A tornado is a violently rotating column of air which is in contact with both a cumulonimbus (or, in rare cases, cumulus) cloud base and the surface of the earth. Tornadoes can come in many shapes, but are typically in the form of a visible condensation funnel, with the narrow end touching the earth. Often, a cloud of debris encircles the lower portion of the funnel.

Tornadoes are the most destructive storms on earth. Most have winds of 112 mph (180 km/h) or less, are approximately 250 feet (75 meters) across, and travel a mile or more before dissipating. However, some tornadoes can have winds of more than 300 mph (500 km/h), be more than 2 miles across, and stay on the ground for dozens of miles (more than 100 kilometers)

They have been observed on every continent except Antarctica; however, a significant percentage of the world's tornadoes occur in the United States.

This is mostly due to the unique geography of the country, which allows the conditions which breed strong, long-lived storms to occur many times a year. Other areas which often experience tornadoes are south-central Canada, northwestern Europe, east-central South America, South Africa, Australia, and south-central Asia



Source: wikipedia

Definitions



A tornado is defined by the National Weather Service (NWS) as "a violently rotating column of air in contact with the ground and...a thunderstorm." A tornado does not necessarily have to be visible; however, the low pressures caused by the fast wind speeds (see Bernoulli's principle) usually cause water vapor in the air to condense into a visible condensation funnel. In strong tornadoes, dirt and debris kicked up at the surface can contribute to the visibility of the tornado, in addition to changing its color depending on the color of the debris.

Stronger tornadoes are often observed to have multiple vortices, or many columns of violently spinning air rotating around a common center. However, a satellite tornado is a term for a weak tornado which forms very near a large, strong tornado, often lasting no more than a minute. The satellite tornado may appear to "orbit" the larger tornado (hence the name), giving the appearence of one, large multi-vortex tornado. However, a satellite tornado is a distinct funnel, and is much smaller than the main funnel.

A waterspout is merely a tornado over water. In general, most tornadoes over land are associated with a severe thunderstorm; however, the National Weather Service in the United States considers all waterspouts—including "fair weather" waterspouts—to be tornadoes. These weaker relatives of classic tornadoes are almost always very weak (F0 on the Fujita Scale), and form from weak, non-rotating thunderstorms, or even regular summer showers. Typically, waterspouts moving onto land cause little or no damage, and dissipate within minutes. However, strong waterspouts from supercells can cause significant damage if they impact land areas. In addition, strong tornadoes can move over lakes or over the ocean, becoming waterspouts, without losing intensity.

A landspout is an unofficial term for a tornado not associated with a mesocyclone. Known officially as a dust-tube tornado, it is usually weak, features a small condensation funnel which often does not appear to reach the ground, and is often marked by a tall tube of dust and/or debris reaching as far up as the parent cloud. Though usually weaker than ordinary tornadoes, they are tornadoes, and can cause serious damage.

A gustnado is a small, vertical swirl associated with a gust front or downburst. Because they are technically not associated with the cloud base, there is some debate as to whether or not gustnadoes are actually tornadoes. These usually cause localized areas of heavier damage among areas of straight-line wind damage caused by the gust front

__________________
||||||||||||||||||||50% Complete

Life Cycle of Tornado:


Most tornadoes follow a recognizable life cycle.The cycle begins when a strong thunderstorm develops a rotating mesocyclone a few miles up in the atmosphere, becoming a supercell. As rainfall in the storm increases, it drags with it an area of quickly descending air known as the rear flank downdraft (RFD). This downdraft accelerates as it approaches the ground, and drags the rotating mesocyclone towards the ground with it.

As the mesocyclone approaches the ground, a visible condensation funnel appears to decend from the base of the storm, often from a wall cloud. As the funnel decends, the RFD also reaches the ground, creating a gust front that can cause damage a good distance from the tornado. Usually, the funnel cloud begins causing damage on the ground (becoming a tornado) within minutes of the RFD reaching the ground.

Initially, the tornado has a good source of warm, moist inflow to power it, so it grows until it reaches the mature stage. During its mature stage, which can last a few minutes to more than an hour, a tornado often causes the most damage, and can be more than one mile across. Meanwhile, the RFD, now an area of cool surface winds, begins to wrap around the tornado, cutting off the inflow of warm air which feeds the tornado.

