Tornadoes strike again. How do they work?

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Tornadoes strike again. How do they work?
aerial view of a neighborhood with flattened houses
Tornado damage in Louisville, Miss., Apr. 29, 2014. As many as 41 tornadoes struck the U.S. midsection in the last days of April.
 

At the end of April, a spasm of tornadoes struck America’s Tornado Alley once again, ripping through Nebraska, Kansas, Iowa, Oklahoma, Arkansas, Louisiana, and Mississippi.

An estimated 35 people died in the outbreak.

As the hard-hit towns clean up, The Why Files is wondering about tornadoes, nature’s most intense storms. Where do these spinning winds originate? What drives tornadoes, and what would protection look like?

  Tornadoes are compact, powerful whirlwinds powered by differences in wind, moisture and temperature. Although tornadoes occur in India, Bangladesh and other regions, they are most intense and devastating in the United States. Tornadoes are most frequent in the afternoon and evening, after the daily buildup of heat powers a violent “supercell” thunderstorm that can produce a tornado.

April 27, 2014 tornado-producing storms seen from space

Time-lapse video of the weather system that spawned tornadoes in seven central and southern U.S. states. Those explosions of white show water rising rapidly, powered by convection, marking tornado tops on this turbulent spring day.
NOAA’s GOES-East, NASA
ENLARGE
tornado alley shown on map of U.S. shows a central area stretching from South Dakota down to the middle of Texas
Tornadoes are most common in “Tornado Alley,” particularly in spring and summer. Alabama, Arkansas, Florida, Georgia, Illinois, Indiana, Louisiana, Mississippi, Missouri and South Dakota also have many tornadoes.
Credit: NOAA

A tornado may stay on the ground for an hour or so; at the extreme, they may be more than a mile wide. Virtually nothing above ground can survive the strongest tornadoes.

Tornado headquarters

Tornadoes most commonly strike Tornado Alley in the American mid-section, where the atmosphere is affected by the Rocky Mountains to the west and the Gulf of Mexico to the south. Howard Bluestein, a veteran storm chaser and professor of meteorology at the University of Oklahoma notes that in the spring, a strong westerly jet stream across the Alley creates instability and a trough of low pressure that draws in warm, moist air from the Gulf. “Conditions for the supercells [large, powerful thunderstorms] that spawn tornadoes require strong vertical wind shear [changes in wind speed and direction with height] and lots of instability,” he says — as happens in Tornado Alley.

Chart shows EF numbers, description of damage, and photos corresponding to those damages. EF-1 is least amount of damage. EF-5 is most damage
The enhanced Fujita scale assesses tornado strength, based on the level of destruction. F5 tornadoes are so rare that in some years, none strikes the United States.
ENLARGE
Illustration shows Tornadoes need: Unstable Atmosphere (for the supercell to develop), faster winds at higher altitude, and rotation
Supercells form where warm, moist air from the Gulf of Mexico meets cool, dry air from the west. The sun warms the moist, air, which rises due to convection, breaking through the cap and releasing latent heat that powers the updraft, the thunderstorm and tornadoes.

The Nuts and Bolts of a Whirlwind

ENLARGE
Tornado diagram shows swirling hot air rising around the tornado, and cold air coming down from above, through the center of the tornado creating a circular funnel of air
A tornado forms when 1. Humid air is trapped beneath cold, dry air; 2. A “cap” between the two air bodies is disturbed by wind or a weather front; 3. Lower air rises and expands toward the reduced pressure aloft, punching a hole in the cap; 4. Moisture condenses, releasing latent heat which warms the air, causing it to rise by convection at up to 150 mph. The cloud has now formed a thunderstorm; 5. The thunderstorm may die out in intense rain and/or hail, or it may spawn a tornado. 6. Interactions between air at various altitudes, humidities and temperatures cause rain, lightning, air circulation and a strong, rotating updraft, now called a “mesocyclone.” This rotation is almost always counter-clockwise (seen from above) in the Northern Hemisphere; 7. A tornado may form below the mesocyclone. As the spinning air column narrows, it rotates faster and the funnel cloud extends higher into the storm.
Adaptation of tornado diagram from Shutterstock.

