Walsh Medical Media

Heading : Tornadoes

A tornado is the most violent windstorm on earth [<a href="#1" title="1">1</a>]. Tornadoes are rapidly rotating columns of air that make contact with the ground. Consider them to be intense, columnar vortices in contact with the ground that are capable of inflicting damage. They are either connected to or are situated underneath a cumuliform, buoyant convective cloud above. Tornadoes are sometimes, but not always, visible as a funnel cloud when the pressure deficit in them is low enough that water vapor condenses and cloud particles form. When the air is too dry or the pressure not low enough for a condensation funnel to form or to extend down to the ground, the tornado may be visualized as a column of rotating dust or debris or not visualized at all if there are no visible particles available to be lofted. In some instances they are encased in precipitation (‘‘rain-wrapped’’) and either not visible at all or visible only from a restricted viewing angle [<a href="#6" title="6">6</a>].

Tornadoes come in many colors, shapes and sizes. Some are gray, white, or even a pale shade of blue. Others are difficult to see until they pick up dirt or other debris, taking on the color of the soil, like brownish-red, or black. Tornadoes often resemble a long, thin tube sweeping across the sky. Some have a classic funnel shape-wide at the top and narrow at the bottom. Still, other tornadoes are as wide as they are tall, and are hard to tell apart from their parent storm cloud [<a href="#1" title="1">1</a>].

All tornadoes are triggered by thunderstorms, but only the very biggest thunderstorms trigger tornadoes. Thunderstorms begin when unstable air rises. To be “unstable,” a deep layer of air must be less dense than the air immediately above it. Being less dense, it rises as denser air sinks beneath it and raises it. It goes on rising until it lies beneath air that is less dense than it, and therefore can rise no higher [<a href="#6" title="6">6</a>-<a href="#31" title="31">31</a>].

Its rise may begin when the ground in a particular location is warmed strongly by the Sun. The warmed air expands and this makes it less dense than the air above it, so it is pushed upward. That is how summer storms begin. Alternatively, moving air may be forced to rise as it crosses hills or mountains, and warm air may be lifted by denser air at an advancing cold front [<a href="#6" title="6">6</a>-<a href="#30" title="30">30</a>].

Just because air is forced to rise, it does not follow that a thunderstorm will develop or, indeed, that any kind of clouds will form. If stable air is made to rise, it will reach a level beyond which it can rise no higher, then sink again. As it rises, the air cools adiabatically and as it sinks it warms [<a href="#6" title="6">6</a>-<a href="#30" title="30">30</a>].

According to the Fujita Scale upon which the “F5” classification is based, such a tornado can flatten homes, turn cars into airborne missiles, and debark trees. The Fujita Scale is a widely accepted “damage scale” for categorizing tornadoes that’s based on the havoc they wreak, and “F5” is used only to identify the most destructive rotating winds - those that spin anywhere from an estimated 261 to 318 miles per hour. Created by University of Chicago meteorologist Tetsuya “Ted” Fujita in 1971, the scale’s other five categories also use damage characteristics and estimated speeds to classify every other tornado: F0 (under 73 miles per hour); F1 (73 to 112 miles per hour); F2 (113 to 157 miles per hour); F3 (158 to 206 miles per hour); and F4 (207 to 260 miles per hour). In 2007, the “F-scale” was supplanted by a slightly modified “Enhanced Fujita Scale or “EF-scale (for consistency I’ve stuck with the F-scale, which was in use for most of my early days of storm chasing, throughout this book). Whichever scale you use, to suggest “F5” is to suggest almost unfathomable power [<a href="#6" title="6">6</a>-<a href="#33" title="33">33</a>].

Tornadoes occur on all continents. In Brazil, they are infrequent and occur mainly in the South and Southeast regions, especially in São Paulo, Rio Grande do Sul, Santa Catarina and Paraná [<a href="#34" title="34">34</a>]. The Meteorological centers are able to warn a few hours in advance of the formation of severe storm clouds because they do not yet have the necessary technologies to warn about tornado formation well in advance [<a href="#34" title="34">34</a>,<a href="#35" title="35">35</a>].

During the period from 1980 to 2007, the total number of natural disasters in all affected municipalities of the State of Santa Catarina, according to data from the Damage Assessment Forms (AVADANs) obtained in the Civil Defense of the State, were: 422 of hail, 549 of vendors and 43 episodes of tornadoes. As of 1998, 28 episodes of storm tides were also recorded, highlighting in 2004 the unprecedented episode of Hurricane Catarina [<a href="#36" title="36">36</a>,<a href="#37" title="37">37</a>].