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How 'F scale' Measures Tornado Strengths

June 6, 2011

By Phil Mulkins, Tulsa World, Okla.

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Shelters and safe rooms routinely are tested to withstand storms of various intensities. The force of a storm usually is measured on the "F scale."

The "Fujita Scale of Tornado Intensity" -- aka the "Fujita scale," the "Fujita-Pearson scale" or the "F scale" -- was the creation of Dr. Tetsuya Theodore Fujita. His lifelong research of severe thunderstorms, tornadoes, hurricanes and typhoons -- and his independent development of advanced techniques for their study -- led him to realize the need for a technical system to categorize tornadoes by their intensities and areas of coverage.

After becoming a professor of meteorology at the University of Chicago in 1953, he and his students visited tornado destruction sites all over the country. He proposed the F-scale rating system in a research paper he published in 1971 with Allen Pearson, then director of the National Severe Storm Forecast Center in Kansas City and now the Storm Prediction Center in Norman. The scale was adopted by the National Oceanic and Atmospheric Administration in 1973.

Here`s what you need to know about the F scale:

The F-Scale still is not the perfect system for linking damage to wind speed, according to The Tornado Project website, but it has distinct advantages over previous rating systems.

Fujita initially set out to form a system for designating tornado-event intensities as "weak," "medium" or "strong." It evolved into the six-category F-Scale system: F0 (gale), F1 (weak), F2 (strong), F3 (severe), F4 (devastating) and F5 (incredible). This was based on visual damage surveys and wind-speed estimates. It is still not possible to measure wind speed inside a moving tornado accurately.

In 1977, National Severe Storms Laboratory technical articles suggested wind speeds attributed to various F-scale rankings were too high. Weather scientists complained it was unlikely wind speeds much above 200 mph were possible near ground level or that they were required to inflict the worst of tornado damages.

In 1999, Texas Tech University`s Wind Science and Engineering Research Center proposed an official examination of the problem that could be used to create an enhanced F-scale meeting the needs of the "meteorological and engineering communities." A Fujita-Scale Forum of 23 scale users, led by Texas Tech`s Kishor Mehta and Jim McDonald, met in March 2001 and developed the "Enhanced Fujita Scale," publishing its recommendations in June 2004. The "Enhanced Fujita Scale (EF Scale) was implemented nationwide Feb. 1, 2007.

The Enhanced Fujita Scale includes 28 damage indicators ranging from (1) small barn or outbuilding to (28) trees, softwood. The softwoods include pine, spruce, fir, hemlock, cedar, redwood and cypress.

Playing the scale: No storm chaser should look out his car window and say, "We`re chasing us an EF-5 out here." The scale is not based on apparent width or even plain wind speed. Only weather scientists on the ground in the days following a tornado event can examine specific "damage indicators" and issue an EF designation.

For most consumers, it`s just best to remember that the Enhanced F-scale still is a set of wind estimates (not actual measurements) based on damage. If you`re shopping for a storm shelter, look for those which have been tested to take the highest amount of punishment. The Enhanced Fujita Scale takes into account the wind doesn`t always blow the same speed, from one second to the next. Each EF ranking considers three variable wind speeds: EXP ("expected") the average of LB (lower bound) and UB (upper bound) speeds, measured over a 3-second period.

Operational EF Scale:

EF-0: 65 -- 85 mph

EF-1: 86 -- 110 mph

EF-2: 111 -- 135 mph

EF-3: 136 -- 165 mph

EF-4: 166 -- 200 mph

EF-5: over 200 mph One of the primary damage indicators in tornado analysis is the "family residences" category. Damage escalation in homes is observed through 10 stages:

No. 1: "threshold of visible damage" caused by EXP 65 mph wind (EF-0), the beginning miles per hour of the EF-Scale.

No. 2: "loss of roof covering material (less than 20 percent), gutters and/or awnings, loss of vinyl or metal siding," 79 mph or EF-0;

No. 3: "broken glass in doors and windows," 96 mph wind (EF-1).

No. 4: "uplift of roof deck and loss of significant roof covering material (more than 20 percent), collapse of chimney, garage doors collapse inward, failure of porch or carport," 97 mph (EF-1);

No. 5: "entire house shifts off foundation," 121 mph winds (EF-2);

No. 6: "large sections of roof structure removed, most walls remain standing," 122 mph wind (EF-2);

No. 7: "exterior walls collapsed," 132 mph wind (EF-2);

No. 8: "most walls collapsed, except small interior rooms," 152 mph wind (EF-3);

No. 9: "all walls collapsed," 170 mph wind (EF-4); and

No. 10: "destruction of engineered and/or well constructed residence, slab swept clean," 200 mph or more (EF-5).

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Copyright (c) 2011, Tulsa World, Okla.

Distributed by McClatchy-Tribune Information Services.

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Story image: This image provided by NASA and captured by the Thematic Mapper on the Landsat 5 satellite on Sunday, June 5, 2011, shows part of the track of a tornado that struck near Sturbridge, Mass., last week. AP Photo/NASA

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