National Geographic News
A photo of a tornado touching down in Kansas.

This tornado touched down a few miles east of Rozel, Kansas.

PHOTOGRAPH BY COLT FORNEY, NATIONAL GEOGRAPHIC YOUR SHOT

Brian Clark Howard

National Geographic

Published April 28, 2014

At least 29 people are dead after a string of tornadoes hit states in the Southern and Midwestern U.S. over the past few days. The National Weather Service predicts more severe weather on Tuesday.

How tornadoes form and how they die is not fully understood, yet scientists probing those mysteries—and aiming to improve warning systems—have pinpointed key risk factors.

A tornado, or twister, is a violently rotating column of air that extends between the Earth's surface and a cloud, usually a cumulonimbus cloud. Most tornadoes last for less than ten minutes, says Harold Brooks, a research meteorologist with the National Oceanic and Atmospheric Administration's (NOAA) National Severe Storms Laboratory (NSSL) in Norman, Oklahoma.

Large tornadoes usually last longer—around 30 minutes, Brooks adds. The most powerful twisters have wind speeds of more than 300 miles (483 kilometers) per hour, which can rip buildings off their foundations. They can be more than two miles (3.2 kilometers) wide, and can spin across the ground for dozens of miles.

The more common tornadoes have wind speeds of less than 110 miles (177 kilometers) per hour, are about 250 feet (76 meters) across, and travel only a few miles before they dissipate.

Tornadoes kill an average of 60 people a year in the U.S., mostly from flying or falling debris, reports NOAA. (See "Interactive: Forces of Nature.") Half of those deaths are caused by the strongest one percent of the most violent storms, says Brooks.

How Tornadoes Form

True tornadoes, unlike smaller swirling winds like dust devils and waterspouts, emerge from what are called supercell thunderstorms. For such a storm to form, you first "need the ingredients for a regular thunderstorm," says Brooks.

Those ingredients include warm moisture near the surface and relatively cold, dry air above. "The warm air will be buoyant, and like a hot-air balloon it will rise," says Brooks.

Animation of a tornado forming.

A supercell requires more: winds that increase in strength and change direction with height. "Then the updraft tends to rotate, and that makes a supercell," explains Brooks.

The supercell churns high in the air and, in about 30 percent of cases, it leads to the formation of a tornado below it. This happens when air descending from the supercell causes rotation near the ground.

Even then, "we still don't know why some thunderstorms create tornadoes while others don't," tornado-chaser Tim Samaras said in early 2013. Samaras was a scientist and National Geographic grantee who was killed by a twister on May 31, 2013, in El Reno, Oklahoma. (Read "The Last Chase" in National Geographic magazine.)

Brooks says scientists believe strong changes in winds in the first kilometer of the atmosphere and high relative humidity are important for the formation of tornadoes. He adds that there also needs to be a downdraft in just the right part of the storm.

Tornado formation also requires a "Goldilocks" situation, in which air must be cold but not too cold. It should be a few degrees more frigid than surrounding air, Brooks says.

He adds, "We don't understand how tornadoes die: Eventually the air gets too cold and it chokes off the inflow of new air into the storm, but we don't know the details."

Graphic on how a tornado forms

Where and When Twisters Strike

Tornadoes have been observed on every continent except Antarctica. They have been most documented in North America, where an estimated 1,200 strike the United States each year, but they frequently appear in many other countries.

The most notoriously affected region in the United States, called "Tornado Alley," includes the Great Plains states of Kansas, Nebraska, and the Dakotas, as well as parts of Texas. Large-scale weather patterns tend to converge on that area, making tornadoes more likely.

Still, the state that receives the highest number of tornadoes per square mile is Florida, according to the American Meteorological Society. Indiana, Illinois, Iowa, and Louisiana also have many tornadoes per square mile.

Tornadoes can happen at any hour of the day and any time of the year, though they are most common in the spring, especially during May and June in North America.

In many countries, including the United States, Canada, and continental Europe, the strength of tornadoes is often measured by the Fujita scale or the updated Enhanced Fujita Scale. An F0 or EF0 tornado damages trees but substantial structures are left unharmed; a tornado in the strongest category—F5 or EF5 —blows away buildings.

Since measuring wind speeds inside a twister is extremely difficult, scientists typically rely on damage to estimate velocities.

The Difficulties of Forecasting

Tornadoes are much harder to forecast than are hurricanes, which are larger storms that last a lot longer. According to NOAA, the average amount of time between a tornado warning and the arrival of a storm is about 13 minutes. (A tornado warning means a twister has been sighted, while a tornado watch means one is possible.)

The National Severe Weather Laboratory's Warn-on-Forecast research project is aiming to improve forecasting, although the work is challenging, says Brooks.

The project uses powerful software to crunch data on temperatures, moisture, and other atmospheric variables. Sometimes the system "makes really good forecasts, and other times it doesn't," says Brooks.

As computers get faster and data improves, accuracy may rise, he suggests. In the meantime, better understanding of the atmosphere will also help with other endeavors, such as planning for wind farms or the placement of solar panels.

Brooks adds, "It's not completely clear that increasing the lead time for tornado [forecasts] is going to benefit the general public, because we're not sure how people are going to respond to that information." Many people ignore current tornado watches, for instance, thinking the threat is unlikely.

But, Brooks says, "there are probably audiences out there that will be able to take good advantage of it, such as emergency managers and vulnerable populations that might take a long time to get prepared."

Predicting the path of a tornado across the landscape can also be challenging. Brooks says tornadoes tend to follow the general movement of the thunderstorm they are associated with, but the route can be erratic.