As the RFD completely wraps around and chokes off the tornado's air supply, the tornado begins to weaken, becoming thin and rope-like. This is the dissipating stage, and the tornado often fizzles within minutes. During the dissipating stage, the tornado becomes highly influenced by the direction of surface winds, and can be blown into fantastic patterns.

Though this is a widely-accepted theory for how most tornadoes form, live, and die, it does not explain the formation of smaller tornadoes, such as landspouts, long-lived tornadoes, or tornadoes with multiple vorticies. These each have different mechanisms which influence their development—however, most tornadoes follow a pattern similar to this one.

__________________
||||||||||||||||||||50% Complete

Shape


Most tornadoes take on the traditional appearence of a narrow funnel, a few hundred yards across, with a small cloud of debris near the ground. However, tornadoes can appear in all manner of shapes and sizes. Small, relatively weak landspouts might only be visible as a small swirl of dust on the ground. Large single-vortex twisters, often violent in nature, can look like a large wedge stuck into the ground, thus they are known as wedge tornadoes or wedges. Wedges can be so wide that they appear to be no moreTornadoes in the dissipating stage can appear like narrow tubes, or ropes, twisting into all manner of curls, twists, and s-shapes. Multiple-vortex tornadoes can appear as a family of swirls circling a common center, or may be completely obscured by debris, appearing to be a single funnel.

In addition to these appearences, tornadoes may be obscured completely by rain or dust. These tornadoes are especially dangerous, as even experienced meteorologists might not spot them.

__________________
||||||||||||||||||||50% Complete

Size


In the United States, an average tornado is around 500 feet across, and stays on the ground for 5 miles.

Weak tornadoes, or strong but dissipating tornadoes, can be exceedingly narrow, sometimes only a few feet across. In fact, a tornado was once reported to have a damage path only 7 feet long.

On the other end of the spectrum, a tornado which affected Hallam, Nebraska on May 22, 2004 was at one point 2.5 miles wide. In terms of path length, some meteorologists believe that the Tri-State Tornado, which affected parts of Missouri, Illinois, and Indiana on March 18, 1925, was on the ground continuously for 219 miles. However, without a modern damage survey, it is impossible to determine whether or not the tornado was in continuous contact with the ground for its entire length. In fact, some scientists believe it was a series of very strong tornadoes. The longest modern-day tornado track was a tornado which was on the ground for 160 miles in northeastern North Carolina on November 22, 1992.

__________________
||||||||||||||||||||50% Complete

Appearence


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. In Utah tornados during the spring are sometimes called "white tornadoes" because they appear white. This happens when the funnel passes through snow-covered land, sucking the white snow into it.




Rotation


Tornadoes normally rotate in a cyclonic direction (counterclockwise in the northern hemisphere). Large-scale storms always rotate cyclonically because of the Coriolis force. The tornadoes usually rotate the same way, however, they are on too small a scale to be directly affected by the rotation of the earth. Therefore, on occasion (about 1 in 100 tornadoes) a tornado will rotate in an anticyclonic direction. Usually only landspouts will rotate anticyclonically. However, on very rare occasions, an anticyclonic supercell will develop, producing a traditional tornado, only anticyclonic.

__________________
||||||||||||||||||||50% Complete

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 recorded tornadoes. The Fujita scale goes all the way up to F12 which represents mach level winds, though F5 is the highest ever recorded. 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 United States of America.

__________________
||||||||||||||||||||50% Complete

Frequency of occurrence


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, 700 or more tornadoes a year are reported in the contiguous United States.

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 winter tornadoes are more frequent 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 (1996), and downtown Salt Lake City, Utah (1999) (see Salt Lake City Tornado). The California Central Valley is an area of some frequency for tornados, albeit of weak intensity. 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 Hurricanes. The state with the highest number of stronger tornadoes per unit area is Oklahoma. The neighboring 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.

__________________
||||||||||||||||||||50% Complete

Social Implications of Tornadoes:



Tornado damage to man-made structures 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% 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 could 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 drama in order to increase their market share. This is especially evident in tornado-prone markets, such as those in the Great Plains.

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.

__________________
||||||||||||||||||||50% Complete

There is animation of Toranado in Encarta ... It would visually tell many things about it.

Thanks!

__________________
Where is the SIGNATURE....????