Tornado Alley, USA: new map defines nation's twister risk.Residents of Pontotoc County Pontotoc County is the name of several counties in the United States:
The science that deals with the phenomena of the atmosphere, especially weather and weather conditions.
[French météorologie, from Greek bull's-eye. Once every 4,000 years or so, a tornado strong enough to rip the roof off a house will sweep across the very spot in southeastern Oklahoma that you call home. That doesn't sound very frequent, does it? Truth be told, it isn't. Fewer than 1,200 tornadoes are spotted in the United States United States, officially United States of America, republic (2005 est. pop. 295,734,000), 3,539,227 sq mi (9,166,598 sq km), North America. The United States is the world's third largest country in population and the fourth largest country in area. each year. A typical twister damages an area of only about 13 square kilometers. In a country that boasts almost 9.4 million square kilometers, that leaves a lot of undamaged real estate.
Even in Tornado Alley--the nickname for the swath of the central United States The Central United States is sometimes conceived as between the Eastern United States and Western United States as part of a three-region model, roughly coincident with the Midwestern United States plus the western and central portions of the Southern United States; the term is where twisters are most common--tornadoes are a rare phenomenon. New models suggest that in this L-shape region, which stretches from western Iowa down through Nebraska and Kansas to southern Oklahoma and then over Arkansas and Louisiana to southeastern Mississippi, any particular spot can expect on average to wait from 4,000 to 10,000 years between roof-ripping twisters.
TWIST AND SPOUT Tornadoes are violently rotating columns of air suspended from so-called cumuliform cu·mu·li·form
Having the shape of a cumulus.
Adj. 1. cumuliform - shaped like a cumulus cloud
circular, round - having a circular shape clouds, which are dense, tall, and characterized by rising mounds, domes, or towers of condensed con·dense
v. con·densed, con·dens·ing, con·dens·es
1. To reduce the volume or compass of.
2. To make more concise; abridge or shorten.
a. water vapor. Twisters are home to the strongest winds on the face of the planet (SN: 5/15/99, p. 308), and they come in various sizes, from tightly whirling whirl
v. whirled, whirl·ing, whirls
1. To revolve rapidly about a center or an axis. See Synonyms at turn.
2. funnels just a few yards across to mile-wide behemoths.
By definition, a swirling vortex of wind isn't a tornado unless it's in contact with the ground. Also, a twister's size isn't correlated with its strength. Large tornadoes can be weak, and some of the smallest funnels are the most destructive. A tornado doesn't even have to have a visible funnel, says Charles A. Doswell III, a meteorologist at the University of Oklahoma University of Oklahoma, abbreviated OU, is a coeducational public research university located in the U.S. state of Oklahoma. Founded in 1890, it existed in Oklahoma Territory near Indian Territory 17 years before the two became the state of Oklahoma. in Norman. The funnel cloud associated with most tornadoes results from moisture condensing con·dense
v. con·densed, con·dens·ing, con·dens·es
1. To reduce the volume or compass of.
2. To make more concise; abridge or shorten.
a. out of humid air as the vortex accelerates and the air pressure inside drops.
Because tornadoes can't be characterized by their apparent size--and because some are never seen at all--they're rated according to according to
1. As stated or indicated by; on the authority of: according to historians.
2. In keeping with: according to instructions.
3. the damage they cause to humanmade structures. The Fujita scale Fujita scale (fjē`tə, f is named for the University of Chicago meteorologist, Ted Fujita Tetsuya Theodore "Ted" Fujita (藤田哲也 Fujita Tetsuya, October 23, 1920–November 19, 1998) was a severe storms researcher of the twentieth century. , who developed it in the 1970s. The damage ratings run from F-O, which causes slight damage to chimneys, to F-5, in which well-built homes are demolished, steel-reinforced concrete structures are badly damaged, and automobiles or objects of similar heft are lofted distances of more than 100 meters.
Meteorologists Atmospheric scientists
What's causing the proliferation? "It's probably not climate change," says Schaefer. "It's people change."
As people move out of cities and disperse over larger areas, they are more likely to spot tornadoes. As increasing media coverage makes travelers and residents of suburbs and exurbs more weather-aware, they are more likely to report twisters. The growing popularity of cellular phones, which enable people on the road or in areas of downed phone lines to call in sightings to emergency centers, will probably continue to fuel the apparent increase, Schaefer speculates.
HEAD FOR COVER! The few decades of reports now available simply don't include enough twisters to permit a reliable analysis of the nation's tornado risk. Therefore, meteorologists such as Harold E. Brooks of the National Severe Storms Laboratory The National Severe Storms Laboratory (or NSSL) is a National Oceanic and Atmospheric Administration weather research laboratory located at the National Weather Center in Norman, Oklahoma. in Norman, have turned to computer simulations. In their most recent study, Brooks and his colleagues started with weather and damage data from 10,000 tornadoes that occurred between 1921 and 1995. Using just five parameters, their simulation generated a realistic mix of 4 million tornadoes over 30,000 years that the team then subjected to statistical analyses.
