FREQUENTLY ASKED QUESTIONS

About Tornadoes...

Enid, Oklahoma tornado in 1966

Where do they come from?

Tornadoes come from the energy released in a thunderstorm. As powerful as they are, tornadoes account for only a tiny fraction of the energy in a thunderstorm. What makes them dangerous is that their energy is concentrated in a small area, perhaps only a hundred yards across. Not all tornadoes are the same, of course, and science does not yet completely understand how part of a thunderstorm's energy sometimes gets focused into something as small as a tornado.

Where do they occur?

Whenever and wherever conditions are right, tornadoes are possible, but they are most common in the central plains of North America, east of the Rocky Mountains and west of the Appalachian Mountains. They occur mostly during the spring and summer; the tornado season comes early in the south and later in the north because spring comes later in the year as one moves northward. They usually occur during the late afternoon and early evening. However, they have been known to occur in every state in the United States, on any day of the year, and at any hour. They also occur in many other parts of the world, including Australia, Europe, Africa, Asia, and South America.

If you'd like to plot tornado tracks, download Severe Plot and the associated data from the NOAA Storm Prediction Center.

What type of damage can they do?

The damage from tornadoes comes from the strong winds they contain. It is generally believed that tornadic wind speeds can be as high as 300 mph in the most violent tornadoes. Wind speeds that high can cause automobiles to become airborne, rip ordinary homes to shreds, and turn broken glass and other debris into lethal missiles. The biggest threat to living creatures (including humans) from tornadoes is from flying debris and from being tossed about in the wind. It used to be believed that the low pressure in a tornado contributed to the damage by making buildings "explode" but this is no longer believed to be true.

How are tornadoes detected?

Today, the development of Doppler radar has made it possible, under certain circumstances, to detect a tornado's winds with a radar. However, human beings remain an important part of the system to detect tornadoes, because not all tornadoes occur in situations where the radar can "see" them. Ordinary citizen volunteers make up what is called the SKYWARN network of storm spotters, who work with their local communities to watch for approaching tornadoes, so those communities can take appropriate action in the event of a tornado. Spotter information is relayed to the National Weather Service, which operates the national Doppler radar network and which issues warnings to the public by radio, TV, and NOAA Weather Radio, using information obtained from weather maps, weather radars, and local storm spotters.

Can tornadoes be predicted?

Yes, but only to a limited extent. Although the process by which tornadoes form is not completely understood, scientific research has revealed that tornadoes usually form under certain types of atmospheric conditions. When forecasters see those conditions, they can predict that tornadoes are likely to occur. However, it is not yet possible to predict in advance exactly when and where they will develop, how strong they will be, or precisely what path they will follow. There are some "surprises" every year, when tornadoes form in situations that do not look like the right conditions in advance, but these are becoming less frequent. Once a tornado is formed and has been detected, warnings can be issued based on the path of the storm producing the tornado, but even these cannot be perfectly precise about who will or will not be struck.

How fast can a tornado go?

We're not really sure what the highest wind speed might be inside a tornado. Since strong and violent tornadoes destroy weather instruments. we really only have measurements of the winds inside weaker tornadoes. Mobile Doppler radars can measure wind speeds in a tornado above ground level -- and the strongest was 318 mph measured on May 3, 1999 near Bridge Creek/Moore, Oklahoma.

What are the people called who study tornadoes?

People who study tornadoes are just research meteorologists. You may have heard another term - storm chaser - but that really refers to people who chase tornadoes for a hobby. Research meteorologists do science. They have to come up with questions they think they can answer by taking certain measurements.

How fast do tornadoes move?

We don't have detailed statistics about this. Movement can range from almost stationary to more than 60 mph. A typical tornado travels at around 10-20 miles per hour.

How long is a tornado usually on the ground?

Detailed statistics about the time a tornado is on the ground are not available. This time can range from an instant to several hours. The average is about five minutes.

Does NSSL do things like they showed in the movie "Twister"?

