A tornado is a violent spinning storm typically shaped like a funnel with the narrow end on the ground. Tornados are known for being extremely destructive and are almost always visible due to water vapor from clouds and debri from the ground. Tornadoes form in storms all over the world, and though they have been recorded in all 50 U.S. states, they form most famously in a broad area of the American Midwest and South known as Tornado Alley. Although, in pure number of incidences, the United States experiences more tornadoes than any other country, the United Kingdom is the most tornado-prone country relative to land area.

The word "tornado" comes from the Spanish or Portuguese verb tornar, meaning "to turn." Some common, related slang terms include: twister, whirlwind, wedge, funnel, willy-willy, or rope.

Cyclone is also another term for a tornado, although it must be noted that in parts of the world (notably Australia) a cyclone refers to what is more correctly known as a tropical cyclone (also known as a hurricane, or a typhoon), and meteorologists use the term cyclone to refer to a wide range of circular weather systems (using adjectives to disambiguate).

In general tornadoes are associated with a thunderstorm however National Weather Service in the United States considers all waterspouts, including "fair weather" waterspouts, to be tornadoes. Larger vortexes not associated with a thunderstorm are sometimes called landspouts.

Dust devils are small vortexes that form near the ground, which may or may not be considered tornadoes.

1 Tornado formation
2 Tornado characteristics
3 Tornado intensity
4 Frequency of occurrance
5 Social implications of tornadoes
6 Tornado awareness and safety

Tornado formation

Tornadoes develop from thunderstorms, most frequently supercell thunderstorms, though they also occur within squall lines and hurricanes. They are believed to be produced when cool air overrides a layer of warm air, forcing the warm air to rise rapidly. Tornadoes, lightning, and sometimes hail are associated with thunderstorms. Many tornadoes appear at the tail end of mesocyclones. On weather radar screens, a characteristic "hook echo" marks the area where tornadoes are likely to exist.

Exactly how tornadoes form is complex and not fully understood. When thunderstorms develop, an increase in wind speed and/or a large change in direction with height ("wind shear") produces a horizontal, spinning area of air. The strong updrafts within the thunderstorm can draw this area of rotation up from horizontal to vertical. Towards the end of this area of rotation (the mesocyclone) is often a lower area of rain-free cloud and can be seen as a rotating "wall cloud". If the rotation intensifies, a funnel cloud can develop where the cloud water vapor is draw down towards the ground. Usually the funnel cloud follows the intensity of the vortex towards the ground and this indicates the formation of a tornado, often referred to as "touching down", however this is not a reliable indicator as tornados can have a partial funnel cloud or be invisible. It is not uncommon for a tornado to suddenly become visible when it fills with debris from the ground. Why the rotation can intensify and form tornadoes is not understood.

Recent Doppler radar studies, such as the Doppler on Wheels project, have shown that at least some tornadoes have "eyes" or "eyewalls" with central downdrafts like hurricanes; this parallel had been modeled as well as reported anecdotally for some time.

A classical "hook echo" as seen in the strongest tornado in the 1999 Oklahoma Tornado Outbreak. At the time of this image, the tornado was crossing Interstate 44 near the Canadian River, after producing F5 damage in Bridge Creek, Oklahoma, and before producing more F5 damage in Moore. The bright red colors at the tornado location represent not rain or hail—but the aggregate signature of car parts, pieces of houses, shredded tree branches, dirt and other debris, hoisted thousands of feet skyward by the tornado vortex. Source: U.S. NOAA National Weather Service

Tornado characteristics

Tornadoes normally rotate in a cyclonic (counterclockwise) direction in the northern hemisphere as the warm air thunderstorms usually form in sweeps north and jet streams come from the west, creating a situation in which the storms rotate. In the northern hemisphere, this rotation is counterclockwise, and in the southern hemisphere, clockwise. The tornadoes usually rotate the same way. Sometimes opposite direction swirls develop under a thunderstorm. About 1 in 100 tornadoes in the northern hemisphere rotate in an anticyclonic direction.

No two tornadoes look exactly alike. Nor have any two tornadoes behaved exactly the same. There are true incidents of tornadoes repeatedly hitting the same town several years in a row. But forecasting the exact position a tornado will strike at a certain time is nearly impossible.