First, the model calculated the probability of a tornado occurring on a particular day. If a tornado appeared, the model computed the probability of companion twisters being spawned that day by other thunderstorms thunderstorms
a storm characterized by thunder and lightning caused by strong rising air currents; identified as agents of animal disease because of their involvement causing (1) spasmodic colic; (2) lightning strike; (3) injuries of cattle acquired in stampedes initiated by storms. . Then, by reference to data from the original group of twisters, each tornado was assigned a Fujita scale rating, a path length over which damage would occur, and a path width.
The simulation mapped the area damaged by each tornado into the 80-km-square grid box where the twister first touched down. The greater the tornado damage in a box at the end of the simulation, the higher the probability that any point within it would have been hit by a twister, says Brooks. From that probability, a simple calculation tells the average length of time that passed before a tornado rated F-2 or larger will hit any given spot. An F-2 rating signifies winds strong enough to rip the roof off a frame-built home.
The researchers presented their analyses at the meteorological meeting in January.
Southeastern Oklahoma fared worst in the simulation, with each point in the area getting hit once every 4,000 years on average. A large portion of the central United States--stretching from the Colorado-Kansas border to western North Carolina Western North Carolina (often abbreviated as WNC) is the region of North Carolina which includes the Appalachian Mountains, thus it is often known geographically as the state's Mountain Region. and from the Gulf Coast to southern Minnesota--suffered a twister approximately once every 10,000 years. Nevada suffered tornadoes so infrequently that points there might get damaged only once every 5 million years.
Brooks suggests that the exact probabilities of being hit by a twister in extremely low-risk areas such as Nevada should be taken with a grain of salt. The number of tornadoes from those regions in 75 years may have been too small to provide accurate input to the model. Tornado hazard in such regions is probably somewhat higher than the model predicts, Brooks speculates.
Other meteorologists think so, too, for slightly different reasons. Doswell points out that many areas of the U.S. West, and even parts of the Great Plains, are so sparsely populated pop·u·late
tr.v. pop·u·lat·ed, pop·u·lat·ing, pop·u·lates
1. To supply with inhabitants, as by colonization; people.
2. that many tornadoes are simply never observed. It's often 100 km or more between the small towns in these areas, he notes. Even if tornadoes are spotted, they're probably underrated on the Fujita scale because there's little for the twisters to damage except crops, rocks, and the occasional tree or telephone pole.
"All we have to go on is the data we have," says Doswell. "We know that it's inaccurate, but it's the best that we've got."
Schaefer agrees: "We've seen what we've seen. We're just not sure we've seen everything."
DAMAGED GOODS DAMAGED GOODS. In the language of the customs, are goods subject to duties, which have received some injury either in the voyage home, or while bonded in warehouses. See Abatement, merc. law. ? Other factors, many of them related to the Fujita damage scale and how it's applied, might significantly affect the results of the tornado-risk assessment conducted by Brooks and his colleagues.
First of all, says Doswell, a tornado's F-scale rating is based on the worst damage found anywhere along its path. In a simulation, that might tend to overestimate o·ver·es·ti·mate
tr.v. o·ver·es·ti·mat·ed, o·ver·es·ti·mat·ing, o·ver·es·ti·mates
1. To estimate too highly.
2. To esteem too greatly. the hazard because the high F-scale rating would apply to the entire swath of damage. On the other hand, many tornadoes may be underrated because the strongest winds in a funnel may occur at a time and place where there's nothing much to damage.
Schaefer notes that many of the F-scale damage ratings assigned in his database to tornadoes between 1950 and 1975 may be too high. His analyses show that even while the average number of tornadoes has appeared to increase each year, the number of twisters rated F-2 and higher has dropped significantly during that period. Schaefer again suggests that people, and not an actual change in climate or tornado behavior, are to blame for the anomaly. In the mid-1970s, soon after the Fujita scale was invented, trained observers went into the field to assess tornado damage. For earlier twisters, researchers have used newspaper accounts to assign F-scale ratings. Exaggerations in reporting, among other factors, may have led to inflated damage estimates.
And maybe the Fujita scale's assessments are too simplistic sim·plism
The tendency to oversimplify an issue or a problem by ignoring complexities or complications.
[French simplisme, from simple, simple, from Old French; see simple , says Schaefer. For one thing, the field personnel should consider the quality of construction of the structures that have been damaged. The destruction of one flimsy building shouldn't skew (1) The misalignment of a document or punch card in the feed tray or hopper that prohibits it from being scanned or read properly.
(2) In facsimile, the difference in rectangularity between the received and transmitted page. the F rating of an entire tornado, he notes.
Likewise, the assessors need to evaluate whether other factors, such as windblown debris, aggravated ag·gra·vate
tr.v. ag·gra·vat·ed, ag·gra·vat·ing, ag·gra·vates
1. To make worse or more troublesome.
2. To rouse to exasperation or anger; provoke. See Synonyms at annoy. damage. A tornado that struck Xenia, Ohio Xenia (pronounced Zeen-yuh) is a city in the U.S. state of Ohio and the county seat of Greene CountyGR6. The municipality is located in southwestern Ohio near Dayton. , on Sept. 20, 2000, is a perfect example, he says. The home that suffered the most damage collapsed when the roof blown off the house across the street landed on it. Houses next-door escaped nearly unscathed.
