how do you rate strength of tornado
There are several scales for rating the strength of tornadoes. The Fujita scale rates tornadoes by damage caused and has been replaced in some countries by the updated Enhanced Fujita Scale. An F0 or EF0 tornado, the weakest category, damages trees, but not substantial structures. An F5 or EF5 tornado, the strongest category, rips buildings off their foundations and can deform large skyscrapers. 
The similar TORRO scale ranges from T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes.
Doppler radar data, photogrammetry, and ground swirl patterns (trochoidal marks) may also be analyzed to determine intensity and assign a rating.
source : https://en.wikipedia.org/wiki/Tornado