Energy and the tornado

Scientifically speaking, energy is the ability to do work (that’s one of the plainest definitions in science). Energy takes many forms, including chemical, kinetic, potential and thermal.

Energy can change forms: Green plants use solar energy to create chemical energy. Plants eventually form petroleum. Your Model T burns petroleum-derived gasoline, creating heat energy, which the engine transforms into kinetic energy. And when your brakes stop the car, the kinetic energy is transformed back into heat energy.

White tower descends from sky, getting narrower as it nears the ground
An F5 tornado struck Elie, Manitoba, Canada, (40 kilometers west of Winnipeg) on June 22, 2007. This was one of the strongest twisters since 1999 and one of only nine to reach EF5 between 1999 and 2011 in North America. Notice the large cloud of flying debris at the base. This mass of dirt, rocks and building parts is a major cause of injury in tornadoes.

A different set of energy transformations powers the furious winds of a twister. The tornado’s ultimate source of energy, the sun, warms the ocean, evaporating water, which carries the latent heat of vaporization (a kind of potential energy) into the atmosphere. When the water vapor rises, it cools and condenses.

That condensation, in turn, releases the latent heat — the biggest single source of energy in a thunderstorm. Latent heat warms the rising air, causing a density difference that lifts the air fast enough to form a tornado.

A tornado with wind speeds of 200 mph releases kinetic energy at the rate of 1 billion watts — equal to the electric output of a large nuclear or coal-fired electric generator.

But that’s just processed cheese compared to the supercells that can spawn tornadoes. These monsters release latent heat at the rate of 40 trillion watts — 40,000 times greater than the twister.

What’s new?

We asked Bluestein about recent advances in tornado prediction, and he pointed to a new system that views the atmosphere in much finer detail, and hence can forecast severe weather, including tornadoes, with greater precision.

map of U.S. shows tornadoes to date this year
How many tornado watches have occurred in your area? Click for today’s data.
Credit: NOAA

The High-Resolution Rapid Refresh system, which will go nationwide later in 2014, will update forecasts hourly over the lower 48 United States at extremely sharp resolution, based on the latest observations from ground and satellite-based sensors, radars and aircraft.

Forecast systems model the atmosphere by looking at conditions at points on imaginary grids; conditions at points between the grid intersections are unknown. The data points in HRRR will be three kilometers apart; for comparison, data points on the system now supporting hourly NOAA forecasts are 13 kilometers apart.

top image shows green hill 'Up the hill (more damage)' with much brown earth showing, indicating where trees were ripped up by tornado. Bottom image is 'Down the hill (less damage)' with much more green foliage
The Apr. 27, 2011 tornado in Tuscaloosa, Ala., caused much more damage on the uphill sections of its path (left) than on the downhill sections.
Google Earth images, courtesy Panneer Selvam, University of Arkansas1

Stan Benjamin, a research meteorologist at NOAA’s Earth System Research Lab in Boulder, Colo. said in a National Oceanic and Atmospheric Administration press release, “When a typical thunderstorm is about 10 to 20 kilometers across, it contains both upward and downward air currents as well as other features that give clues to its potential to create dangerous weather, so it’s important to be able to see what’s happening inside the storm.” Benjamin, who leads the team that developed HRRR, says, “It’s a game-changer to go from 13 to three kilometers for model resolution.”

With that accuracy, HRRR can identify rotating storms, which are more likely to produce tornadoes. It can also predict damaging straight-line windstorms called “derechoes” and detect changes in storm severity.

The benefits of HRRR are already evident to Bluestein, who had to cut our interview short to go out storm chasing on Wednesday. We asked about the increase in prediction accuracy, and he said, “I can’t give you a good quantitative answer, but 20 years ago, when we went out, we would see a tornado maybe one time in 10. These days that’s a lot higher. It’s helped us quite a bit, and it also reduces the false alarm rate.”