Many factors influence the extent of damage that tornadoes inflict, says Doswell. Besides the strength of the structure that's hit and the quantity of windblown debris, these factors include how long the winds blow on the structure and the rate at which the winds intensify. "We don't know Don't know (DK, DKed)
"Don't know the trade." A Street expression used whenever one party lacks knowledge of a trade or receives conflicting instructions from the other party. a lot of the details of how winds behave around tornadoes," Doswell laments.
Although scientists know nature's overall ingredients for tornadoes (see box, page 297), mystery enshrouds the small-scale phenomena that cook up a twister. In fact, says Doswell, the real question is why there aren't more? Only a tiny fraction of the storms that occur each year produce tornadoes, and the ones that end up spawning damaging funnels aren't much different from those that don't.
Tornado Scarcity Twister total through April is one of the lowest on record For the first 3 months of this year, Tornado Alley--indeed, the whole nation--caught a break. Through March 31, only 40 tornadoes had struck the continental United States. This year's start is tied for the ranking of third lowest since 1950, says Joseph T. Schaefer, director of the National Weather Service's Storm Prediction Center in Norman, Okla. The record low count, 16, occurred during the first quarter of 1969, and only 18 tornados spun across the lower 48 states during the same period in 1951. As of May 1, this year is still running well behind normal with only 154 tornados, as compared with an average of 270 for Jan. 1 through April 30. For this year's dearth of funnels, twisterphobes can thank the high-altitude river of air known as the jet stream. Its southerly position this spring prevented masses of warm, moist air in the Gulf of Mexico from moving north to the U.S. plains, where they could collide with cold air spilling down from Canada. That mixing creates the thunderstorms that spawn most tornadoes, explains Schaefer. Instead, the jet stream this year steered most storms out over the Gulf or along the coast. Could this good luck last the rest of the year? Statistically, it's likely. From 1955 through 1999, the 11 years that tallied the lowest January-to-March tornado totals also experienced fewer-than-average twisters in the following 9 months, says Harold E. Brooks, a meteorologist at the National Severe Storms Laboratory, also in Norman. When totals were adjusted to account for the gradual increase in tornado reports since the 1950s, each of the 11 years with the fewest first-quarter tornadoes had an average of just 71 twisters. That period for the other 33 years registered, on average, a whopping 155 funnels. Brooks and his colleagues found that this disparity carried through to the rest of the year. The 11 years with low-tornado winters counted, on average, 943 twisters in the period from April 1 through Dec. 31, while the other years racked up 1,064 funnels.--S.P.
FREE-RANGE FUNNELS While tornadoes are most common on the Great Plains and throughout the Mississippi River Mississippi River
River, central U.S. It rises at Lake Itasca in Minnesota and flows south, meeting its major tributaries, the Missouri and the Ohio rivers, about halfway along its journey to the Gulf of Mexico. valley, they can occur almost anywhere in the United States. Rough terrain may inhibit the formation of twisters, but it doesn't prevent them altogether. That's because the storm clouds that produce tornadoes are often 12 to 15 km from top to bottom, says Doswell. "The atmosphere doesn't know about the calendar, the clock, or a map, and an atmospheric feature that size isn't going to care about a bump on the ground," he adds.
The idea that tornadoes don't cross mountain ranges, rivers, or other geographical features simply isn't true, says Doswell. In July 1987, an F-4 tornado roared across the Continental Divide in Yellowstone Park at an elevation of at least 3,000 m. The twister that struck Salt Lake City in August 1999 dropped down one side of a canyon and climbed up the other. The deadliest tornado in U.S. history is the so-called Tristate tornado of March 18, 1925, which killed 695 people. It swept across the Mississippi River as it carved a 350-km corridor of devastation across Missouri, Illinois, and Indiana.
Likewise, it's only a myth that twisters avoid large cities. Salt Lake City, Oklahoma City Oklahoma City (1990 pop. 444,719), state capital, and seat of Oklahoma co., central Okla., on the North Canadian River; inc. 1890. The state's largest city, it is an important livestock market, a wholesale, distribution, industrial, and financial center, and a farm , Miami, Nashville, and Fort Worth, Texas Fort Worth is the fifth-largest city in the state of Texas, 18th-largest city in the United States, and voted one of "America’s Most Livable Communities. , are just a few of the downtown areas that have been damaged by tornadoes in recent years. The only thing that protected them previously had been good luck and their small size, says Doswell.
Even though tornadoes are extremely rare events, they wreak wreak
tr.v. wreaked, wreak·ing, wreaks
1. To inflict (vengeance or punishment) upon a person.
2. To express or gratify (anger, malevolence, or resentment); vent.
3. havoc when they do strike. "All the statistics in the world don't help you if you're in the path of a tornado," he notes. "Somebody's going to be unlucky every year."