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TORNADO SCALES Dr. T. Theodore Fujita
developed a damage scale (Fujita 1971, Fujita and Pearson 1973) for winds,
including tornadoes, which was supposed to relate the degree of damage to the
intensity of the wind. This scale was
the result. The original F-scale should not be used anymore, because it
has been replaced by an enhanced
version. Even with all its flaws, the original F-scale was the only
widely used tornado rating method for over three decades. The enhanced F-scale takes effect 1
February 2007. Fujita Tornado Intensity Scale Category F0: Gale tornado (40-72 mph) light damage. Some
damage to chimneys; break branches off trees; push over shallow-rooted trees;
damage to sign boards. The Enhanced F-scale is a much more precise and robust way to assess tornado damage than the original. It classifies F0-F5 damage as calibrated by engineers and meteorologists across 28 different types of damage indicators (mainly various kinds of buildings, but also a few other structures as well as trees). The idea is that a "one size fits all" approach just doesn't work in rating tornado damage, and that a tornado scale needs to take into account the typical strengths and weaknesses of different types of construction. This is because the same wind does different things to different kinds of structures. In the Enhanced F-scale, there will be different, customized standards for assigning any given F rating to a well built, well anchored wood-frame house compared to a garage, school, skyscraper, unanchored house, barn, factory, utility pole or other type of structure. In a real-life tornado track, these ratings can be mapped together more smoothly to make a damage analysis. Of course, there still will be gaps and weaknesses on a track where there was little or nothing to damage, but such problems will be less common than under the original F-scale. As with the original F-scale, the enhanced version will rate the tornado as a whole based on most intense damage within the path
Enhanced F Scale for Tornado Damage: An update to the original F-scale by a team of meteorologists and wind engineers, to be implemented in the U.S. on 1 February 2007.
IMPORTANT NOTE ABOUT ENHANCED F-SCALE WINDS:The Enhanced F-scale still is a set of wind estimates (not measurements) based on damage. Its uses three-second gusts estimated at the point of damage based on a judgment of 8 levels of damage to the 28 indicators listed below. These estimates vary with height and exposure. Important: The 3-second gust is not the same wind as in standard surface observations. Standard measurements are taken by weather stations in open exposures, using a directly measured "one minute mile" speed. Not necessarily. There is a statistical trend (as documented by NSSL's Harold Brooks) toward wide tornadoes having higher F-scale damage. This can be out of more strength or out of greater opportunity for targets to damage - or some blend of both. However, the size or shape of any particular tornado does not say anything conclusive about its strength. Some small "rope" tornadoes can still do violent damage of F4 or F5; and some very large tornadoes over a quarter-mile wide have produced only weak damage of F0 to F1. That depends on what
it is hitting, its size, intensity, closeness and other factors. The most
common tornado sound is a continuous rumble, like a close by train. Sometimes a
tornado produces a loud whooshing sound, like that of a waterfall or of open
car windows while driving very fast. Tornadoes, which are tearing through
densely populated areas, may be producing all kinds of loud noises at once,
which collectively may make a tremendous roar. Just because you may have heard
a loud roar during a damaging storm does not necessarily mean it was a tornado.
Any intense thunderstorm wind can produce damage and cause a roar. |
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