Tornadoes and Other Whirling Winds

Abstract

If the reader will excuse some wanderings down memory lane, let me recall an early spring afternoon when the writer was just a child living with his parents on the Central Coast of New South Wales.

Appendix: The Fujita and Enhanced Fujita Scales of Tornado Damage

The Fujita Scale was derived, by Tetsuya Fujita of the University of California, in 1971 to help researchers determine the strength of tornadic winds necessary to cause different levels of damage. Principally, it is a scale of wind damage rather than wind strength per se but from assessment of the degree of damage, wind velocities can be derived. The minimum wind speed for each step in the scale may be calculated by;

• V  =  14.1(F  +  2) ^1.5 miles per hour, where V is wind velocity and F is the scale value. The answer can be multiplied by 8/5 to convert to kilometers per hour.

The Fujita Scale is as follows;

• F0 Weak Tornado (40–72 mph; 64–116 kph). Light damage. Signboards damages; some windows broken; twigs snapped of trees and shallow-rooted trees pushed over.

• F1 Moderate Tornado (73–112 mph; 117–180 kph). Moving automobiles pushed off road; light trailers overturned. Outbuildings demolished and trees broken.

• F2 Strong Tornado (113–158 mph; 181–253 kph). Roofs torn from frame houses; mobile homes destroyed and automobiles blown off roads. Large trees snapped or uprooted.

• F3 Severe Tornado (159–207 mph; 254–332 kph). Walls torn from frame houses; locomotives derailed; automobiles lifted off ground. Most forest trees uprooted.

• F4 Devastating Tornado (208–262 mph; 333–419 kph). Frame houses destroyed; bark stripped from trees; automobiles and locomotives thrown and rolled; trees in forest uprooted and carried some ­distance.

• F5 Incredible Tornado (263–320 mph; 420–512 kph). Whole frame houses tossed from foundations; automobiles thrown through air and asphalt torn from roads. Trees debarked.

No tornado has been recorded as greater than F5, although the one in Oklahoma on May 3, 1999 packed wind strengths as high as 318 mph at its strongest; just shy of the “unbelievable” F6 rating. As there is no reason to think that this was the most violent tornado ever, it is very likely that tornadoes of F6 rating have occurred at rare intervals and it is not unlikely that some that have been rated toward the top end of the F5 category have actually fallen over the line into the low F6 category.

More recently, the scale has been updated by a team of U.S. meteorologists and wind engineers. This Enhanced Fujita Scale was implemented in The U.S. on 1st February, 2007. This enhanced scale is based upon the level of damage caused by three-second wind gusts. Wind speed within the gusts is estimated according to eight levels of damage to 28 types of structure, e.g. homes, automobiles, trees, towers and so forth. The Enhanced Scale is as follows;

• EF0 65–85 mph (104–136 kph)

• EF1 86–110 mph (137–176 kph)

• EF2 111–135 mph (177–216 kph)

• EF3 136–165 mph (217–264 kph)

• EF4 166–200 mph (265–320 kph)

• EF5  >200 mph

No value greater than 5 exists on the EF Scale, unlike the original F Scale which is, in theory at least, open ended.