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Tornadoes

A tornado is a violently rotating column of air extending from within a thundercloud, down to ground level. The strongest tornadoes may sweep houses from their foundations, destroy brick buildings, toss cars and school buses through the air, and even lift railroad cars from their tracks. Tornadoes vary in diameter from tens of meters to nearly 2 km (1 mi), with an average diameter of about 50 m (160 ft). Most tornadoes in the northern hemisphere create winds that blow counterclockwise around a center of extremely low atmospheric pressure. In the southern hemisphere the winds generally blow clockwise. Peak wind speeds can range from near 120 km/h (75 mph) to almost 500 km/h (300 mph). The forward motion of a tornado can range from a near standstill to almost 110 km/h (70 mph).

A tornado becomes visible when a condensation funnel made of water vapor (a funnel cloud) forms in extreme low pressures, or when the tornado lofts dust, dirt, and debris upward from the ground. A mature tornado may be columnar or tilted, narrow or broad—sometimes so broad that it appears as if the parent thundercloud itself had descended to ground level. Some tornadoes resemble a swaying elephant's trunk. Others, especially very violent ones, may break into several intense suction vortices—intense swirling masses of air—each of which rotates near the parent tornado. A suction vortex may be only a few meters in diameter, and thus can destroy one house while leaving a neighboring house relatively unscathed.

Scientists study tornadoes to gain a better understanding of their formation, behavior, and structure. Scientists who study tornadoes have a variety of powerful research tools at their disposal. Advances in computer technology make it possible to simulate the thunderstorms that spawn tornadoes using computer models running on desktop computers. Doppler radars, which detect the rain in clouds, allow meteorologists, scientists who study weather, to 'see' the winds inside the storms that spawn tornadoes. Modern video camera footage and reports from trained storm-spotters provide an unprecedented amount of high-quality tornado documentation. These tools all contribute greatly to the scientific understanding of tornadoes. This information may eventually lead to increased tornado warning times, better guidelines for building construction (especially schools), and improved safety tips.

Formation

Many tornadoes, including the strongest ones, develop from a special type of thunderstorm known as a supercell. A supercell is a long-lived, rotating thunderstorm 10 to 16 km (6 to 10 mi) in diameter that may last several hours, travel hundreds of miles, and produce several tornadoes. Supercell tornadoes are often produced in sequence, so that what appears to be a very long damage path from one tornado may actually be the result of a new tornado that forms in the area where the previous tornado died. Sometimes, tornado outbreaks occur, and swarms of supercell storms may occur. Each supercell may spawn a tornado or a sequence of tornadoes.

The complete process of tornado formation in supercells is still debated among meteorologists. Scientists generally agree that the first stage in tornado formation is an interaction between the storm updraft and the winds. An updraft is a current of warm, moist air that rises upward through the thunderstorm. The updraft interacts with the winds, which must change with height in favorable ways for the interaction to occur. This interaction causes the updraft to rotate at the middle levels of the atmosphere. The rotating updraft, known as a mesocyclone, stabilizes the thunderstorm and gives it its long-lived supercell characteristics. The next stage is the development of a strong downdraft (a current of cooler air that moves in a downward direction) on the backside of the storm, known as a rear-flank downdraft. It is not clear whether the rear-flank downdraft is induced by rainfall or by pressure forces set up in the storm, although it becomes progressively colder as the rain evaporates into it. This cold air moves downward because it is denser than warm air. The speed of the downdraft increases and the air plunges to the ground, where it fans out at speeds that can exceed 160 km/h (100 mph). The favored location for the development of a tornado is at the area between this rear-flank downdraft and the main storm updraft. However, the details of why a tornado should form there are still not clear.

The same condensation process that creates tornadoes makes visible the generally weaker sea-going tornadoes, called waterspouts. Waterspouts occur most frequently in tropical waters.

Classification

Direct measurements of tornado wind speeds are difficult (and dangerous) to obtain. In 1971 Theodore Fujita, a meteorology professor at the University of Chicago, devised a classification system linking the degree of damage to humanmade structures to possible wind speeds. In 1973 this system, known as the Fujita or F-scale system, was adopted as the official tornado classification system of the National Weather Service (NWS). Using the F-scale system, the NWS ranks tornado damage as light to moderate (F0 and Fl), considerable to severe (F2 and F3), or devastating to incredible (F4 and F5). The weakest tornadoes (F0) may damage chimneys and signs, whereas the most violent tornadoes (F5) can blow houses completely off their foundations. Although media reports may sometimes refer to a tornado as an F6, this ranking is not recognized by the NWS.

However, the F-scale system applies only in regions where humanmade structures exist. Also, scientists are able to correlate F-scale values only roughly with wind speeds. For instance, a wind speed of 145 km/h (90 mph) might do minor F0 damage to a well-constructed building but significant F2 damage to a poorly constructed building. Scientists estimate that F0 tornadoes may have wind speeds up to 110 km/h (70 mph), while F5 tornadoes may have wind speeds somewhere in the range of 420 to 480 km/h (260 to 300 mph). While the F-scale system remains a convenient means for scientists to classify and discuss the intensity of tornadoes, the limitations of the scale have inspired attempts to design a more sophisticated modeling system.

