In 2011, an outbreak—not of disease, but of tornadoes—slammed through a large swath of the Southern and Eastern U.S. The region saw 64 individual twisters on April 25, and another 50 the next day, but the worst still lay ahead. A head-spinning 199 tornadoes touched down on April 27, killing 316 and injuring almost 3,000. To compare this cluster to others, meteorologists use the Destruction Potential Index. DPI is the sum of each individual storm's destructive capacity, which weather-folk quantify by multiplying each tempest's wind power by the area it hits. April 27's outbreak reached 21,980. That's three times the next-most devastating event in 2010.