The movie Twister was based upon work NSSL did in the mid-1980s using a 55-gallon drum outfitted with various meteorological sensors. It was called TOTO (TOtable Tornado Observatory). NSSL tried for several years to put it in the path of an oncoming tornado, but had minimal success. It did not have the sensors that fly up into the tornado, like in the movie. However, that is not a bad idea and with all the advances being made in computer technology, we might be able to do that someday.

Has every state had a tornado?

Yes, although some states have many more tornadoes than others.

Are there tornadoes in the Arctic Circle?

We are not aware of any tornadoes occurring in the Arctic Circle. Tornadoes need moisture and warm air to form, which is unusual at that lattitude. Plus tornadoes or their evidence have to be observed by someone, and the Arctic Circle has few residents!

Do tornadoes really stay away from gullies, rivers and mountains?

A gully could actually make a tornado more intense, just as an ice skater spins faster when he or she stands up tall and stretches their arms up straight over their heads. Every major river east of the Rockies has been crossed by a significant tornado, and high elevations in the Appalachians, Rockies, and Sierra Nevada have all experienced tornadoes. A violent tornado crossed the Continental Divide in Yellowstone National Park.

Do tornadoes always come from a wall cloud?

A wall cloud is not always present. It is possible, though, that you cannot see a wall cloud because of your viewing angle.

What does a tornado sound like?

People who have been in a tornado say it sounds like a jet engine or a freight train and is very loud. They said it hurt their ears but they were more worried about what might happen to them than they were about the pain in their ears.

Can tornadoes be stopped?

You have to consider that the tornado is part of something bigger - the supercell thunderstorm. Unless you disrupt the supercell thunderstorm itself, you would likely have another tornado, even if you were able to destroy the first. The thunderstorm's energy is much greater than the tornado. No one has tried to disrupt the tornado because the methods to do so could likely cause even more damage than the tornado. Detonating a hydrogen bomb, for example, to disrupt a tornado would be even more deadly and destructive than the tornado itself. Lesser things (like huge piles of dry ice or smaller conventional weaponry) would be too hard to deploy in the right place fast enough, and would likely not have enough impact to affect the tornado much anyway.

How much advance warning can forecasters give us before a tornado strikes?

The current average lead-time for tornado warnings is 11 minutes. NSSL is working to increase tornado warning lead-times to 20 minutes.

What is the difference between a tornado watch and a tornado warning?

A tornado watch defines a box-shaped area where tornadoes and other kinds of severe weather are possible in the next several hours. It means that you need to be alert, and be prepared to go to safe shelter if tornadoes happen or a warning is issued. If you have a NOAA Weather Radio and have it set up correctly it will alert you to the watch. Tune in to local TV or radio for more information. A tornado warning means that a tornado has been spotted, or that Doppler radar shows a thunderstorm circulation which can spawn a tornado. When a tornado warning is issued for your area, seek safe shelter immediately. The Storm Prediction Center issues tornado and severe thunderstorm watches. Your local National Weather Service offices issue tornado warnings.

Can a tornado dig up the ground?

There have been reports of tornadoes blowing dirt and creating a trench 3 feet deep, but it is very uncommon. Tornadoes have been known to strip asphalt pavement.

What is the worst tornado that you know of?

The deadliest US tornadoes [more than one?] were in 1925 on March 18. It is called the "Tri-state" tornado and killed 695 people along its 219 mile track through Missouri, Illinois and Indiana.

Do tornadoes have a positive effect on the environment?

Not that we know of yet...but the thunderstorms that produce tornadoes release energy that builds up in the atmosphere.

How many people are killed by tornadoes each year?

On the average tornadoes kill about 60 people each year, mostly from flying or falling debris.

What would it be like to be in the eye of a tornado?

The very center of the tornado is probably almost calm, but may have some downward motion in it. There have not been any direct measurements of the winds because the instruments used to measure wind speed can't survive long enough to measure the eye.

How many tornadoes hit the US each year?

About 1000

How often do F4/F5 tornadoes occur?

Of the 1000 tornadoes that occur each year, about 2% of them are rated F4 or F5. That means that there are as many as 20 devastating tornadoes each year. It is possible that meteorologists have underestimated the number of violent tornadoes that occur each year. Tornadoes are rated only by damage they do to man-made structures. Therefore, if a tornado doesn't hit a structure of some kind, we cannot estimate its strength. Also, a tornado varies in strength during its lifetime and could be its strongest while between areas of houses or other buildings.

