Measuring Up

The Beaufort, Fujita, and Saffir-Simpson scales all share one thing in common. They all measure strength (intensity) of winds based on empirical observation of the effect of the wind on land or sea surfaces. The oldest of these, the Beaufort Scale, was the first time wind strength was quantified in an orderly, consistent format that anyone can use easily.

The Beaufort scale is a system used to estimate and report wind speeds. Invented by British Admiral Sir Francis Beaufort in 1806, it was originally based on the effect of various wind speeds on the amount of canvas that a fully rigged frigate of the era could carry. It has been modified and modernized several times since it was first introduced and now includes three elements, wind speed, descriptive terms for wind speeds, and descriptions of wind effects on land objects or sea surfaces.

The scale ranges from 0 to 12; 0 represents a dead calm and 12 represents hurricane force winds. For example, a Beaufort rating of 4 at sea would be a wind speed of 11-16 knots in a moderate breeze. Small waves become longer with fairly frequent white horses (foam on the wave tops). A fishing smack would have a good working breeze; under all sail heels considerably. On land, the wind would be 13-18 mph (note the change from knots to mph) in a moderate breeze that raises dust and loose paper. Small branches move.

The Fujita Scale, invented in 1971 by Dr. Tetsuya (Ted) T. Fujita and Dr. Allen Pearson, measures the intensity and likely wind speed of tornadoes. It does this by assigning an "F Scale" (Sometimes known as an FFP Scale) number by the estimated wind in the tornado. Damage a tornado produces is not a good criterion because damage depends on where and when it touches down. An F0 is the weakest tornado and an F5 is the strongest. The F Scale defines several parameters for tornado intensity, but the main criterion is wind speed. An F2 or F3 tornado has maximum winds of 100-200 mph, a life span of 3-20 minutes, a path width of 150-500 yards, and a path length of 1-10 miles. These storms cause less than 4 percent of all tornado deaths.

The Saffir-Simpson Scale is the most recent of the three scales. It first appeared in public announcements in 1975 and measures the intensity of hurricanes. Developed by Herbert Saffir and Robert Simpson, it assigns hurricane strength ratings according to the amount of damage the storm produces or could potentially produce at landfall and is based mainly on wind speed. Storm strength can be rated from satellite or radar imagery or from reports from hurricane research aircraft long before the storm gets near land, but the potential damage is based on winds and flooding that could occur on the coast on landfall.

A hurricane assigned a Category 1 rating is the weakest, a minimal hurricane, and one assigned a 5 is the strongest, a catastrophic storm. A Category 2 hurricane has winds of 96-110 mph (83-95 knots), a storm surge 6-8 feet above normal high water, and causes no real damage to most structures but damages trees, shrubs, mobile homes, and poorly constructed signs and piers. Small watercraft in unprotected anchorages could break moorings and low-lying coastal areas will flood 2-4 hours before the storm center arrives.

All three scales are simple ways of defining the parameters they measure in a way that cam be easily understood by the lay public. Understanding them thoroughly so they can be used properly is the job of the weather professional. To review these scales, visit the following site: http://www.friesian.com/wind.html

Written by Melody L. Higdon, 14 WS/DOPA