Volunteers help clean out Jean McAdams' mobile home after it was overturned by a tornado May 20, 2013 near Shawnee, Okla.
Deadly tornadoes have lashed the United States for centuries. Most of the worst occurred before modern warning systems existed, although one occurred almost exactly two years before the deadly twister that struck Oklahoma on May 20.
The deadliest tornado in U.S. history, the Tri-State Tornado of March 18, 1925, ravaged 219 continuous miles of Missouri, Illinois and Indiana. Nearly 700 people lost their lives in that single tornado, according to NOAA. However the twister was not officially classified by NOAA as an EF5 -- the most damaging type -- because of a lack of data, nor were there official records of wind speeds.
On May 22, 2011, the deadliest tornado yet recorded by the new Enhanced Fulita Scale struck Joplin and killed 158 Americans, making it the seventh deadliest in U.S. history. Winds exceeded 200 miles per hour as the EF5 tornado demolished a path that was 22.1 miles long and up to 1 mile wide.
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The second deadliest tornado in U.S. history whipped along the Mississippi River on May 2, 1840, ending 317 lives, according to NOAA’s Storm Prediction Center. The tornado killed hundreds on boats and barges on the river until striking Natchez, where the storm killed dozens more. Like all tornadoes from before 1950, NOAA lacks sufficient data to classify the Natchez tornado as F5 or EF5.
St. Louis, Mo.
St. Louis suffered a tornado’s wrath on May 27, 1896, when at least 255 people died. A study published in Weather and Forecasting estimated that the tornado cost $2.2-$2.9 billion in 1997 dollars when adjusted for inflation and wealth increases, making it the costliest tornado in American history. The death toll made it the third deadliest.
On April 5, 1936, the fourth deadliest tornado in U.S. history struck Tupelo, Miss., and killed 216. A total of 436 people died in the outbreak of 17 tornadoes that included the Tupelo twister. Tetsuya Fujita of the University of Chicago and Tom Grazulis, head of the Tornado Project, retroactively rated the Tupelo tornado as an F5 on the scale invented by Fujita.
The same storm system that lashed Mississippi in 1936 continued on to Georgia where it unleashed the fifth deadliest tornado in U.S. history and killed 203 people in Gainesville. Fujita and Grazulis rated this tornado an F4, meaning winds reached 207-260 miles per hour.
Oklahoma is no stranger to tornadoes. The sixth deadliest on record struck on April 9, 1947. The storm nearly destroyed the towns of Higgins and Glazier. In Woodward, Okla., 100 city blocks were destroyed and 107 lives lost, according to the Tornado Project. A total of 181 people died in the tornado.
The largest outbreak of F5 super-tornadoes occurred April 3-4, 1974. Seven F5 tornadoes struck in a single 24-hour period. In total, 147 tornadoes whirled through the central portion of the United States on that day.
On May 22, 2004, the largest tornado ever recorded hit Hallum, Neb. The twister stretched nearly 2 1/2 miles across. No one died in the massive twister.
Groups of tornadoes or outbreaks can cause as much or more damage than a single giant storm. The “Dixie Outbreak” of April 27, 2011, killed 316 people according to NOAA’s Storm Prediction Center. More Americans died in tornadoes that day than any other in this century.
Tornadoes are as much of a given in the Midwest as cornfields and county fairs. In an average year, twisters kill 80 people and injure more than 1,500, so you can imagine the excitement when a physics professor proposed an end to the annual misery: three great walls, each about 1,000 feet (300 meters) tall, that could potentially block the deadly storms of Tornado Alley.
But the idea didn't impress meteorological scientists. Never mind the huge cost, ecological consequences and engineering difficulties involved in the scheme, weather experts say it just wouldn't work.
"The first time somebody mentioned it to me, I thought they were actually joking," said Paul Markowski, a professor of meteorology at Penn State. "There are crazy ideas that could at least work, and then there are crazy ideas that wouldn't even work theoretically." [5 Wild Weather Control Ideas]
The basic goal of the proposal, put forth in the International Journal of Modern Physics B by Rongjia Tao, a physicist at Temple University, is to thwart the "violent air mass clashes" that spawn punishing tornadoes.
Tao envisions three east-west walls, one at the northern edge of Tornado Alley, maybe in North Dakota, another in the middle, perhaps in Oklahoma, and the last stretching across southern Texas and Louisiana. In theory, these barriers would stop the warm, moist air that flows north from colliding at high speeds with cold air flowing southward. (Tornado Alley refers to the stretch of land between the Rocky Mountains and Appalachian Mountains that's particularly prone to tornadoes.)
Tao said he got the idea while working as a professor at Southern Illinois University at Carbondale, where he studied the differences in tornado risk between nearby Washington and Gallatin counties. He theorized that Gallatin County was better protected from tornadoes thanks to a small range at its southern border known as the Shawnee Hills, which only reach about 820 feet (250 m) in altitude. He saw a parallel in the Jiang-Huai Hills of China, which only stretch about 984 feet (300 m) off the ground. According to Tao, the Jiang-Huai Hills and two other east-west mountain ranges make the plains of China largely tornado-proof. He thinks he could mimic that quiet landscape in the United States with artificial walls.