Could there be an F-6 tornado?

Dr. Ted Fujita, the inventor of the "F-scale", did plot hypothetical winds at an "F6" level. But, it was untested theory and therefore he was making educated guesses. The F-scale is based on damage to man-made structures, not precise wind speeds. Scientists have very few measurements of tornado winds. F5 is the most intense possible damage level - total destruction. Fujita said that although he speculated that a tornado could be stronger than an F5, even a structural engineer could not distinguish F6 damage from F5. F5 is the highest scientifically accepted rating for tornadoes. The highest recorded windspeed in a tornado was 318 mph, still within the bounds of the F5 description. These precise wind speed numbers are actually educated guesses and have never been scientifically verified. As we obtain more measurements on tornadoes, we may actually learn that the wind estimates on the Fujita scale are wrong! Damage can vary from place to place or even building to building based on construction, even if the wind speeds were the same.

What effect do El Niño/La Niña have on tornadoes?

There have only been two strong La Niña events. Although scientists have looked for a correlation between La Niña and tornadoes, there just isn't enough data to make any conclusions.

I have a theory about tornadoes, who do I talk to?

We receive literally hundreds of ideas for observing, controlling, or stopping destructive storms. Our scientists are likely to look at ideas that are investigated by a researcher who publishes the results in a peer-reviewed journal. In this way they can review, and if necessary, replicate the results, which then will suggest the next step to move the science forward.

How do I become a storm spotter?

Visit www.skywarn.org . SKYWARN is a cooperative effort between the National Weather Service and communities to organize spotters. On that site there is a link to local SKYWARN groups. If your area is not listed, contact your local National Weather Service Office.

I would like to volunteer to help NSSL during a tornado intercept field project.

Unfortunately, government regulations make it impossible to accept offers from the public to do volunteer field work for any tornado intercept programs. Legal liability questions prevent NSSL from accepting volunteers, even at their own risk.

I want to chase tornadoes on my vacation – what time of year and what locations are best?

Check our Severe Weather Climatology page at www.nssl.noaa.gov/hazard/

Which is stronger, a hurricane or tornado?

The winds from a strong tornado (F4 or F5 - 207 mph or higher) are significantly stronger than the highest category of hurricane (Saffir-Simpson Scale Category 4 or 5 - 131 mph and higher). However, hurricanes tend to cause much more destruction than tornadoes because they cover a much larger area, last longer, and have a variety of destructive forces (the eyewall, storm surge, flooding, and sustained strong winds). Tornadoes, in contrast, tend to be a mile or smaller in diameter, last for minutes and primarily cause damage from their extreme winds.

Where is the most common birthplace for a violent tornado?

According to research done by Dr. Harold Brooks here at NSSL, the most common birthplace for a violent tornado (F4 or greater) is in south-central Oklahoma.

How is the strength of a tornado determined?

The most common and practical way to determine the strength of a tornado is to look at the damage it caused. From the damage, we can estimate wind speeds. Professor Theodore Fujita developed a scale to help us evaluate damage and classify tornadoes. In summary, the Fujita Scale ranges from F0-F5. F0 tornadoes have winds of 40-72 mph and can break branches off of trees and damage signs. F5 tornadoes have winds from 261-318 mph, which can lift houses off foundations, and make automobiles as dangerous as a missile flying through the air. A more complete listing of the scale can be found at -- www.tornadoproject.com , and click on "The Fujita Scale" either in the menu to the left or at the bottom.

An "Enhanced Fujita Scale" will be implemented by the National Weather Service by February, 2007. In a nutshell, the enhanced F scale was developed to rate tornadoes in a more consistent and accurate manner. It will still rate tornado categories from F0 to F5, but the ranges of wind speed in each category are now more accurate. It takes into account more variables than the original scale did when assigning a wind speed rating to a tornado, incorporating 28 damage indicators such as building type, structures, and trees. For each damage indicator, there are 8 degrees of damage ranging from the beginning of visible damage to complete destruction of the damage indicator. The original F scale did not take these details into account. The original F Scale historical data base will not change. An F5 tornado rated years ago is still an F5, but the wind speed associated with the tornado may have been somewhat less than previously estimated. A correlation between the original F Scale and the EF Scale has been developed. This makes it possible to express ratings in terms of one scale to the other, preserving the historical database.Go to: www.spc.noaa.gov/faq/tornado/ef-scale.html

Do tornadoes target mobile home parks?

