Tornado Facts: Causes, Formation & Safety

Tornadoes are the most violent storms in nature. An average of 800 tornadoes are reported each year, resulting in 80 deaths and 1,500 injuries. Tornadoes are a worldwide phenomenon, touching down in every continent save Antarctica.

Tornadoes form when different temperatures and humidity meet. In the United States, warm, wet winds travel north from the Gulf of Mexico in the spring and summer, where they meet cold, dry, south-moving Canadian fronts.

Generally, warm air rises, but when the two fronts meet, the cold air can trap the warm air beneath it. Because the warm air cannot move upward, it begins to rotate. As the sun heats the ground, more warm air continues to rise, until finally the mass is strong enough to push through the cold air barrier. The rising warm air pushes the cold air beneath it, creating a rotating column that can span up to 10 miles, while twisting at speeds exceeding 200 mph (322 km/h)

The spinning air may remain unseen until it picks up enough dust and debris for its shape to be visible. Tornadoes can last for a minute or an hour, and they can tear a damage path up to 10 miles (16 kilometers) long.

Tornadoes forming over warm water are known as waterspouts, and are most common in southern states and along the Gulf Coast. Occasionally, waterspouts can move inward.

The region lying between the Rocky and Appalachian Mountains — known informally as Tornado Alley — has the highest number of tornadoes in the United States each year; however, no place is safe. Tornadoes can occur anywhere in the United States, as well as in other countries. In fact, the United Kingdom reports the most tornadoes by land area.

In 1987, a tornado tore through Yellowstone National Park in Wyoming at elevations reaching up to 10,000 feet (3,048 meters), making it the highest altitude violent tornado in the United States.

Tornado wind and debris cause most of the structural damage suffered, but nearly half of the injuries from such disasters occur after the tornado has left, during rescue work and cleanup. According to the Federal Emergency Management Agency, a third of these injuries come from stepping on nails. As such, it is important to remember to exercise caution even after the danger appears to have passed.

Tornadoes can occur at any time during the year, but they form most frequently in the spring and summer, depending upon your location. They are most likely to appear between 3 p.m. and 9 p.m., but again, they can occur at any time in the day or night.

Tornados were once ranked by wind speed on the Fujita scale. It was upgraded to the Enhanced Fujita scale in 2007 and ranges from EF0 to EF5. An EF0 tornado may damage trees but not buildings, with winds ranging up to 85 mph (137 km/h). An EF5 tornado is devastating; winds exceed 200 mph (322 km/h), and buildings can be annihilated.

During strong thunderstorms, keep an ear on local weather alerts. FEMA encourages families to have a disaster plan in place for all contingencies, not only for tornadoes; this not only can keep you safe, but can allow for some peace of mind when families are separated. [Infographic: Tornado Season: What to Expect]

If your television or radio announces a tornado watch, this means that conditions are favorable for a tornado to form. Continue to listen for further updates. If a tornado warning is announced, it means a tornado has been sighted or indicated by radar. Move to a safe place immediately.

If a tornado strikes and you are inside a sturdy building, go to the lowest floor, such as the basement or storm cellar. If the building has no rooms beneath the ground, get to the lowest level possible and find an interior room, putting as many walls as possible between you and the tornado. Don’t bother opening windows to equalize pressure; this will accomplish nothing except to allow more debris inside.

If you are in a mobile home or trailer, leave immediately. Find the lowest floor of a sturdy shelter, or a ditch or depression. Lie down flat and cover your head. Do not seek refuge under an overpass or bridge.

Never try to outrun a tornado. Instead, find a safe place to ride it out. Watch for flying debris, which causes the most fatalities and injuries while the tornado is in process.

Remember that almost half of tornado-related injuries occur after the tornado has ended. Be careful when entering buildings that have been damaged. Wear sturdy shoes and appropriate clothes to avoid scrapes. Monitor your radio for emergency information. Do not touched downed power lines or anything in contact with such lines. Watch (and smell) for gas line breaks.


Historical Records and Trends

One of the main difficulties with tornado records is that a tornado, or evidence of a tornado must have been observed. Unlike rainfall or temperature, which may be measured by a fixed instrument, tornadoes are short-lived and very unpredictable. If a tornado occurs in a place with few or no people, it is not likely to be documented. Many significant tornadoes may not make it into the historical record since Tornado Alley was very sparsely populated during the 20th century.

