Although it's mentioned by any character, a tornado is measured by its destructive force with the Fujita's Scale. It was named after Tetsuya Fujita, who in 1971 in collaboration with Allan Pearson created a scale to differentiate a twister according the wind speed:
F0: 60-117 km/h or 45-72 mph (light damage).
F1: 117-181 km/h or 73-112 mph (moderate damage).
F2: 181-250 km/h or 113-157 mph (significant damage).
F3: 250-320 km/h or 158-206 mph (severe damage).
F4: 320-420 km/h or 207-260 mph (devastating damage).
F5: 420-510 km/h or 261-308 mph (incredible damage).
F6: 510-610 km/h or 309-379 mph (altough initially Fujita scale have five marks, in 1999 a tornado located in Bridge CreekMoore, Oklahoma, devastated with a force more powerful never seen before. It was the only one F6 registered in history, despite The United States National Weather Service officially maintains that the Bridge Creek-Moore tornado was a F5, not F6).