With all the tornado video these days, you get the impression that tornadoes are more frequent now than they used to be. The numbers certainly support this. In the United States, the average annual number of tornadoes has increased from around 450 in the 1950s to 1200 in the 1990s!

A clue to explaining this emerges if we categorize tornadoes as either weak or strong. If we only consider the strong ones, the story changes. If anything, there are fewer powerful tornadoes now than there used to be. So it must be the weak tornadoes that are increasing in number.

And that's the key. In the past, many weak tornadoes escaped detection. They were there, but so weak that we missed them! Now, with Doppler radar and with a larger population that's more spread out, we're better observers. Few tornadoes go unreported. The conclusion: the increase in observed tornadoes most likely results from changes in the way we're doing things, not changes in the weather.