- Tornado intensity and damage
Tornadoes vary in intensity regardless of shape, size, and location. While strong tornadoes are typically larger than weak tornadoes, there are several instances of F5 tornadoes with damage paths less than 500 feet (150 m) wide. Data from the Storm Prediction Centerarchives, which are accessible through [http://www.spc.noaa.gov/software/svrplot2/ SeverePlot] , free software created and maintained by John Hart, lead forecaster for the SPC.]
History of tornado intensity measurements
For many years, before the advent of home movies and doppler radar, scientists had nothing more than educated guesses as to the speed of the winds in a tornado. The only evidence indicating the wind speeds found in the tornado was the damage left behind by tornadoes which struck populated areas. Some thought they might exceed 500 mph, and perhaps even be
In 1971, Dr. Tetsuya Theodore Fujita introduced the idea for a scale of tornado winds. With the help of colleague
Allen Pearson, he created and introduced what came to be called the Fujita scalein 1973. This is what the F stands for in F1, F2, etc. The scale was based on a relationship between the Beaufort scaleand the Mach numberscale; the low end of F1 on his scale corresponds to the low end of B12 on the Beaufort scale, and the low end of F12 corresponds to the speed of sound at sea level, or Mach 1. In practice, tornadoes are only assigned categories F0 through F5.
TORRO scale, created by the Tornado and Storm Research Organisation (TORRO), was developed in 1974, and published a year later. The TORRO scale has 12 levels, which cover a broader range with tighter graduations. It ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes. T0-T1 roughly correspond to F0, T2-T3 to F1, and so on. While T10+ would be approximately an F5, the highest tornado rated to date on the TORRO scale was a T8.cite web| url = http://www.torro.org.uk/TORRO/about/history.php| title = A Brief History of TORRO (to 1985)| accessdate = 2006-11-01| last = Meaden| first = Dr. Terence| year = 1985| publisher = Tornado and Storm Research Organisation (TORRO)] cite web| url = http://www.torro.org.uk/TORRO/britwxextremes/summary.php| title = British Weather Extremes Summary| accessdate = 2006-11-02| author = Various| publisher = TORRO] There is some debate as to the usefulness of the TORRO scale over the Fujita scale—while it may be helpful for statistical purposes to have more levels of tornado strength, often the damage caused could be created by a large range of winds, rendering it hard to narrow the tornado down to a single TORRO scale category.
Research conducted in the late 1980s and 1990s suggested that, even with the implication of the Fujita scale, tornado winds were notoriously overestimated, especially in significant and violent tornadoes. Because of this, in 2006, the
American Meteorological Societyintroduced the Enhanced Fujita Scale, to help assign realistic wind speeds to tornado damage. The scientists specifically designed the scale so that a tornado assessed on the Fujita scale and the Enhanced Fujita scale would receive the same ranking. The EF-scale is more specific in detailing the degrees of damage on different types of structures for a given wind speed. While the F-scale goes from F0 to F12 in theory, the EF-scale is capped at EF5, which is defined as "winds ≥ 200 mph (≥ 320 km/h)". cite web| url = http://www.spc.noaa.gov/faq/tornado| title = The Online Tornado FAQ| accessdate = 2006-09-08| last = Edwards| first = Roger| date = 2006-04-04| publisher = Storm Prediction Center] In the United States, the Enhanced Fujita scale went into effect on February 2, 2007for tornado damage assessments and the Fujita scale is no longer used.
The first observation which confirmed that F5 winds could occur happened on
April 26, 1991. A tornado near Red Rock, Oklahoma was monitored by scientists using a portable Doppler radar, an experimental radar device that measures wind speed. Near the tornado's peak intensity, they recorded a wind speed of 115-120 m/s (257-268 mph or 414-432 km/h). Though the portable radar had uncertainty of ± 5-10 m/s (± 11-22 mph or ± 18-36 km/h), this reading was probably within the F5 range, confirming that tornadoes were capable of violent winds found nowhere else on earth.
Eight years later, during the
1999 Oklahoma tornado outbreakof May 3, 1999, another scientific team was monitoring an exceptionally violent tornado (one which would eventually kill 36 people in the area near Moore, Oklahoma). At about 7:00 pm, they recorded one measurement of 301±20 mph (484±32 km/h), cite web| url = http://cswr.org/dow/DOW.htm| title = Doppler On Wheels| accessdate = 2006-12-29| author= [http://cswr.org/ Center for Severe Weather Research] | year = 2006] 50 mph faster than the previous record. Though this reading is just short of the theoretical F6 rating, the measurement was taken more than 100 feet in the air, where winds are typically stronger than at the surface. In rating tornadoes, only surface wind speeds, or the wind speeds indicated by the damage resulting from the tornado, are taken into account. Also, in practice, the F6 rating is not used.