"It's kind of like walking a dog," he says. "You get down the block, but in the middle the dog goes back and forth."

Follow Brian Clark Howard on Twitter and Google+.

19 comments
dj pressley
dj pressley

Actually, it isn't true that all tornadoes rotate clockwise in the northern hemisphere.  The majority rotate counterclockwise, but there are also tornadoes that rotate clockwise.  They are rare but they do exist.  They even have a special name - anticyclonic.  Also, the reason tornadoes in the northern hemisphere mostly rotate counterclockwise and those in the southern hemisphere clockwise is explained by the coriolis force.

Joel Chick
Joel Chick

In response to F.P. Maggio. The difference in rotation of the earth from northern to southern hemispheres is purely an observational one. A simple trick to grasping this concept is to look down on a dinner plate and slowly turn it in a counter-clockwise direction (as the Earth turns to the east). As if looking down on the Earth from above the north pole. Now pick up the plate and carefully keep it turning in the same counter-clockwise direction, but now slowly raise it above your head so you can see it from underneath. As if looking up from below the south pole. Although the plate/earth is spinning in the same direction as before, the change in observation point permits the viewer to now observe a clockwise-rotation. A tunnel (or tornado) running right through the centre of the planet from the northpole to the southpole would not change direction as it crossed through the core/equator, it would spin in the same direction - but one's view and the subsequent direction of the spin would change depending on your location (ie which hemisphere you're in).

Devon Michael
Devon Michael

What is more practical and safer to do? 1- Destroying the tornado before it is big enough to do damage. 2- Having an adequate warning system that allows citizens to be properly prepared.

Satellites can be used to heat up the cold air a tornado needs to form. By using microwaves a fleet of Satellites could bring down a forming tornado. The European Space Agency has funded studies of such satellites. http://www.popsci.com/scitech/article/2003-07/how-destroy-tornado by stopping the tornadoes before serious damage occurs lives and infrastructure can be saved. 

marlvin chiungwe
marlvin chiungwe

I have never been in a tornado, twister or hurricane before and I think I would really panic if ever I was caught in one given my fear of thunderstorms... the weather patterns are not that severe in Southern Africa.

robbie butler
robbie butler

tornadoes are awesome  weather type  just they spawn in the wrong places

Byron Scott
Byron Scott

Tornadoes are actually "charged sheath vortices", it's an electric discharge phenomena, just as lightning is...

Mark Chandler
Mark Chandler

This is incorrect.  If the rotation of the tornado is formed as this article shows, then there is no preference for clockwise or counter clockwise rotation.  Hence with hundreds of  tornados observed, looking up at them from the ground, roughly half would rotate clockwise, and half counter clockwise.   But this distribution is not observed.  In the northern hemisphere, all rotate clockwise, in the southern hemisphere all rotate counter clockwise.  Why is that?   Same phenomenon observed with the flush of the toilet.   No matter how long the water is allowed to sit calmly with no initial rotation, it always rotates.... why?  And in the northern hemisphere it always rotates the same direction, in the southern hemisphere it always rotates in the opposite direction.   When the data of experiment in the real world disagrees with the theory, the theory is to be discarded.  The pseudo force caused by the earths rotation is the cause of the rotation of the rapidly rising or falling fluid/gas.  The speed of the rise, the viscosity of the fluid, and the strength of gravity (mass of the planet) determines the rate of rotation.  Check the data to confirm.


I expect more from Nat Geo.  Step up your game, get a better staff.

Preeti Sharma
Preeti Sharma

Tornadoes are scary and they are dangerous sign of destruction. Let's pray that they never take form on our planet "Earth". 

Ignignokt Mooninite
Ignignokt Mooninite

@dj pressley It can , and does happen .

I remember when Dr Fujita was studying whether or not some of the tornadoes that formed during Hurricane Andrew were anticyclonic , as some of the damage was extreme , such as 1 ton+ concrete tie beams being flung over 300 yards . His theory was the anticyclonic rotation of the tornadoes exacerbated the already Cat 5 winds and caused such incredible forces .

Ignignokt Mooninite
Ignignokt Mooninite

@Mark Chandler Actually , the Coriolis effect in a sink or toilet is too small to cause rotation one way or the other . The construction of the drain and the way it is introduced is much more of an influence .

F. P. Maggio
F. P. Maggio

@Mark Chandler I read your comment and I tend to agree with - re: counter rotational forces coupled with magnetics.  Why though when you cross the equator is rotation reversed when obviously, the entire planet spins in one direction? I ask the same question.  Why the difference in rotation from northern to southern hemisphere?.

Philip Olson
Philip Olson

@F. P. Maggio @Mark Chandler

The equator rotates at a higher speed and the poles effective rotational speed drops to zero.  The edge of any rotating mass is traveling more rapidly closer to the equator.  The acceleration from this effect will cause the edges close to the equator to move in the same direction as if they were gears with the cogs meshed.

This leads to counter rotation when viewed from above.

The grain in trees reflect this same bias, as does the bands on the gas giant planets when viewed with telescopes.  This is a documented physical result of rotating a sphere and works on both the large scale of planetary atmosphere and in the small scale of your kitchen sink.

The question is the mechanics of a vortex reaching the surface from the top of the cloud formation, the devil is in the details.

Share

Popular Stories

The Future of Food

  • Why Food Matters

    Why Food Matters

    How do we feed nine billion people by 2050, and how do we do so sustainably?

  • Download: Free iPad App

    Download: Free iPad App

    We've made our magazine's best stories about the future of food available in a free iPad app.

See more food news, photos, and videos »