The plains that freeze so hard in winter can turn violent in summer. When warm, humid air flows north and meets the restless upper winds over the region, the atmosphere grows unstable, and on the worst afternoons it builds the towering, rotating thunderstorms called supercells — the engines that spawn the tornado. The plains sit on the northern edge of the continent's great tornado country.

Watching the Green Sky

The signs are written in the sky for those who can read them: the anvil-topped tower climbing to enormous height, the strange green cast of a hail-laden cloud, the lowering wall cloud from which a funnel may descend. The plains dweller keeps half an eye on the summer sky and an ear on the warnings, and knows the way to the basement.

From the almanac's reckoning:

• Rotating supercell storms are the engines that spawn tornadoes.
• A green-tinged sky often signals a hail-laden storm.
• Radar warnings now give a margin the settlers never had.

Modern radar and warning systems give a precious margin the old settlers never had, turning what was once pure terror into a manageable, if still fearsome, hazard. The almanac counts tornado season among the plains' summer trials, and counsels a watchful eye whenever the afternoon sky begins to build.