Erwan Koch

Every spring, the sky over the US Great Plains starts behaving like a giant steam boiler. Warm, moist air from the Gulf of Mexico collides with dry air from the west, and in that mix, thunderstorms are born that can rip the roof off a barn in minutes. Meteorologists have long tracked the 'fuel' for such storms, but so far they have mostly looked at average values.

A thunderstorm front is like a rubber band stretched between two fingers. The harder you pull, the wider the strip where lightning strikes. Meteorologists have long been able to measure storm intensity at a single point, but until now they didn't understand why severe weather sometimes compresses into a compact blob and sometimes covers half a continent.