“Today, weather radar can detect even very weak tornadoes from distances of more than 160 km (100 miles). Before radar was widely deployed, however, weak tornadoes didn’t always make it into the record. While strong tornadoes leave an evident trail of destruction, weaker tornadoes can come and go without a trace, particularly in sparsely populated areas. To see if there’s been a real change in the number of tornadoes over the past seventy years, we’ve got to correct for the observing bias in favor of strong storms early in the record.”