Meteorologists rank tornado intensity on what’s called the Fujita Scale, or F-scale. It was developed in the early 1970s by Dr. Theodore Fujita, the world’s most renowned tornado expert. The lowest ranking is F0, reserved for the weakest tornadoes. A ranking of F5, with winds exceeding 260 mph, is assigned to only the strongest. Three-quarters of all tornadoes are rated F0 or F1, while only about 1% achieve F4 or F5.

Now, we don’t have wind measurements from most tornadoes. So F-scale rankings and wind speeds are inferred based on a survey of the damage caused by the tornado. It’s a great example of meteorological detective work.

For everything tornado on the web, I highly recommend tornadoproject.com. The site is run by Dr. Tom Grazulis, a highly respected tornado researcher. You’ll find lists of tornadoes by county, a great tornado FAQ, information on tornado safety and myths, and lots more.