While scientists have long theorized that extremely low pressures might occur in the center of tornadoes, there were no measurements to confirm it. A few home
barometers had survived close passes by tornadoes, recording values as low as 24 in Hg (810 mbar), but these measurements were highly uncertain. Lyons, Walter A. The Handy Weather Answer Book. Detroit, MI: Visible Ink Press, 1997.] However, on June 24, 2003, a group of researchers successfully dropped devices called "turtles" into an F4 tornado near Manchester, South Dakota, one of which measured a pressure drop of more than 100 mbar as the tornado passed directly overhead. [http://magma.nationalgeographic.com/ngm/0404/feature1/fulltext.html Chasing Tornadoes @ National Geographic Magazine ] ] Still, tornadoes are widely varied, so meteorologists are still conducting research to determine if these values are typical or not.
In the United States, F0 and F1 (T0 through T3) tornadoes account for 80% of all tornadoes. The rate of occurrence drops off quickly with increasing strength—violent tornadoes (stronger than F4, T8), account for less than 1% of all tornado reports.cite web| url = http://www.nws.noaa.gov/om/brochures/basicspot.pdf| title = Basic Spotters’ Field Guide| accessdate = 2006-11-01| author = Edwards, Moller, Purpura et al| year = 2005| format = PDF| publisher = [http://www.commerce.gov/ US Department of Commerce] , [http://www.nws.noaa.gov National Weather Service] ] Worldwide, strong tornadoes account for an even smaller percentage of total tornadoes. Violent tornadoes are extremely rare outside of the United States, Canada and Bangladesh.
F5 and EF5 tornadoes are exceptionally rare, occurring on average once every few years. The last confirmed F5 tornado anywhere in the world was the
Elie, Manitoba Tornadoin Canada, on June 22, 2007. Before that, the last confirmed F5 was the Moore, Oklahoma tornado, which killed 36 people on May 3, 1999. Two EF5 tornadoes have occurred in the United States, in Greensburg, Kansason May 4, 2007and in Parkersburg, Iowaon May 25, 2008.
As stated in the lede section, a typical tornado has winds of 110 mph (175 km/h) or less, is approximately 250 feet (75 meters) across, and travels a mile (1.6 km) or so before dissipating. However, tornadic behavior is extremely variable; these figures represent only statistical probability.
Two tornadoes that look almost exactly the same can produce drastically different effects. Also, two tornadoes which look very different can produce similar damage. This is due to the fact that tornadoes form by several different mechanisms, and also that they follow a life cycle which causes the same tornado to change in appearance over time. People in the path of a tornado should never attempt to determine its strength as it approaches. Between 1997 and 2005 in the United States, 38 people were killed by F1 tornadoes, and 3 were killed by F0 tornadoes. " [http://www.spc.noaa.gov/climo/historical.html Climatological or Past Storm Information and Archived Data] ." " [http://www.spc.noaa.gov Storm Prediction Center] ". 2006.] Even the weakest tornado can kill.
*Weak tornadoesThe vast majority of tornadoes are designated F1 or F0, also known as "weak" tornadoes. However, weak is a relative term for tornadoes, as even these can cause significant damage. F0 and F1 tornadoes are typically short-lived—since 1980 almost 75% of tornadoes rated weak stayed on the ground for one mile or less. However, in this time, they can cause both damage and fatalities.
F0 (T0-T1) damage is characterized by superficial damage to structures and vegetation. Well-built structures are typically unscathed, sometimes sustaining broken windows, with minor damage to roofs and
chimneys. Billboards and large signs can be knocked down. Trees may have large branches broken off, and can be uprooted if they have shallow roots.
F1 (T2-T3) damage has caused significantly more fatalities than that caused by F0 tornadoes. At this level, damage to mobile homes and other temporary structures becomes significant, and cars and other vehicles can be pushed off the road. Permanent structures can suffer major damage to their roofs.
F2 (T4-T5) tornadoes are the lower end of "significant", and yet are stronger than most
tropical cyclones (though tropical cyclones affect a much larger area). Well-built structures can suffer serious damage, including roof loss and collapse of outer walls. Mobile homes, however, are almost totally destroyed. Vehicles can be lifted off the ground, and lighter objects can become small missiles, causing damage outside of the tornado's main path. Wooded areas will have a large percentage of their trees snapped or uprooted.
F3 (T6-T7) damage is a serious risk to life and limb. Few parts of affected buildings are left standing; well-built structures lose outer and inner walls. Cars are lifted off the ground, and can be tossed through the air for some distance. Wooded areas will suffer almost total loss of vegetation.
F4 (T8-T9) damage typically results in a total loss of the affected structure. Well-built homes are reduced to a short pile of debris. Even heavy vehicles, including airplanes,
trains, and large trucks, can become airborne, with other large projectiles being flung some distance.
F5 (T10+) damage is almost always total. F5 tornadoes demolish well-built houses and sweep the foundation clean. The official description of this damage states that "incredible phenomena will occur". The damage they cause is an extreme hazard to life and limb—since 1950 in the United States, only 50 tornadoes (0.1% of all reports) have been designated F5, and yet these have been responsible for more than 1000 deaths and 11,000 injuries (21.5% and 13.6%, respectively). In recorded history, F5 tornadoes have performed awesome displays of power, including twisting skyscrapers, levelling entire communities, and stripping asphalt from the ground.
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