Tornado

Union City, Oklahoma tornado (1973)

A tornado is a violently spinning column of air in contact with both a cumiliform cloud base and the surface. A tornado is typically shaped like a funnel with the narrow end on the ground. Tornadoes are known for being extremely destructive and are usually visible due to water vapor from clouds and debris from the ground. Tornadoes form in storms all over the world, and though they have been recorded in all fifty U.S. states, they form most famously in a broad area of the American Great Plains, Midwest, as well as South known colloquially as Tornado Alley. In pure number of incidences, the United States reports more tornadoes than any other country, however, the Netherlands is the most tornado-prone country relative to land area.

Terminology

The word "tornado" comes from the Spanish word for "turned", which in turn comes from the Latin word torqueo, meaning "to twist." Some common, related slang terms include: twister, whirlwind, wedge, funnel, willy-willy, or rope. However, willy-willy usually refers to a dust devil in Australia.

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. Waterspouts commonly form from rapidly growing cumulus clouds that have not become thunderstorms. They grow by stretching an already existing vortex. USA Today The Weather Book helps in defining a Waterspout as a tornado-like rotating column of air under a cumuliform cloud occurring over water;they are most common over tropical and subtropical waters and dissipate upon reaching shore/land.

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.

Formation

The formation of a tornado

Tornadoes develop from thunderstorms, most frequently supercell thunderstorms, though they also occur within squall lines. They are part of what meterologists call a “mesoscale convective system”. They are believed to be produced through the concentration of angular momentum, typically by the tilting and stretching of vorticity beneath a thunderstorm. 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.

The Supercell is what can be refered to as the king of thunderstorms. These are the most organized form of thunderstorm and also the most dangerous. A mesocylcone, a rotating column of rising air, is the primary aspect in creating a supercell’s long and powerful lifespan. It organizes the flow of warn, humid air close to the ground and the cool, dry air from above and supplies the spil that strong tornadoes require. As cool dry air enters the storm, raindrops evaporate into it which help cool the air and make the storm heavier, which causes its rear to descend. As it is descending it catches the descending air and creates a cold front (known as the gust front). As the storm advacnes, the gust front pushes up the warn humid air which feeds into the mesocyclone. Water vapor supplies the water needed for cloud and rain droplets. The latent heat released by the condensation of the water vapor is the catalyst in creating the storms intense energy. Through evaporation, the high-altitude air cools. Tornadoes have a tendency to form where cool downdraft air spins into the mesocyclone.

Exactly how tornadoes form is complex and not fully understood. In pre-thunderstorm environments, an increase in wind speed and/or a large change in direction with height ("wind shear") produces a horizontal, spinning area of air. Strong updrafts can draw this area of rotation up from horizontal to vertical, producing a mesocyclone. Towards the lower portion of the mesocyclone, a lowered, rapidly rotating area in the rain-free cloud base may exist and is known as a " wall cloud". If the rotation intensifies further, a funnel cloud can develop where water vapor condenses closer to 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 tornadoes 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. [1] [2]

A classical " hook echo" as seen in the strongest tornado in the 3 May 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

Characteristics

Tornadoes normally rotate in a cyclonic (counterclockwise) direction in the northern hemisphere, as the warm air in which thunderstorms usually form 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 in exactly the same way. There are true incidents of tornadoes repeatedly hitting the same town several years in a row; however, forecasting the exact position a tornado will strike at a certain time is presently 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 1,200 tornadoes a year are reported in the contiguous United States.

There are two general types of tornadoes, which are the ones that form in the outflow of air from a thunderstorm. These have a shallow, and very localized vortex and are generally weak storms that rarely exceed an F-2 on the fujita scale (about 157 mph). Waterspouts can be associated with this first kind of tornado.

The next type of tornado is the strongest and most dangerous. These are storms found when air is flowing into a thunderstorm, in the updraft area. Tornadoes begin at the storms middle levels and grow up into the storm and down towards the ground. These types generally form the categories F-4 and F-5 tornadoes which range from 207 mph to above 261mph.

Intensity

In the United States (and predominately worldwide), 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 and vegetative damage indicators compared to engineering data, as well as ground swirl patterns or photogrammetry / videogrammetry. 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 tornadoes. The Fujita scale is effectively a damage scale, wind speeds are estimates and have never been confirmed or fully tested, and there is no upper bound wind speed in the Enhanced Fujita Scale which has replaced the original Fujita scale.

The TORRO scale, developed in the United Kingdom by the Tornado and Storm Research Organisation (TORRO) and used primarily in the U.K., covers a broader range with tighter graduations, and TORRO says it is based solely on wind speed, though in practice damage analysis is used to infer wind speeds. It ranges 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 approximately 0.1% of all tornadoes in the U.S.

Frequency of occurrence

Oldest known tornado photograph (1884)

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. There is a secondary peak in autumn, especially in the southeastern U.S. where tornadoes also occur more frequently during winter there than in other areas.

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 California Central Valley is an area of increased tornado occurrence. More tornadoes occur in Texas than in any other US state. The 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. A number of Florida's tornadoes occur along the edge of Hurricane s. The state with the highest number of stronger tornadoes per unit area is Oklahoma. The neighbouring state of Kansas is another particularly notorious tornado state. It should be mentioned that states such as Oklahoma and Kansas have much lower population densities than Florida and that tornadoes may go unreported.

On average, the United States experiences 100,000 thunderstorms each year, resulting in more than 1,200 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 24 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. A series of continuous tornado outbreaks is known as a tornado outbreak sequence, with significant occurrences in May 1917, 1930, 1949, and 2003.

Canada also experiences numerous tornadoes, although fewer than the United States. In Canada, an average of 80 tornadoes occurs 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 Netherlands, followed by the United Kingdom (especially England). Bangladesh, India, Argentina, Italy, Australia, New Zealand, Germany, Estonia, and portions of Uruguay also have pockets of high tornadic activity. Occasional strong tornadoes occur in Russia, France, Spain, Japan, and portions of Paraguay and Brazil. Tornadoes have recently hit South Africa and parts of Pakistan in 2001 as well. Approximately 170 tornadoes are reported per year on land in Europe. Perhaps the most notorious tornado of recent years was that which struck Birmingham , England in July 2005 which destroyed a row of houses though - amazingly - without fatalities.

Social implications of tornadoes

Salt Lake City Tornado, August 11, 1999. (Orange fireball is substation exploding)

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 in excess of 300 mph (480 km/h). Tornado season in North America is generally March through November, 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 and state troopers, fulfil 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.

Due to the relative rarity and large scale of destructive power that tornadoes possess, their occurrence or the possibility that they may occur can often create what should be considered sensationalism in their reporting. 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.

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.

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.

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

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, channeling the wind into a tighter space. Furthermore, 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.

Tornado as metaphor

Cyclone as metaphor for political revolution; the Aunt-Em-type farm woman is labelled 'Democratic Party' and wears exactly the same dress as Dorothy in The Wizard of Oz; Puck magazine (1894)

The tornado has been used by cartoonists for over 100 years as a metaphor for political upheaval. For example, according to political interpretations of The Wizard of Oz, the tornado takes Dorothy to a utopia (Oz), and kills the Wicked Witch of the East who had oppressed the little people (the Munchkins). The storm cellar has also been used as a metaphor for seeking safety, as the 1894 cartoon at right shows.

A household cleanser advertising campaign in the 1960's claimed that Ajax "Cleans like a white tornado".

Motion pictures with a tornado theme

• The Wizard of Oz, 1939
• Night of the Twisters (TV), 1996
• Tornado! (TV), 1996
• Twister, 1996
• The Day After Tomorrow, 2004