The Fujita Scale is used to rate the intensity of a tornado by examining the damage caused by the tornado after it has passed over a man-made structure.
The "Percentage of All Tornadoes 1950- 1994" pie chart reveals that the vast majority of tornadoes are either weak or do damage that can only be attributed to a weak tornado. Only a small percentage of tornadoes can be correctly classed as violent. Such a chart became possible only after the acceptance of the Fujita Scale as the official classification system for tornado damage. It is quite possible that an even higher percentage of all tornadoes are weak. Each year the National Weather Service documents about 1000 tornado touchdowns in the United States. There is evidence that 1000 or more additional weak tornadoes may occur each year and go completely undocumented.
The "Percentage of Tornado-Related Deaths 1950-1994" pie chart shows that while violent tornadoes are few in number, they cause a very high percentage of tornado-related deaths. The Tornado Project has analyzed data prior to 1950, and found that the percentage of deaths from violent tornadoes was even greater in the past. This is because the death tolls prior to the introduction of the forecasting/awareness programs were enormous: 695 dead(Missouri-Illinois-Indiana, March 18, 1925); 317 dead(Natchez, Mississippi, May 7, 1840);.255 dead(St. Louis, Missouri and East St. Louis, Illinois, May 27, 1896); 216 dead(Tupelo, Mississippi, April 5, 1936); 203 dead(Gainesville, GA, April 6, 1936). In more recent times, no single tornado has killed more than 50 people since 1971.
The Fujita Scale (also known as the Fujita-Pearson Scale) may not be a perfect system for linking damage to wind speed, but it had distinct advantages over what had gone on before its inception. And it was simple enough to use in daily practice without involving much additional expenditure of time or money. From a practical point of view, it is doubtful that any other system would have found its way into widespread accepted use, even to this day. The entire premise of estimating wind speeds from damage to non- engineered structures is very subjective and is difficult to defend from various meteorological perspectives. Nothing less than the combined influence and and prestige of the late Professor Fujita and Allen Pearson , director of NSSFC(National Severe Storm Forecast Center) in 1971 could have brought this much needed system into widespread use. The FPP scale rates the intensity of the tornado, and measured both the path length and the path width. The Fujita part of the scale is as follows:
F-Scale Wind Intensity Phrase Type of Damage Done Number Speed Some damage to chimneys; breaks branches off 40-72 F0 Gale tornado trees; pushes over shallow-rooted trees; damages mph sign boards. The lower limit is the beginning of hurricane wind Moderate 73-112 speed; peels surface off roofs; mobile homes pushed F1 tornado mph off foundations or overturned; moving autos pushed off the roads; attached garages may be destroyed. Considerable damage. Roofs torn off frame houses; 113- Significant mobile homes demolished; boxcars pushed over; F2 157 tornado large trees snapped or uprooted; light object mph missiles generated. 158- Roof and some walls torn off well constructed F3 Severe tornado 206 houses; trains overturned; most trees in fores mph uprooted 207- Well-constructed houses leveled; structures with Devastating F4 260 weak foundations blown off some distance; cars tornado mph thrown and large missiles generated. Strong frame houses lifted off foundations and 261- carried considerable distances to disintegrate; Incredible F5 318 automobile sized missiles fly through the air in tornado mph excess of 100 meters; trees debarked; steel re- inforced concrete structures badly damaged. These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F4 and F5 wind that would surround the F6 winds. 319- Inconceivable Missiles, such as cars and refrigerators would do F6 379 tornado serious secondary damage that could not be directly mph identified as F6 damage. If this level is ever achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies A key point to remember is this: the size of a tornado is not necessarily an indication of its intensity. Large tornadoes can be weak, and small tornadoes can be violent. A good example of a relatively "small" tornado would be the Pampa, Texas tornado of 1995. Eyewitnesses to this tornado claim to have seen as many as 6 vehicles in the air at the same time when it passed over a parking lot.
The Fujita Scale is based on damage , not the appearance of the funnel. Storm spotters, storm chasers and other weather observers often try to estimate the intensity of a tornado when they are in the field, basing their judgement on the rotational speed and amount of debris being generated as well as the width. However, the official estimate is made after the tornado has passed. Personnel from the National Weather Service office that issued the warning survey the site to determine the F-Scale rating. Sometimes they call in experts from out of the area. Aerial surveys are occasionally done after violent tornadoes to determine the exact damage track. Insurance companies may also call in wind engineers to do their own evaluations, but the official rating is set by the NWS. A few of the things they all look for are:
• attachment of the walls and floor to the foundation of the building • attachment of the roof to the rafters and walls • whether or not there are steel reinforcing rods in concrete or cinder block walls • whether there is mortar between the cinder blocks
The Fujita Scale is very subjective, and varies according to how experienced the surveyor is. We have many readers who have tried to do their own "surveys" of tornado damage when storms have occurred in their area. However, the less experienced the surveyor is, the more likely he/she is to be awed by the damage, and the more likely they are to give it a high rating. Media hype and inexperience with tornado damage also plays a big part in exaggerated F-Scale claims seen on television or in the paper. A reporter may see a collapsed concrete block home and be very impressed, never noticing that there was no mortar between the blocks. They may be aghast to see a park whose trees have been leveled, but not know that the species had very shallow roots, planted in soil that was soft and soggy from torrential rains, and thus easily toppled. They may see a roof that had been blown a quarter of a mile from its house, and not know that the roof was attached to the house with only a few nails, and when lofted into the air, acted as a "sail." They may see a light post that is bent at a 30 degree angle and think that it must have taken a 600 mph wind to do that, not knowing that a van had been blown into the pole, bending it, then been towed off to help clear the streets. For some of the media, the exaggerations make for a better story than the actual facts. Fortunately, they often make up for this by printing helpful stories about aid available and inspirational human interest stories.
Questions
1. What is the name of the scale used to measure tornado intensity? What exactly does it measure.
2. What percentage of tornadoes fall in the “weak” (F0 – F1) range? How many are violent?
3. What has been introduced that has reduced the number of tornado related deaths?
4. Explain what classifies a tornado as “significant”.
5. What wind speed is needed to level a well-constructed house?
6. Does the size of a tornado always relate to its Fujita scale rating? Explain.
7. What are some limitations of the Fujita scale?
8. List 2 examples of “mistakes” that could be made when trying to rate a tornado.