In the realm of basic physics, Bluestein says, atmospheric scientists are closing in on a fundamental question: Where does the rotation originate? He says the answer seems to lie in a zone of extreme pressure differential in the forward flank of the supercell.

A path to greatness?

Do tornadoes lumber aimlessly across the countryside? Not always, according to a recent study of catastrophic 2011 tornadoes in Tuscaloosa, Ala., and Joplin, Mo. After tracking the destruction on aerial photos, Panneer Selvam at the University of Arkansas found that twisters preferred going uphill, and also caused more destruction while ascending. “We wanted to understand the impact of terrain on damage magnitude and tornado path,” said Selvam, professor of computational mechanics. “Information about this interaction is critical. It influences decisions about where and how to build, what kind of structure should work at a given site.”

The proclivity for uphill travel seemed to be the deciding factor when the two titanic tornadoes changed direction. The researchers also noticed the tornadoes hopping over valleys and causing significant damage only on the top of the bordering hills.

A record of death and destruction

Rollover image to see average deaths per state.
Both NOAA

The two tornadoes were astonishingly deadly. Tuscaloosa destroyed parts of that city and Birmingham, Ala., on Apr. 27, 2011, killing 64 and causing roughly $2.2 billion in property damage. For a few weeks, it was the costliest single tornado in U.S. history. Then the EF5 Joplin tornado damaged or destroyed roughly a third of that city, killing 162 and causing $2.8 billion in damage.

Between 2001 and 2010, the average annual U.S. death toll was 58.

In a computer model of hills and tornadoes,2 Selvam modeled the relationship between tornado radius and hill height. A low-level tornado vortex was “significantly disrupted” if the hill is at least as tall as the tornado’s radius. Shorter hills provided less protection.

Computer models, Selvam notes, “don’t have much validity unless you have a way to verify them. We are trying to look at both sides. Students are modeling different heights from the ground to see what is happening,” and then studying the effects of real tornadoes. “If they find a similar pattern, that gives the model more veracity.”

A question of safety

In the United States 20 years ago, only 35 percent of tornadoes were preceded by a warning, says Daniel Sutter, a professor of economics at Troy University in Troy, Ala. By 2012, 74 percent had a warning.

Sutter says warnings have grown more timely, credible and effective due to:

The publicity around tragedies like Joplin;

the improved forecasting ability with increased use of Doppler radar;

a greater understanding of the mechanisms of tornadoes; and

A reduction in false warnings due to improved data collection and analysis.

“Warnings are such short-fuse events,” Sutter says. So how effective are the tornado watches that are intended to spread the word that a tornado is possible?

In a study of tornadoes between 1986 and 20043 Sutter found that a watch preceding a warning did not save lives. “We were trying to see if the watch had a life-saving benefit in addition to the warning, and we were not able to find any evidence of that.”

Sutter admits he was surprised by the result.

“There are a lot of things people might do to prepare during a watch,” he says, especially considering that 26 percent of twisters were unwarned, and sometimes the funnel cloud comes right on the heels of the warning.

Sutter does not favor deep-sixing tornado watch. They can, he says, trigger early efforts to organize community assistance, and notify tornado spotters, weather helicopters, and television crews trying to cover the storm. “We did not attempt to quantify any benefit from these.”

Your house: How safe?

If tornadoes are undeniably dangerous, why not build houses able to withstand a direct hit by an EF4 or EF5? Because it’s impossible — the extreme fluctuation in air pressure in these tornadoes can cause a house to explode.

Fortunately, direct hits from such monster storms are extremely rare. Even in Tornado Alley, a twister hits a given square mile only once every 700 years, and unless you want to live underground, it makes no economic sense to build a house to survive such a cataclysm.

