In
the U.S., tornadoes are classified according
to a system known as the Fujita
or Fujita-Pearson scale, named
after Professor Theodore Fujita, retired
professor at the University of Chicago,
and Dr. Alan Pearson, former director
of the National Severe Storm Forecast
Center (NSSFC), who devised the system
in 1971. This scale relates the
wind speed of a tornado to the amount
of damage done. This 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.
There
are seven levels in the Fujita scale,
F0 - F6. |
| 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. Roof torn off frame houses;
mobile homes demolished; boxcars pushed
over; large trees snapped or uprooted;
light object missiles generated |
| F3 |
Severe
tornado |
159-206
mph |
Roof
and some walls torn off well constructed
houses; trains overturned; most trees
in forest 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 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 that the size
of a tornado is not necessarily an indication
of its intensity. Large tornadoes can
be weak, and small tornadoes can be violent.
The Fujita Scale is based on damage, not the
appearance of the funnel. Although storm
spotters, storm chasers and other weather observers
often try to judge the intensity of a tornado
when they are in the field, the official estimate
or classification is made after the tornado
has passed.
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