Much early work on tornado climatology in the United States was done by John Park Finley in his book Tornadoes, published in 1887. While some of Finley’s safety guidelines have since been refuted as dangerous practices, the book remains a seminal work in tornado research. The University of Oklahoma created a PDF copy of the book and made it accessible at John Finley’s Tornadoes.

Today, nearly all of the United States is reasonably well populated, or at least covered by NOAA’s Doppler weather radars. Even if a tornado is not actually observed, modern damage assessments by National Weather Service personnel can discern if a tornado caused the damage, and if so, how strong the tornado may have been. This disparity between tornado records of the past and current records contributes a great deal of uncertainty regarding questions about the long-term behavior or patterns of tornado occurrence. Improved tornado observation practices have led to an increase in the number of reported weaker tornadoes, and in recent years EF-0 tornadoes have become more prevelant in the total number of reported tornadoes. In addition, even today many smaller tornadoes still may go undocumented in places with low populations or inconsistent communication facilities.

With increased National Doppler radar coverage, increasing population, and greater attention to tornado reporting, there has been an increase in the number of tornado reports over the past several decades. This can create a misleading appearance of an increasing trend in tornado frequency. To better understand the variability and trend in tornado frequency in the United States, the total number of EF-1 and stronger, as well as strong to violent tornadoes (EF-3 to EF-5 category on the Enhanced Fujita scale) can be analyzed. These tornadoes would have likely been reported even during the decades before Doppler radar use became widespread and practices resulted in increasing tornado reports. The bar charts below indicate there has been little trend in the frequency of the stronger tornadoes over the past 55 years.

EF1-EF5 Tornado Counts

EF3-EF5 Tornado Counts



Because most tornadoes are related to the strength of a thunderstorm, and thunderstorms normally gain most of their energy from solar heating and latent heat released by the condensation of water vapor, it is not surprising that most tornadoes occur in the afternoon and evening hours, with a minimum frequency around dawn (when temperatures are lowest and radiation deficits are highest). However, tornadoes have occurred at all hours of the day, and nighttime occurrences may give sleeping residents of a community little or no warning.

Tornado Occurrence by Hour of Day for the United States
U.S. Tornado Occurrence by Hour

Tornado Occurrence by Hour by Region of the United States

Tornado Alley
U.S. Tornado Occurrence by Hour
Dixie Alley
U.S. Tornado Occurrence by Hour
Northeast Climate Region
U.S. Tornado Occurrence by Hour
Southeast Climate Region
U.S. Tornado Occurrence by Hour
Central Climate Region
U.S. Tornado Occurrence by Hour
East North Central Climate Region
U.S. Tornado Occurrence by Hour
South Climate Region
U.S. Tornado Occurrence by Hour
West North Central Climate Region
U.S. Tornado Occurrence by Hour
Southwest Climate Region
U.S. Tornado Occurrence by Hour
Northwest Climate Region
U.S. Tornado Occurrence by Hour
West Climate Region
U.S. Tornado Occurrence by Hour

In addition, tornadoes occur throughout the year. Because a tornado may occur at any time of the day or year somewhere in the United States, there really is no national tornado “season” (as there is with Atlantic hurricanes). Instead, each region may experience increased tornadic potential at different times of the year. Like with the diurnal pattern, for the United States (and hemisphere) as a whole, the months in which tornadoes are most likely correspond to the times of year with increased solar heating and strong frontal systems. Regionally, the frequency of tornadoes in the United States is closely tied with the progression of the warm season when warm and cold air masses often clash. Most of the early spring tornadoes in the United States tend to occur in the Southeast and South Central regions. Gulf States, such as Mississippi and Louisiana, are the frequent recipients of tornadoes from February to April. Late spring tornadoes generally spread a bit farther north, often into Kansas, Nebraska, and the Tennessee Valley region. By midsummer, most of Tornado Alley is active and tornadoes may occur throughout the United States. Late summer tends to bring some of the stronger tornadoes into the upper Midwest and Ohio valleys, and the pattern shifts back southward into the late autumn. The fewest tornadoes are documented during the winter months. Although rare, deadly winter outbreaks do occur.