In the wake of the tornado that ripped Moore, Okla. in 2013, NPR’s News Apps team put together a map of the twister’s path.Brian Boyer, Christopher Groskopf, Alyson Hurt, and Matt Stiles
Scientists not involved in the study said Tao's proposal is based on outdated ideas about how tornadoes form and ignores important geographic features.
"He's got the basics almost completely wrong," said Harold Brooks, a research meteorologist at the National Oceanic and Atmospheric Association's (NOAA) National Severe Storms Laboratory in Norman, Oklahoma. "The reason there are so many tornadoes in the central United States is southerly flow at the surface bringing warm, moist air northward, and westerly winds from the Rockies bringing relatively cool, dry air above that, not some surface 'air flow clash location.'"
Markowski also chafed at the notion that stopping "air mass clashes" could stop tornadoes. (In fact, he recently co-wrote a paper in the Bulletin of the American Meteorological Society that offers a long argument about why people should drop the term "clash of air masses" when explaining tornadoes.) Air masses clash all the time, and they don't always spawn tornadoes, Markowski told Live Science. What's more, tornadoes can develop even without an air mass collision. Rather, storms with tornado potential form when warm, humid air near the ground gets trapped under drier air. This instability can produce a spinning updraft of air, which sometimes leads to a tornado.
Semantics aside, the scheme ignores already existing east-west mountain ranges, such as the Wichitas, Arbuckles and Ouachitas in Oklahoma, Brooks said. And roving air masses that go on to seed tornadoes would be able to clear a wall of the size Tao proposed, other scientists said.
"Air masses routinely pass over the Appalachian Mountains. This is true in winter, when the air masses are much colder and heavier than they are in the summer," said Matthew Parker, a storm researcher and associate professor at North Carolina State University.
One of Parker's graduate students at N.C. State, Brice Coffer, actually put Tao's proposal to the test in computer simulations. Coffer used the Weather Research and Forecasting Model, a system commonly used to make high-resolution thunderstorm forecasts, to re-create a storm that spawned deadly tornadoes in Oklahoma in May 2013. He watched how the storm virtually unfolded in three different scenarios: one with the type of walls Tao proposed, one control with no walls and one with 1.6-mile-high (2.5 km) walls — just to play devil's advocate. [Skyscraper Storms: 7 Big City Tornadoes]
Coffer found that the 1,000-foot walls had no significant impact on the formation of storms in the region; that simulation looked nearly identical to the control. The exaggerated, 1.6-mile walls, however, do block the air — but with unintended consequences.
"If you put a rock in a stream, the same type of thing happens," Coffer told Live Science.
If the rock is small enough, the water will just flow over the rock. But plop a big enough rock into the water, and the stream will just go around it on the sides. That's would happen with a mile-high wall. The air would move around it instead of over it, shifting the storms eastward to the Mississippi River Valley instead of the plains and turning much of Texas into a desert, according to Coffer's model. What's more, strong circulations would occur at the edges of the walls, producing landspout tornadoes, which don't emerge from a supercell like regular tornadoes do, Coffer found.
Shown is the rubble of a new school in Tupelo, Miss., which was damaged when an F5 tornado hit the town on April 5, 1936. Corbis
Coffer's work hasn't been put through a peer review yet, but he recently submitted it to the Electronic Journal of Severe Storms and Meteorology.
Despite advances in engineering and scientists' understanding of tornadoes, humans are still largely powerless against the threat of these natural disasters. One only needs to look at recent history for proof. The National Weather Service ranked 2011 the fourth deadliest year for tornadoes on record, with 550 deaths and 1,691 tornadoes reported across the country, including the catastrophic storm that hit Joplin, Missouri.
Tao, whose work was in part funded by a grant from the U.S. Naval Research Lab, said he hopes to do a field test with a small wall protecting a localized, high-risk area to prove his idea is feasible.
"Once this field test is successful, people will accept the idea, and the wall will be gradually extended to eliminate the major tornado threat for the entire Tornado Alley," Tao told Live Science in an email. He added that he welcomes comments of skeptics and hopes that his proposal will at least spur more research into tornado-stopping plans.
Tao isn't the first to come up with a tornado-blocking concept, and experts think there are more theoretically plausible options than physical walls. One possibility might be to kill tornadoes by freezing them.
"If you have a bad storm approaching and you could suddenly make the downdrafts of the storm really cold, it would probably have a disruptive effect," Markowski said.
Markowski doesn't expect to see something like that happen in his lifetime. Fast forward 1,000 years, however, and perhaps engineers will have the technology to make a giant refrigerator to cool a whole town by several degrees just before a tornado barrels in, he said.
But who knows what the future will hold? Just a few years ago, an inventor was awarded a patent for a scheme to send suicide drones into a tornado with an ultra-cold substance such as liquid nitrogen to thwart the storm.
More from LiveScience:
Infographic: Tornado! How, When & Where Twisters Form
Destructive U.S. Tornadoes of April 2014: Gallery
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