While it may appear tornadoes target mobile home parks, they actually do not. An F1 tornado might do significant damage to a mobile home, and cause minor damage to a site built home -- looking like the tornado "skipped" the house. Mobile homes are, in general, much easier for a tornado to damage and destroy than well-built houses and office buildings. A mobile home, or manufactured home, by definition, is built at a factory and taken to the place they will occupy--so they are much more affordable than a house built on-site. Also, they are often built with lighter-weight materials, which do not hold up well in tornadic winds.

Straight-line winds can also destroy a mobile home as easily as a tornado, especially one that is not anchored. Any wind gust that is sustained for 3 seconds over 50mph can cause damage to mobile homes.

These websites may be of interest to you:
http://www.spc.noaa.gov/faq/tornado/mhdeadly.htm
http://www.ametsoc.org/POLICY/statement_2004_mobilehomes.html
http://sciencepolicy.colorado.edu/zine/archives/1-20/29/guest.html

Some states are beginning to require storm shelters for their residents. The statistics definitely support this --7% of our population lives in mobile homes, and almost half of tornado fatalities in the U.S. occur in mobile homes. The problem of warning and sheltering mobile home residents has become the biggest obstacle to continuing to reduce death tolls from tornadoes.

Do wider tornadoes cause more damage?

There is a statistical trend toward wide tornadoes having higher F-scale damage. This can be because of more strength or because of greater opportunity for targets to damage - or a combination of both. However, the size or shape of any particular tornado does not say anything conclusive about its strength. Some small tornadoes can still do very violent damage of F4 or F5. And, some very large tornadoes over a quarter-mile wide have produced only weak damage.

What is the difference between a tornado and a cyclone?

A tornado is a small-scale cyclonic circulation, and in the past, has been referred to as a cyclone.

The term cyclone was used to describe anything that rotated counterclockwise, so often tornado (a small-scale cyclonic circulation) and cyclone were interchangeable. However, modern meteorology now restricts the use of the term to the larger-scale circulations -- usually also accompanied by low pressure and bad weather. But, people still use it both ways.

Are there electromagnetic or magnetohydrodynamic explanations for the development of tornadoes?

As far as scientists understand, tornadoes are formed and sustained by a purely thermodynamic process. As a result, their research efforts are towards that end. They have spent a lot of time modeling the formation of a tornado and measuring many parameters in and around a tornado when it is forming and going through its life cycle. They have not seen any evidence to support magnetism or electricity playing a role.

Can my TV signal detect tornadoes?

You may have read about a technique called "the Weller Method" of tornado detection. The idea was to be able to use your TV as a lightning detector to detect the radio waves emitted by a lightning flash, with the assumption that tornadic thunderstorms were very active lightning producers. But, not all tornadic storms produce large amounts of lightning. And, TV's are all different and have different sensitivities, and some are even made to filter out lightning signals. Plus, if you are connected to cable, it won't work. The method was found to be completely unreliable and it has mostly been abandoned.

Do tornadoes occur when it is cold?

There is no particular temperature at which tornadoes form. It is more about what the surface temperature is in relation to the temperature higher up in the atmosphere. Even if it is cold near the surface, as long as it is colder higher up, the winds are right to set up low-level wind shear, along with other necessary ingredients, a tornado is possible.

What direction do tornadoes spin?

Most tornadoes (but not all) in the Northern Hemisphere spin counterclockwise.

Do rocks, hills, or trees increase or decrease the wind speeds in a tornado?

