Why does the United States consistently lead the world in the number of tornadoes each year?

Dear Tom,
Why does the United States consistently lead the world in the number of tornadoes each year?
— Mick Bulakowski, Palatine
Dear Mike,
The unique geography of the central U.S. that provides a barrier-free collision course between polar, tropical and desert air masses makes this nation home to about 75 percent of the world’s tornadoes. Tornado expert Tom Grazulis speculates that tornadoes have occurred in almost every nation of the world, with the exception of the polar regions. Significant tornado activity occurs each year in Europe, Australia, Canada, Russia, Argentina, China and Japan. Bangladesh has logged at least a dozen twisters deadlier than this nation’s deadliest, the 1925 Tri-State Tornado that killed nearly 700. Most scientists believe that the number of tornadoes around the world is poorly documented because many are small and weak and go undetected.