In the United States, 75 percent of the tornadoes rate F0 or F1 in strength. Most remaining tornadoes rate F2 or F3, with only 1 percent rating F4 or F5. Usually no more than one or two tornadoes per year reach F5 strength. On the other hand, the few F4 and F5 tornadoes account for 67 percent of the fatalities caused by tornadoes.

Occurrence

The United States has the highest average annual number of tornadoes in the world, about 800 per year. Outside the United States, Australia ranks second in tornado frequency. Tornadoes also occur in many other countries, including China, India, Russia, England, and Germany. Bangladesh has been struck several times by devastating killer tornadoes.

In the United States, tornadoes occur in all 50 states. However, the region with the most tornadoes is “Tornado Alley,” a swath of the Midwest extending from the Texas Gulf Coastal Plain northward through eastern South Dakota. Another area of high concentration is “Dixie Alley,” which extends across the Gulf Coastal Plain from south Texas eastward to Florida. Tornadoes are most frequent in the Midwest, where conditions are most favorable for the development of the severe thunderstorms that produce tornadoes. The Gulf of Mexico ensures a supply of moist, warm air that enables the storms to survive. Weather conditions that trigger severe thunderstorms are frequently in place here: convergence (flowing together) of air along boundaries between dry and moist air masses, convergence of air along the boundaries between warm and cold air masses, and low pressure systems in the upper atmosphere traveling eastward across the plains.

In winter, tornado activity is usually confined to the Gulf Coastal Plain. In spring, the most active tornado season, tornadoes typically occur in central Tornado Alley and eastward into the Ohio Valley. In summer, most tornadoes occur in a northern band stretching from the Dakotas eastward into Pennsylvania and southern New York State.

The worst tornado disasters in the United States have claimed hundreds of lives. The Tri-State Outbreak of March 18, 1925, had the highest death toll: 740 people died in 7 tornadoes that struck Illinois, Missouri, and Indiana. The Super Outbreak of April 3-4, 1974, spawned 148 tornadoes (the most in any known outbreak) and killed 315 people from Alabama north to Ohio.

American Tornado History
A brief history of Tornadoes.

Typically shaped like a funnel with the small end on the ground, a tornado is a violently spinning column of air that comes in contact with both cumulus clouds and the surface of the earth. Storms all over the world spawn tornadoes, but they are most famous for forming in the broad area of the central and southern United States known as Tornado Alley. The US reports more tornadoes per year than any other country in the world, although in relation to land area, the Netherlands is more tornado-prone.

The first tornado on record in the United States occurred in 1671 in Rehobeth, Massachusetts. Very little is known about the aftermath of that tornado. However, in the last 325 years the US has seen thousands of tornadoes tear across every part of the country, with every state experiencing at least one tornado. According to the National Weather Service, in an average year, tornadoes are responsible for 80 deaths and for injuring more than 1,500 people.

There have been many killer tornadoes in American history, but the Tri-State Tornado that tore through northeastern Missouri, south central Illinois and southwest Indiana on March 18, 1925, is the deadliest. The tornado left a path of destruction more than 300 miles long, traveling farther than any other tornado in recorded history. Initially, it was feared that the tornado had taken as many as 4,000 lives, yet the twister was still considered the deadliest in US history after it took 695 people. The deadliest single tornado in world history struck in Bangladesh in 1989, claiming more than 1,300 lives.

Tornadoes routinely cause millions of dollars in property damage; the 10 costliest tornadoes alone have caused nearly $6 billion in damage. The costliest tornado outbreak occurred in 1999 in Oklahoma. From May 3 to May 6, more than 66 tornadoes coursed through the state. The largest single tornado, an F5 that at 301 mph contained the highest wind speed ever recorded, tore through Oklahoma cities of Moore, Newcastle, Bridge Creek, Midwest City, Del City and Oklahoma City, destroying more than 10,500 buildings and killing 42 people. The tornado caused $1.1 billion in damages.

Fortunately, tornadoes cause significantly fewer deaths today than in the previous century. For instance, the last tornado to kill more than 100 people occurred over 50 years ago in Flint, Michigan. While many American tornadoes are still killers, the number of tornado related deaths in the US has dropped dramatically. However, tornadoes still cause massive amounts of property damage, which has not changed over time. For instance, the Tri-State Tornado of 1925 caused property damage which would have amounted to $1.65 billion in 2005 dollars.

While tornadoes often cause significantly less damage than other natural disasters, such as hurricanes, they remain very dangerous in that they often strike without adequate warning. While meteorologists have shown a significant ability to predict the path of a hurricane, predicting when and where a tornado will strike is very difficult. While weather professionals can forecast whether or not storm systems are likely to produce tornadoes and use radar to aid their predictions, there is no fool proof way for meteorologists to predict when and where a tornado will actually strike.

Natural disasters are an ever present part of life for people the world over. Every year they account for thousands of deaths and immeasurable amounts of property damage. In the last year alone, we have witnessed two extreme natural disasters with Hurricane Katrina and the Tsunami that struck multiple nations in the Indian Ocean, while experts are predicting yet another highly active weather year in 2006. The US has already witnessed several tornado outbreaks in 2006, one of which took 27 lives in early April. The history of tornadoes proves to us that we must be prepared to deal with severe weather at any given time.

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