho" to describe long-lived con- vective windstorms, they equated the term with the scale of winds known as a "family of downburst clusters". Thus, to qualify as a "derecho" the series of microbursts and downbursts must be greater than 240 miles (400 km) in length.