Figure 16.29: The frequency of F3, F4 and F5 tornadoes in the United States. The red region
that starts in Texas and covers Oklahoma, Nebraska and South Dakota is called Tornado
Alley because it is where most of the violent tornadoes occur. ( 44 )
and a warning is issued to people in that area. The exact path is unknown because tornado
movement is not very predictable. The intensity of tornadoes is measured on the Fujita Scale
(seeTable16.3), which assigns a value based on wind speed and damage.
Table 16.3: The Fujita Scale (F Scale) of Tornado IntensityF Scale (km/hr) (mph) Damage
F0 64-116 40-72 Light - tree branches
fall and chimneys
may collapse
F1 117-180 73-112 Moderate - mobile
homes, autos pushed
asideF2 181-253 113-157 Considerable - roofs
torn off houses, large
trees uprootedF3 254-332 158-206 Severe - houses
torn apart, trees
uprooted, cars lifted