Interesting Facts about Tornado


Tornado is a violent storm commonly known as a twister that brings destruction to people and property. Tornadoes mostly hit the United States of America although they might come anytime and anywhere. Tornadoes are high speed winds that hit the place generally in the months of March to May. The speed and size of the tornado can vary to any extent. Tornadoes are generally accompanied with thunderstorms. Some common signs of tornadoes are dark, blackish green sky, hailstorm and loud sounds like that of a yelling train etc. The main reason of occurrence of a tornado is the meeting of warm air that rises up the ground with the cool air that rushes down to descend. A tornado may rotate either in clockwise or anticlockwise direction. The speed of rotation of this funnel shaped tornado is so high that it destroys everything that comes in its way. The strength of a tornado is so high that it can uproot trees, blow the roofs of the buildings or even crumple down buildings to ashes. The color of a tornado depends upon two factors the surroundings and the time of the day. The tornado is a noisy wind; the sound of which can be either high or low depending upon the obstructions in the way. It has been recorded that almost eight hundred to one thousand tornadoes hit America every year killing 80 people and injuring around fifteen hundred. Other tornado prone areas are south of Canada, south of Africa, some regions of Australia, New Zealand and Asia. “Tri-state” tornado that hit United States of America in March 18, 1925 was the deadliest tornado and tornado in Oklahoma in 1999 was the most destructive tornado known to men. Interesting fact about tornado is that some people survived being in the middle of tornado.

What is a tornado?


A tornado is a violently rotating column of air which descends from a thunderstorm to the ground. No other weather phenomenon can match the fury and destructive power of tornadoes. Tornadoes can be strong enough to destroy large buildings, leaving only the bare concrete foundation. In addition, they can lift 20-ton railroad cars from their tracks and they can drive straw and blades of grass into tree and telephone poles.

How do tornadoes form?

The truth is that scientists don’t fully understand how tornadoes form. Typically, tornadoes develop several thousand feet above the earth’s surface inside of a severe rotating thunderstorm. This type of storm is called a supercell thunderstorm. The spinning of these supercell thunderstorms is visible via Doppler radar.

What is a supercell thunderstorm?

A supercell is an organized thunderstorm that contains a very strong, rotating updraft. This rotation helps to produce severe weather events such as large hail, strong downbursts, and tornadoes. Supercell storms are usually isolated from other thunderstorms because it allows them to have more energy and moisture from miles around. These storms are rare, but always a threat to life and property.

What is the difference between a funnel cloud and a tornado?

A tornado begins as a rotating, funnel-shaped cloud extending from a thunderstorm cloud base. A funnel cloud is made visible by cloud droplets, however, in some cases it can appear to be invisible due to lack of moisture. When the funnel cloud is half-way between the cloud base and the ground, it is called a tornado. The tornado’s high-speed winds rotate about a small, relatively calm center, and suck up dust and debris, making the tornado darker and more easily seen.

What is the path length of tornadoes? How long do they last? How fast do they move?

Tornado paths range from 100 yards to 2.6 miles wide and are rarely more than 15 miles long. They can last from several seconds to more than an hour, however, most don’t exceed 10 minutes. Most tornadoes travel from the southwest to northeast with an average speed of 30 mph, but the speed has been observed to range from almost no motion to 70 mph.

When and where do tornadoes occur?

Most tornadoes occur in the deep south and in the broad, relatively flat basin between the Rockies and the Appalachians, but no state is immune. Peak months of tornado activity in the U.S. are April, May, and June. However, tornadoes have occurred in every month and at all times of the day or night. A typical time of occurrence is on an unseasonably warm and sultry Spring afternoon between 3 p.m. and 9 p.m.

What causes tornadoes?

Tornadoes form under a certain set of weather conditions in which three very different types of air come together in a certain way. Near the ground lies a layer of warm and humid air, along with strong south winds. Colder air and strong west or southwest winds lie in the upper atmosphere. Temperature and moisture differences between the surface and the upper levels create what we call instability. A necessary ingredient for tornado formation. The change in wind speed and direction with height is known as wind shear. This wind shear is linked to the eventual development of rotation from which a tornado may form.

A third layer of hot dry air becomes established between the warm moist air at low levels and the cool dry air aloft. This hot layer acts as a cap and allows the warm air underneath to warm further…making the air even more unstable. Things start to happen when a storm system aloft moves east and begins to lift the various layers. Through this lifting process the cap is removed, thereby setting the stage for explosive thunderstorm development as strong updrafts develop. Complex interactions between the updraft and the surrounding winds may cause the updraft to begin rotating-and a tornado is born.

The Great Plains of the Central United States are uniquely suited to bring all of these ingredients together, and so have become known as “Tornado Alley.” The main factors are the Rocky Mountains to the west, the Gulf of Mexico to the south, and a terrain that slopes downward from west to east.

