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 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 currently used Fujita
Scale standards are as follows:
|
F-Scale
Number |
Intensity Phrase |
Wind
Speed |
Type of
Damage Done |
|
F0 |
Gale tornado |
40-72
mph |
Some
damage to chimneys; breaks branches off trees; pushes over
shallow-rooted trees; damages sign boards. |
|
F1 |
Moderate tornado |
73-112
mph |
The
lower limit is the beginning of hurricane wind speed; peels surface off
roofs; mobile homes pushed off foundations or overturned; moving autos
pushed off the roads; attached garages may be destroyed. |
|
F2 |
Significant tornado |
113-157
mph |
Considerable
damage. Roofs torn off frame houses; mobile homes demolished; boxcars
pushed over; large trees snapped or uprooted; light object missiles
generated. |
|
F3 |
Severe tornado |
158-206
mph |
Roof
and some walls torn off well constructed houses; trains overturned; most
trees in fores uprooted |
|
F4 |
Devastating tornado |
207-260
mph |
Well-constructed
houses leveled; structures with weak foundations blown off some
distance; cars thrown and large missiles generated. |
|
F5 |
Incredible tornado |
261-318
mph |
Strong
frame houses lifted off foundations and carried considerable distances
to disintegrate; automobile sized missiles fly through the air in excess
of 100 meters; trees debarked; steel re-enforced concrete structures
badly damaged. |
|
F6 |
Inconceivable tornado |
319-379
mph |
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. Missiles, such as cars and
refrigerators would do serious secondary damage that could not be
directly identified as F6 damage. If this level is ever acheived,
evidence for it migh 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. Large tornadoes can also be strong and small tornadoes can be weak. The Fujita Scale is based on damage, not the appearance of the funnel. Weather observers often try to judge the intensity of a tornado when they are in the field, but the official F-Scale estimate is made after the tornado has passed and the damage has been surveyed.
This Homepage was prepared by Kenneth F. Hambrick, Polk County Coordinator. Contact at E-Mail address:
webmaster@pcoem.org for additional information about our program.
The
information contained in this Homepage is considered public
domain and the Office of Emergency Management encourages
interested persons to utilize any portions of it that might be of educational
benefit or enhance their local programs.
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