Tornadoes are ranked according to the damage they produce, since it's currently impossible to measure a tornado's wind speed directly. The official measure of tornado strength since 2007 is the Enhanced Fujita Scale, an improvement over the original Fujita Scale developed by meteorologist T. Theodore Fujita at the University of Chicago. The scale uses the following 6-tier numerical value system.
The wind speeds are estimated three-second wind gusts, based on how much force it would take to produce a given area of damage.
EF-0 - Wind 65-85 mph
EF-1 Wind 86 - 110 mph
EF-2 Wind 111 - 135 mph
EF-3 Wind 136 - 165 mph
EF-4 Wind 166 - 200 mph
EF-5 Wind Over 200 mph
When reviewing the aftermath of a tornado, investigators will note specific damage (everything from broken windows to freight trains blown off their tracks) and match the damage to the wind gusts on the EF Scale to arrive at the tornado's classification.