During the spring and summer months southerly winds prevail across the plains. At the origin of those south winds lie the warm waters of the Gulf of Mexico, which provide plenty of warm, humid air needed to fuel severe thunderstorm development. Hot dry air forms over the higher elevations to the west, and becomes the cap as it spreads eastward over the moist Gulf air. Where the dry air and the Gulf air meet near the ground, a boundary known as a dry line forms to the west of Oklahoma. A storm system moving out of the southern Rockies may push the dry line eastward, with severe thunderstorms and tornadoes forming along the dry line or in the moist air just ahead of it.

What is the Fujita Tornado Damage Scale?

Dr. T. Theodore Fujita, a pioneer in the study of tornadoes and severe thunderstorm phenomena, developed the Fujita Tornado Damage Scale (F-Scale) to provide estimates of tornado strength based on damage surveys. Since it is extremely difficult to make direct measurements of tornado winds, an estimate of the winds based on damage is the best way to classify them. The new Enhanced Fujita Scale (EF-Scale) addresses some of the limitations identified by meteorologists and engineers since the introduction of the Fujita Scale in 1971. Variability in the quality of construction and different local building codes made classifying tornadoes in a uniform manner difficult. In many cases, these inconsistencies led to overestimates in the strength of tornadoes. The new scale identifies 28 different free standing structures most affected by tornadoes taking into account construction quality and maintenance. The range of tornado intensities remains as before, zero to five, with ‘EF0’ being the weakest, associated with very little damage and ‘EF5’ representing complete destruction, which was the case in Greensburg, Kansas on May 4th, 2007, the first tornado classified as ‘EF5’. The EF scale was adopted on February 1, 2007.

The Storm Prediction Center has a brief description of the new Enhanced Fujita Scale. Here’s the full report submitted by the Wind Science and Engineering Center at Texas Tech University in PDF format.

A modification of the original Fujita Scale developed by “Dr. Tornado”, T. Theodore Fujita of the University of Chicago.

New EF Scale: Old F-Scale: Typical Damage:
EF0 (65-85 mph) F0 (65-73 mph) Light damage. Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over.
EF1 (86-110 mph) F1 (73-112 mph) Moderate damage. Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors; windows and other glass broken.
EF2(111-135 mph) F2 (113-157 mph) Considerable damage. Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.
EF3 (136-165 mph) F3 (158-206 mph) Severe damage. Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations blown away some distance.
EF4 (166-200 mph) F4 (207-260 mph) Devastating damage. Whole frame houses Well-constructed houses and whole frame houses completely leveled; cars thrown and small missiles generated.
EF5 (>200 mph) F5 (261-318 mph) Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 m (109 yd); high-rise buildings have significant structural deformation; incredible phenomena will occur.
EF No rating F6-F12 (319 mph to speed of sound) Inconceivable damage. Should a tornado with the maximum wind speed in excess of F5 occur, the extent and types of damage may not be conceived. A number of missiles such as iceboxes, water heaters, storage tanks, automoblies, etc.will create serious secondary damage on structures.
What should I do to prepare myself for a tornado?

Continued vigilance and quick response to tornado watches and warnings are critical, since tornadoes can strike virtually anywhere at any time. Most tornadoes are abrupt at onset, short-lived and often obscured by rain or darkness. The best way to deal with them is preparedness. Every individual and business should have a tornado emergency plan for their homes and places of work, and should learn how to protect themselves in cars, open country, and other situations that may arise.

Remember if a tornado warning is issued for your area, a tornado is imminent. Know what to do–have an emergency plan to protect yourself and those for whom you are responsible. Quick response when a tornado approaches can save many lives. There may be only seconds in which to take action.


There are 5 Basic Steps in the National Weather Service warning system. Every part of the system has to work for the greatest number of people to get the warning in time.

  1. The Tornado Watch

    Meteorologists using the latest in computers, radar and satellite data are always monitoring the weather elements. When a high probability of severe weather exists, a tornado or severe thunderstorm watch is issued. Watches may be issued hours before any severe storm hits the area. The forecasters at the National Severe Storms Forecast Center in Kansas City, Missouri use every tool available including satellite pictures, radar reports, and numerous weather charts to predict the areas where severe thunderstorms and tornadoes are likely to occur.

  2. Spotters

    Severe weather spotters are constantly being trained under the Skywarn training program. Spotting severe weatheris serious business and requires specific training. However, once an observer is trained in severe weather spotting procedures, they are one of the most reliable tools of the National Weather Service meteorologist. Spotters serve as the National Weather Service’s eyes in the field.

