Seventy-five percent of the world's tornadoes occur in the United States, yet few people who live in "Tornado Alley" bother with the trouble and expense of a proper shelter from the storms.

Mel Evridge, 69, a retired builder who experienced both Monday's twister in this Oklahoma City suburb that killed 24 and a still deadlier one in May 1999, is a proud member of that minority.

Not only did he put a storm cellar in the smart single-level house he built for his family in the 1970s, but he also opted to use Arkansas Hackett stone tough enough to withstand the worst of Oklahoma's climatic extremes.

Even then, Evridge told AFP as he collected debris from his front lawn, "I was just about as scared (Monday) as I was the first time."

"Ever heard a jet throttle up when holding its brakes?" he asked in describing what a tornado feels like from inside a shelter. "That's what it sounded like. Just one big roar."

Yet few homes in the tornado-vulnerable Great Plains that stretch from Texas to the Canadian border are fitted with tornado shelters -- and public buildings even less so.

In the Oklahoma City area, perhaps 10 to 20 percent of homes have some kind of formal shelter, said John Snow, a professor of meteorology at the University of Oklahoma and an authority on the Great Plains' often unforgiving climate.

No state or local law in Oklahoma, the "bullseye" of Tornado Alley, mandates the installation of residential storm shelters -- and homeowners who do opt for them have to shell out upwards of $4,000 for the most basic option.

"Storm shelters are a good idea. That's a fundamental message," Snow told AFP in a telephone interview. "But there are awesome challenges to building them."

Besides the expense, the region's flat open terrain consists of clay, which expands when wet, contracts when dry and renders the kind of basements common in the eastern United States vulnerable to cracking and crumbling.

There's also the fact that tornadoes as potent as those seen in Moore, with winds of 200 miles (321 kilometers) per hour, are few and far between, compared to the far more common ones that are half as strong.

For those twisters at the low end of the five-step Enhanced Fujita scale, taking cover in the bathtub, under a staircase or inside a closet is typically good enough, Snow said.

Schools like Plaza Towers Elementary, in which at least seven children died Monday, also don't require underground shelters. Instead, teachers are trained to huddle their young charges in hallways and other places well away from windows.

In Oklahoma, only about 100 schools have safe rooms built with federal funding that is no longer available, The Wall Street Journal reported.

"Unfortunately, people prefer homes with swimming pools rather than storm shelters," said Oklahoma State Representative Joe Dorman, who is proposing a $500 million state bond issue to pay for more school shelters.

Public buildings like office and shopping complexes are subject to more stringent construction and engineering codes than homes, and so structurally have a better chance of standing up to a tornado, according to Snow.

"If you look closely, all the washrooms (in shopping malls) have little signs that say, 'This is a tornado shelter,'" he said.

Indeed, the Warren multiplex cinema -- part of a western US theater chain whose owner takes pride in the high quality of his real estate -- took the fury of the tornado so well that it was commandeered as a first aid center.

And there's the character of the people who chose to live in Oklahoma, a land of cowboys, oil workers, wide open skies and endless prairie that encapsulates the American ideal of rugged individualism.

"It traces back to the people who settled this part of the country" in the late 1800s, Snow said. "If you didn't like it here, you left. What you have is a pretty tough lot. Come back (to Moore) in 18 months. You'll see it rebuilt."