Geoscience Reference
In-Depth Information
highest concentration of tornadoes occurs in the United States. On average each
year, more than 800 tornadoes are reported nationwide, resulting in more than
80 deaths and 1500 injuries.
10
The strongest tornadoes can have rotating winds
that exceed 250 mph. Their paths can be up to one mile wide and several miles
long. These extreme tornadoes only make up about 2% of all reported tornadoes.
However, they account for about 70% of all tornado deaths. Tornadoes are asso-
ciated with severe thunderstorms, called supercells, which can occur across the
United States. Most times tornadoes come with very little notice, although advances
in Doppler radar technologies have the ability to recognize developing tornadoes
before they form on the ground.
In February 2007, the National Weather Service Storm Prediction Center
switched from classifying tornadoes using the Fujita Scale (F-Scale) to using the
Enhanced Fujita Scale (EF Scale). The reasons for the switch was that the F-Scale
did not take into account differences in construction of structures, the catego-
ries were only based on damage in worst-case location, and it was felt that the
wind speeds were overestimated in F-3 tornadoes and stronger. Most of the time,
the classification of a tornado is done after the fact, and after damages have been
assessed. The Enhanced Fujita scale uses 28 damage factors that take into account
building construction and object (tree, flagpole, etc.) type, and allows for more
accurate estimation of winds (Table 5.2).
One tornado of historical record is the Oklahoma/Kansas Tornado Outbreak
on May 3, 1999 (Figure 5.4). During a period of 21 hours, 74 tornadoes touched
down across the two states. At one point, there were four tornadoes reported to be
on the ground at the same time. The strongest tornado was an F-5 on the Fujita
Tornado Scale (the EF scale had not yet been adopted). This F5 tornado tracked for
Table 5.2
Fujita versus Enhanced Fujita Scale
Fujita Scale
Operational EF Scale
Fastest 1/4
Mile (mph)
3-s Gust
(mph)
3-s Gust
(mph)
F Number
EF Number
0
40-72
45-78
0
65-85
1
73-112
79-117
1
86-110
2
113-157
118-161
2
111-135
3
158-207
162-209
3
136-165
4
208-260
210-261
4
166-200
5
261-318
262-317
5
Over 200
Source:
National Weather Service, Storm Prediction Center.
