Knowledge of the wind speeds required to upset various types of motor vehicles can be useful in safety and preparedness programs for tornadoes, hurricanes, and other severe windstorms. That knowledge may also be applied to rating tornado intensity through the Fujita Scale, especially where vehicles were struck but no sturdy buildings were struck. There are scattered reports in the literature of estimates of the wind speed required to lift or upset various vehicles. Examples include 80 m/s to lift a 5,443 kg farm truck, 51 m/s to lift a 1,818 kg auto, 40 m/s for a passenger rail car, 33 m/s for a 13,600 kg semi-truck trailer, and 29 m/s for a 9 m motorhome. Some of these estimates of overturning wind speeds used only wind pressure on the side of the vehicle. Others also used aerodynamic lift forces across the top and others used wind tunnel models.

Our research on 291 passenger vehicles struck by tornadoes while parked outdoors near a house showed that vehicles were rarely tipped over in F2 damage and only 18% were tipped in F3 damage. Our wind tunnel experiments on a sedan and a minivan showed the angle of the wind was important in determining the upset wind speed. A minimum uniform steady wind speed of 50 m/s was required to upset the sedan and 58 m/s to upset the minivan. Gusts, turbulence, and debris could alter these values.

Suggestions will be made for wind speed ranges required to upset common vehicle types for incorporation into the Fujita scale. The impact of wind on a single vehicle is not very informative, so data should be obtained on several vehicles for an F-scale assessment. Whether a severe wind will upset a vehicle will depend on the details of vehicle weight, progressive damage, impacts of debris, wind gusts, direction and duration of wind, exposure, and whether the vehicle was moving when struck. There are similar concerns and debates when assigning F-scales to damage to frame houses and other buildings.