Tornadoes can be nearly invisible, marked only by swirling debris at the base of the funnel. While tornadoes are invisible at night, some nocturnal tornadoes have been observed glowing diffusely due to lightning activity. Verified observations by Hall and others suggest a cellular structure inside tornadoes. Some tornadoes are composed of several mini-funnels. A tornado must by definition have both ground and cloud contact. Thus, the oft-mentioned exclamation "Tornado on the ground!" is indeed redundant.

Not every thunderstorm, supercell, squall line, or hurricane will produce a tornado. Luckily, it takes exactly the right combination of atmospheric variables (wind, temperature, pressure, humidity, etc.) to spawn even a weak tornado. On the other hand, roughly 1,000 tornadoes a year are reported in the contiguous United States.

Even though no two tornadoes are exactly alike, they always have the same general characteristics that classify them as tornadoes. First, a tornado is a microscale rotating area of wind, from a few feet to a few miles wide. A thunderstorm can rotate, but that does not mean it is a tornado. Secondly, the vortex, rotating wind, must be attached to a convective cloud base, and be in contact with the ground. Some of those are thunderstorms embedded in squall lines, supercell thunderstorms, and also not to exclude the outer fringes of landfalling hurricanes. Third, a spinning vortex of air must have caused enough damage to be classified by the Fujita scale as a tornado.

Tornado intensity

In the United States (and sometimes in other countries, as well), the intensity of a tornado is measured on the Fujita-Pearson Tornado Scale (also known simply as Fujita scale). The intensity can be derived directly with high resolution Doppler radar wind speed data, or empirically derived from structural damage compared to engineering data. Note that intensity does not refer in any way to the size, or width, of a tornado. The scale ranges from F0 for the weakest to F5 for the most powerful known tornadoes. No F6 tornado has yet been detected.

The TORRO scale, developed in the United Kingdom and used primarily in Europe, covers a broader range in finer detail, and is based solely on wind speed. It ranges in a similar way from a T0 to T11 for the most powerful known tornado in the United States.

Of all tornadoes formed in the U.S., F0 and F1 tornadoes account for a large percentage of occurrences. On the other end of the scale, the massively destructive F5s account for fewer than 1% of all tornadoes in the U.S.

Frequency of Tornado occurrance

The United States experiences by far the most tornadoes of any country, and has also suffered the most intense ones. Tornadoes are common in most states in spring and summer, especially those east of the Rocky Mountains. However, tornadoes can occur in the West as well, although they are usually very small and relatively weak. Recently tornadoes have struck the Pacific coast town of Lincoln City, Oregon, in 1996 and downtown Salt Lake City, Utah, in 1999 (see Salt Lake City Tornado). The U.S. state which has the highest number of tornadoes per unit area is Florida, although most of the tornadoes in Florida are weak tornadoes of F0 or F1 intensity.

On average, the United States experiences 100,000 thunderstorms each year, resulting in more than 1,000 tornadoes and approximately 50 deaths per year. The deadliest U.S. tornado on record is the March 18, 1925, Tri-State Tornado that went across southeastern Missouri, southern Illinois and southern Indiana, killing 695 people. More than six tornadoes in one day is considered a tornado outbreak. The biggest tornado outbreak on record—with 148 tornadoes, including six F5 and 23 F4 tornadoes—occurred on April 3, 1974. It is dubbed the Super Outbreak. Another such significant storm system was the Palm Sunday Tornado Outbreak, which affected the United States Midwest on April 11, 1965.

Canada also experiences numerous tornadoes, although less than the United States. In Canada, an average of 80 tornadoes occur annually, killing 2, injuring 20 and causing tens of millions of dollars in damage. The last killer tornado in Canada struck Pine Lake, Alberta, on July 14, 2000, killing 11.

Tornadoes do occur throughout the world as well; the most tornado-prone region of the world (outside North America), as measured by number of reported tornadoes per unit area, is the United Kingdom, especially England. Australia, New Zealand, Bangladesh and portions of Uruguay also have pockets of strong tornadic activity. Tornadoes have recently hit parts of Germany in 2003 and Pakistan in 2001 as well.

Social implications of tornadoes

Tornado damage to man-made structures from a tornado is a result of the high wind velocity and windblown debris. Tornadic winds have been measured well in excess of 300 (480 km/h). Tornado season in North America is generally March through October, although tornadoes can occur at any time of year. They tend to occur in the afternoons and evenings: over 80 percent of all tornadoes strike between noon and midnight.