But it’s a different story for houses under weaker tornadoes, or those near the path of an EF4 or EF5. These houses, which can be damaged or destroyed by wind, rain and flying debris, can benefit from simple, cheap fixes, especially regarding the roof, which is needed not only to keep out torrential rain, but to prevent the walls from collapsing:

Roof shingles are usually the first to go. Shingles near roof edges face the strongest winds and should be set in special mastic during reshingling.

Roof sheathing, usually made of oriented strand board or plywood, should be nailed or stapled securely to the rafters. Fasteners should be 6 inches apart at the edges, with a 12-inch spacing elsewhere.

Flat steel sheet on garage floor open to reveal shelter.
The FlatSafe tornado shelter seems to meet the extreme requirements of an effective shelter, and carries some significant endorsements, but it looks expensive.
Photo: Wesley Fryer/flickr

Rafter fastening is critical. These angled beams support the roof sheathing and were traditionally nailed to the walls. This nailing is extremely weak, as the nails tend to split the rafters. Simple, cheap, steel “hurricane ties” are many times stronger.

Anchor bolts prevent a house from blowing off the foundation. Even though building codes usually require anchor bolts, builders sometimes deem them optional. A tornado may get the last word on the subject.

The tornado shelter

Protecting the occupants is even more important than protecting the house. A new home in tornado country can be fitted with a tornado room in an interior, windowless room. Steel doors and reinforced concrete walls and ceiling can form a room that’s nearly bombproof, except in a direct hit.

The conventional wisdom is that existing houses cannot be retrofitted with a safe room, but Robert Falk of the U.S. Department of Agriculture’s Forest Products Laboratory in Madison, Wis. wants to change that.

“I’m not sure how many houses have safe rooms, but not nearly enough do,” Falk says. To expand their use, he is overseeing development of a practical, affordable safe room that would:

use commodity building material and hardware;

be constructed by a do-it-yourselfer who knows how to use a nail gun and a saw;

be built in an existing basement; and

be affordable. “I think we can do it for $3,000 to $4,000, plus the homeowner’s sweat equity,” says Falk, “or even less if we can make a wooden door work.”

A 2 x 4 fired at 100 mph strikes a test wall for a safe room at the U.S. Department of Agriculture Forest Products Laboratory in Madison, Wis. The wall, composed of a triple layer of 2 x 8 lumber nailed loosely together, is being evaluated for a safe room that can be built inside an existing basement. Dots record wall movement during the shot, which was gauged a success because the 2 x 4 did not penetrate the wall.
Courtesy Robert Falk, FPL

Falk expects to finish development in a year or two, and hopes to persuade stores in Tornado Alley to sell a complete kit of materials and instructions.

Shelter is the name of the game, says tornado expert Bluestein. “Warnings may be getting better, but people still have to react in the right way. If everyone hears about a tornado and gets in the car to try to outrun it, they will get caught in traffic, and they are going to get hurt. If it’s an EF5, it’s going to wipe out everything, so if people have not gotten to shelter, or don’t have an underground shelter, they could be out of luck.”

– David J. Tenenbaum

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Terry Devitt, editor; S.V. Medaris, designer/illustrator; Yilang Peng, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. The Effect of Terrain Elevation on Tornado Path R. Panneer Selvam & Nawfal Ahmed, 12th Americas Conference on Wind Engineering, Seattle, Washington, USA, June 16-20, 2013
  2. Three-Dimensional Simulation of Tornado over Complex Terrain, Piotr Gorecki & R. Panneer Selvam 12th Americas Conference on Wind Engineering, Seattle, Washington, USA, June 16-20, 2013
  3. Preparing for Danger: On the Impact of Tornado Watches on Tornado Casualties, International Journal of Mass Emergencies and Disasters, Daniel Sutter and Kevin Simmons, March 2014, Vol. 32, No. 1, pp. 1–25
  4. FEMA’s tornado safety page
  5. NOAA’s Storm Prediction Center
  6. The Weather Channel’s Tornado Central, severe weather forecast.
  7. Six of the worst twisters in US history