The whirling winds of tornadoes can produce damage that ranges from broken tree limbs to a block of houses swept from their foundations. They can inflict utter devastation across a wide swath of land or, remarkably, destroy one house and leave others on either side largely untouched. In this interactive activity from NOVA Online, you will evaluate scenes of destruction utilizing the most widely used scale for assessing a tornado's intensity, the Fujita scale.
Rate Tornado Damage (HTML version) (Document)
It is difficult to take direct measurements of tornadoes due to their unpredictable behavior. In the 1970s, meteorologist Ted Fujita proposed a scale to classify a tornado's intensity. The scale gauged the tornado's maximum wind speed according to the maximum damage that it inflicted. The Fujita scale, or F-scale, has six ratings ranging from F0 to F5. F0 tornadoes have weak winds that inflict light damage. Powerful F5 tornadoes can strip the bark from trees or tear whole houses from their foundations.
One problem with a scale that associates damage with wind speed is that two tornadoes with different wind speeds can cause similar damage. Likewise, two tornadoes with the same wind speeds can cause different levels of damage. Factors including differences in building construction, wind direction, and wind duration may account for this variability. Moreover, the process of rating the damage itself is largely subjective. Two highly experienced damage surveyors may assign different F-scale ratings for the same disaster scene. In spite of its shortcomings, however, the F-scale continues to be the most widely used method for rating tornado damage.
In recent years, new technologies have improved the accuracy of weather predictions. Using Doppler radar, forecasters can now spot the large-scale rotation from which many tornadoes form and measure the strength of the vortex by tracking raindrops carried inside a developing tornado. In an effort to reduce tornado-related casualties, the National Weather Service (NWS) now issues tornado watches and warnings over local radio and television broadcasts as soon as tornadoes are indicated on weather radar or spotted from the ground -- before they actually strike.
Still, the high death, injury, and property damage tolls from tornadoes have prompted some structural engineers to look at ways to construct tornado-safe buildings and houses. Research has shown that a tornado exerts an upward force on a structure up to 10 times stronger than the force of gravity. This can tear a roof from its supporting walls and lead to complete structural collapse. It also appears that the dimensions of a structure can alter a tornado's path, causing the twister to veer to one side. By understanding how tornadoes interact with structures, engineers hope to design or modify buildings accordingly to minimize tornado damage.
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