A tornado is devastating, but an outbreak — when six or more tornadoes develop within a limited time — causes the most deaths and economic damage. And they're on the rise.

From 1954 to 2014, the number of outbreaks has remained relatively constant. However, say two university researchers, the number of tornadoes per outbreak has increased by about 50%. That means storms have become more intense.

The statistical analysis researchers used is pretty wonky. If you'd like an explanation a layperson can understand, check out Time's explanation.

To learn how to become tornado-ready, check out these tips from public works leaders who've lived through major events.