Trained weather spotters are often on alert to look for tornadoes and notify local weather agencies when severe weather is occurring or predicted to be imminent. In the United States, skywarn spotters, often local sheriff's deputies, fulfill this role. Additionally, some individuals, known as storm chasers, enjoy pursuing thunderstorms and tornadoes to explore their many visual and scientific aspects. Attempts have been made by storm chasers to drop probes in the path of oncoming tornadoes in an effort to analyze the interior of the storms, but only about five drops have been successful since around 1990.

The relative rarity and large scale of destructive power that tornadoes provide, their occurence or the possibility that they may occur creates should be considered sensationalism. This results in so-called weather wars, in which competing local media outlets, particularly TV news stations, engage in continually escalating technological one-upsmanship and dramaticism in order to increase their market share. This is especially evident in tornado-prone markets, such as those in the Great Plains. So intense are the weather wars in these locations that news outlets have been known to concoct footage and endanger public safety themselves to promote their public mission of "notifying their viewers".

The sensationalistic nature of tornado occurrence also results in some skewing of scientific data. As those affected by severe weather would like to have their "15 minutes of fame", regardless of the weather phenomenon that caused their damage, if it was high winds of any sort, it is often claimed by the victims that they saw a tornado, even if they did not.

According to Environment Canada, the chances of being killed by a tornado are 12 million to 1 (12,000,000:1). One may revise this yearly and/or regionally, but the probability may be factually stated to be low. Tornadoes do cause millions of dollars in damage, both economic and physical, displacement, and many injuries every year.

Some common myths about tornadoes which people should not rely upon to protect them are given in the article on The Super Outbreak of 1974, in which some of the most dangerous tornadoes formed near rivers and crossed them, and crossed over steep hills, mountains and deep valleys. Other misconceptions and science fiction, concerning tornado formation can be found at the article for Tornado myths.

Tornado awareness and safety

With each tornado season, schools and media outlets in tornado-prone areas spend time educating the public about the dangers and what can be done to improve the chances of surviving a storm. In the United States, citizens are often advised to purchase NOAA Weather Radios. They are relatively inexpensive devices costing as little as $20 in U.S. currency, which will activate whenever the National Weather Service issues severe weather warnings. Warnings are also carried on radio and television, and most communities have civil defense sirens that will activate when severe weather is believed to be approaching.

Tornadoes can cause serious damage, injury or death. One should always heed official watches and warnings.

When tornado warnings are issued, members of the public are advised to get into sheltered areas. In most buildings, it is recommended to seek shelter in a central, windowless room or corridor, below ground if possible. If a tornado does strike a building, it can cause debris to rain down on people inside, so it is advisable for those caught near a tornado to crouch under strong beams, in doorways, or under strong furniture. However, light structures such as mobile homes are in severe danger when tornadoes and strong winds appear. Residents of such structures are advised to evacuate them whenever severe weather is imminent and seek shelter in sturdier buildings, whether they are designated shelters or the homes of nearby friends. Storm cellars are also common places of refuge in some regions.

Vehicles are extremely dangerous in a tornado. If the tornado is visible, far away, and the traffic is light, one may be able to drive out of its path by moving at right angles to the tornado. Otherwise, one should park the vehicle as quickly and safely as possible, out of the traffic lanes and seek shelter in a sturdy building or ditch. One should not, under any circumstances, stay in a vehicle if the vehicle is in or near the path of a tornado. Vehicles are easily tossed around by the extreme winds created by a tornado.

As the result of a concocted news story that involved two newscasters being overtaken by a tornado on a Kansas highway and being forced to take refuge under an overpass, some people have been led to believe that taking shelter under overpasses is good practice. Regardless that the newscasters survived the near miss of the tornado as it passed the bridge, underpasses are not considered safe places to take shelter. Bridges vary in construction, and many do not provide any significant protection from the wind and flying debris. They also act as a funnel, channling the wind into a tighter space. Finnaly the congestion of vehicles from several people parking their vehicles under and around the bridge can block the progress of other vehicles, potentially keeping them from having a chance to reach safety.

The National Weather Service, has created a presentation discussing the use of bridges as protection during the Oklahoma Tornado Outbreak that occurred on May 3, 1999, in the region of Oklahoma City where tornadoes passed over three different bridges—at least one person was killed in each instance.

Types of Tornados

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