Unfortunately, there is no clear answer. Both observations (of real tornadoes), computer simulations, and laboratory studies (in tornado vortex chambers) have shown that the "surface roughness", i.e., the measure of how disrupted the wind near the ground is by objects such as dirt, rocks, hills, trees, and even houses, can either increase or decrease the wind speeds in a tornado. How can trees increase the wind speeds? Well, the strongest winds in a tornado occur when air from outside the tornado can flow closest to the center of the vortex. The conservation of angular momentum, e.g., the rotation in the air, requires that as the air flows toward the center of the tornado (as it spirals in) its rotation must increase. Depending on the configuration of the airflow outside of the tornado, sometimes there is not ENOUGH "inflow" toward the center, and so blobs of air outside the tornado do not get very close to the center of rotation before they are lifted upward off the ground. In this case, INCREASING the surface roughness helps get these blobs of air closer to the center of the tornado, where they rotate even faster than before. So occasionally we see in tornado videos the vortex increasing in intensity when it travels from one type of ground surface (say a field) into a grove of trees or a housing subdivision. It does not always happen, but often enough that we are aware of it. This is a case where "friction", which people normally think of slowing things down, actually speeds them up!

Can you recommend a good storm shelter manufacturer?

We do not endorse any particular company or type of storm shelter. Consider checking out the Federal Emergency Management Agency website at fema.gov . They work with communities on tornado preparedness.

How can I keep my kids safe during a tornado?

Educate yourself and your kids about tornado safety. Being informed and having a plan of action is the best way to keep you and your family safe. Work out together where you will go in case a tornado is approaching and why. Have the kids put together a tornado kit, with games, books, cars, and even snacks to keep them busy while they are taking shelter – make them a part of the process. Use a NOAA Weather Radio and show the kids how it can alert your family to danger. NSSL developed coloring books for kids a few years ago – it may be helpful to print them out and review them with your kids.

Most tornado injuries are caused by flying debris, so the best thing for you to do is probably lay on top of your child and cover both of you with a mattress or a couch cushion. Strap infants in their car seat (in the house of course), and put bicycle helmets on the other children to protect their heads. Make sure everyone had sturdy shoes (not sandals) and jeans. See our severe weather safety page.

Check out books from the library about tornadoes. It seems the more we understand something, the less we fear it.

Plus, there are much more dangerous weather events than tornadoes. Flash floods are the number one weather-related killer in the U.S., followed by lightning. There are an average of 1000 tornadoes a year, killing about 40-50 people each year.

Is it safe to take shelter under an overpass?

Absolutely not! Stopping under a bridge to take shelter from a tornado is a very dangerous idea, for several reasons:

  1. Deadly flying debris can still be blasted into the spaces between bridge and grade -- and severely injure any people taking shelter there.
  2. Even when strongly gripping the girders (if they exist), people may be blown loose, out from under the bridge and into the open -- possibly well up into the tornado itself. Chances for survival are not good if that happens.
  3. The bridge itself may fail, peeling apart and creating large flying objects, or even collapsing down onto people underneath. The structural integrity of many bridges in tornado winds is unknown -- even for those that may look sturdy.
  4. Whether or not the tornado hits, parking on traffic lanes is illegal and dangerous to yourself and others. It creates a potentially deadly hazard for others, who may plow into your vehicle at full highway speeds in the rain, hail, and/or dust. Also, it can trap people in the storm's path against their will, or block emergency vehicles from saving lives.

What should I do if there is a tornado warning while I am at work at a hospital or nursing home?

Your medical director or nursing home administrator should have a disaster plan for you to review. Most accredited medical facilities are required to have a disaster plan on file, including procedures during tornadoes, and to review that plan annually with the staff.

Are crawl spaces safe tornado shelters?

Crawl spaces are generally not considered safe shelter from tornadoes because it is hard to know how well the house is attached to the foundation. If a tornado moved the house a foot or two, and this happens a lot, the entire house could drop on you. Plus, lots of unwelcome critters live in crawl spaces and it doesn't sound very safe to spend time with spiders and snakes!

Where is tornado alley?

"Tornado Alley" is a just a nickname made up by the media for an area of relatively high tornado occurrence - it is not a clearly defined area. Is tornado alley the area with the most violent tornadoes, or is it the area with the most tornado related deaths, or the highest frequency or tornadoes? It depends on what kind of information you want!

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