In terms of damage severity that title would probably go to the Jarrell, Texas tornado of May 27, 1997. This F5 tornado essentially erased a portion of the Double Creek Estates subdivsion. The tornado destroyed everything; houses, trees, grass, streets, and driveways were all blown away and pulverized, leaving a field of mud. In some areas the tornado removed up to 18 inches of topsoil. The only indicator that there had been anything there were the concrete slabs that ones supported houses, and the discolored earth where roads once were. Of the 27 people in this area of complete destruction only one survived and an additional death occurred just outside of Jarrell. Although an F5 tornado can be expected the remove a houses from their foundations, no other tornado has been known to cause destruction this complete. See the related link below for pictures and more information.
Since the tornado did not hit a large population center, the actual amount of damage was limited, only a few dozen buildings were destroyed, though most of those that were got completely obliterated. Other tornadoes that were less intense, but still very violent,have hit larger developed areas and destroyed thousands of buildings amounting to a greater total cost.
See the related question for a list of the costliest U.S. tornadoes.
It depends on the tornado, but in the most destructive tornadoes, the worst damage is usually done by a series of smaller subvortices that revolve withing the main circulation.
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The intensity of a tornado is rated based on damage done on the ground.
Damage is done to varying degrees depending on the intensity of the tornado on the Fujita scale.
The damage that the most recent tornado has done is estimated to be worth over $10 million. These are the twin tornadoes that hit Nebraska on June 17, 2014.
The effect of the 1979 Red River tornado outbreak included widespread destruction and 58 deaths. The worst damage was done by the F4 tornado that hit Wichita Falls, Texas destroying a large part of the city and killing 42 people.
A detailed study done on a single tornado in order to understand it better.
The most destructive tornado in Ohio history was the Xenia tornado of April 3, 1974. This F5 tornado killed 34 people and caused $439 million in property damage (in 2011 U.S. dollars).
Typical damage from a tornado costs a few hundred thousand dollars. Costs in the millions are less common but tend to attract more attention.
The is no Fajita scale. The Fujita scale provides basic standards by which to assess the damage done by a tornado. Based on the severity of the damage a tornado is assigned a rating, which can range from F0 for the weakest tornadoes to F5 for the strongest.
The Fujita scale uses the damage done by tornadoes to estimate their intensity.
Meteorologists and engineers examine the damage done by a tornado a rate if from EF0 to EF5 based on how severe the damage is. EF0 and EF1 (light to moderate damage) tornadoes are considered weak. EF2 and EF3 tornadoes are classified as strong. EF4 and EF5 tornadoes are classified as violent.