The cruelty of this April, with 297 confirmed tornadoes recorded, is without equal in the United States. Preliminary assessments say that of the four biggest clusters ever recorded, two have occurred in the past three weeks.

What is happening? Despite all of the precise digital tools to clarify the inner heart of a storm cell in rampage, the natural world can still seem unpredictable when it roars into full-throated chaos.

"There's a large crapshoot aspect," said Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo. "A little quirky thing can set one off at one time, and another time not."

Tornadoes need warm, moist air interacting with faster, cooler air. That much, scientists know for sure.

"There's a lot we understand about tornadoes," Trenberth said. "They're tied to thunderstorms and also require something that will cause the rotation to occur, a wind shear."

The prevalence of hurricanes, droughts and floods has been linked in many climate models to the impact of a warming planet. And they are also often slower in building or wreaking havoc, which gives scientists more time to marshal their evidence.

"Tornadoes are tougher," said Jeff Masters, director of meteorology at Weather Underground, an Internet-based weather service.

Many climate models, for example, predict a weakening upper atmosphere jet stream over time on a warming planet, Masters said, which would presumably create less energy for tornado formation. But some of those same models also suggest wetter conditions in tornado country, which is the other key ingredient in storm formation.

At the same time, more people are also living in areas where tornadoes strike, across the broad swath called Tornado Alley in the nation's midsection and South, so the number of recorded tornadoes has gone up.

The South's population grew 14.3 percent over the past decade, according to the Census Bureau, compared with 9.7 percent for the nation as a whole. Of those states hardest hit by tornadoes this year, some were among the fastest growing, notably Texas and North Carolina.

Much of the new construction took place on flat areas of flood plain, meteorologists say, where rains from storms in years past spread across the earth and either evaporated or were absorbed. The water now runs across pavement, seeking lower ground rendered more vulnerable to flood.

Also, jet stream forces in April were among the strongest ever recorded, possibly because of La Nina conditions in the Pacific Ocean. These lead to cooler water around the Equator in the Pacific Ocean and are associated with wetter and stormier conditions through the middle of the country as cooler air from Canada surges into warm moisture heading north - made warmer, many climate scientists say, by climate change.

And yet, that crucial element of weather prediction is missing - when and where the next tornado might come.