  3. Civil Defense and the State Police

    Any information that the National Weather Service issues is relayed to individual cities and towns through state and local civil defense, and the State Police.

  4. The Media

    The vast majority of people are reached through the cooperation of the media. A direct line between the National Weather Service and local media offices insures that severe weather information is relayed quickly and broadcast within the shortest time possible.

  5. The Users

    Users include everyone within the severe thunderstorm or tornado warning area. We want to reach the greatest number of people possible and provide a concise, yet persuasive message of necessary action. Even if every other step in the warning system works, it does little good unless the users know what to do, and act.

What was the deadliest tornado outbreak in the U.S.?

The deadliest tornado outbreak in U.S. history occured on March 18, 1925. 747 people were killed and 2,027 were injured in Missouri, Illinois, and Indiana when several twisters touched down on this day. The largest of these tornadoes, named the “Tri-State”, took 695 lives and was classified as an F5. It moved over 215 miles of land at 60-73 mph.

What was the longest path length of a tornado? What was the shortest?

The longest tornado path was the Tri-State tornado of March 18, 1925, with a path length of 215 miles. It was on the ground for 7 hours and 20 minutes. Because there was no damage report filed, its difficult to know if this tornado actually stayed on the ground for the entire time, or if it was the result of several tornadoes. The shortest reported tornado was 7 feet long.

What are the strongest winds in a tornado?

Mobile Doppler radar reported one twister on May 3, 1999 as having winds as fast as 302 mph, plus or minus 16 mph, at an altitude of 100 feet. Scientists have found that the strongest winds typically occur about 300 feet above the ground. Most tornadoes, however, fail to have wind speeds in excess of 113 mph.

Should I open my windows and doors during a tornado?

NO! Opening your windows and doors may in fact increase the damage to your house and make you suseptible to being struck by flying glass. Instead, use that time to find a safe spot under heavy furniture and away from windows. Any openings, including garage doors, that allow wind to enter a building increases the chance for damage.

Are the number of tornadoes increasing?

The number of tornadoes that occur each year is not increasing, but the number of spotted and reported tornadoes is. The reason for this is that more people live in or travel through tornado prone areas than used to. This has led to better communication and reportings of severe weather.

How are tornadoes detected?

In addition to the thousands of National Weather Service trained severe weather spotters, NEXRAD Doppler radars detect severe weather. These radars spot large scale rotation from which many tornadoes form. NEXRAD does not detect every tornado, but it is likely that they will provide advance warning for large twisters.

Tornado Safety Rules

We can do little to prevent a tornado from occurring, but by knowing the safety rules we can minimize the number of deaths and injuries.

A tornado watch means that tornado development is possible. Keep a watchful eye on the sky for threatening weather and stay tuned to radio and television and listen for weather bulletins.

A tornado warning means that a tornado has been sighted or indicated by radar. Persons in the path of the storm should seek shelter immediately – preferably in a storm cellar, underground excavation, or in a steel-framed or concrete reinforced building.

A severe thunderstorm warning means that either spotters or radar have indicated that severe weather is occuring, and is expected to be heading towards you soon. This warning is issued by the National Weather Service local office, and usually covers a few counties, lasting about an hour or so. A thunderstorm is classified as severe because it can contain hail three-quarter inches or larger, and/or wind gusts of 58 mph or higher, and/or a tornado. When a warning is issued, persons should remain indoors until the storm has passed.

In homes,the basement offers the greatest safety. Seek shelter under sturdy furniture if possible. In homes without basements take cover in the center part of the house, on the lowest floor, in a small room such as a closet or bathroom, or under sturdy furniture. Stay away from windows!

In schools, hospitals, and shopping centers move to pre-designated shelter areas. Interior hallways on lowest floors are best. If the building is not of reinforced construction, go to a nearby one that is, or take cover outside on low, protected ground. Stay out of auditoriums, gymnasiums, and other structures with wide free-span roofs.

In open country, move away from the tornado’s path at right angles. If there is not time to escape,lie flat in the nearest ditch or ravine.

In your car, do not try to outrun a tornado. If available take shelter in a sturdy structure. Otherwise, get in the nearest ditch or depression until the tornado passes.

Mobile homes are writing acknowledgement thesis particularly vulnerable to overturning during strong winds and should be evacuated when strong winds or tornadoes are forecast. Damage can be minimized by securing trailers with cables anchored in concrete footing. Trailer parks should have some community storm shelters. If there is no shelter nearby, leave the trailer park and take cover on low-protected ground.