Tornadoes
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What is a tornado? [Top]
A tornado is a violent whirling wind that is surrounded by a funnel-shaped cloud that extends down from a cumulonimbus cloud. Many people wonder why tornadoes suck up everything in their path, and the answer is quite simple. The air pressure at the base of the funnel is very low. When this low pressure region comes in contact with the ground, it acts like a giant vacuum. Most tornadoes occur on land, but some can even form over water. These tornadoes are called waterspouts. They form over lakes and oceans with a diameter of only about .4 kilometers. The length of an average tornado's path is 6 kilometers, but it varies depending on a number of factors. The speed of the winds inside the funnel travel at an average of 480 km/h (300 mph), but winds of up to 800 km/h (500 mph) have been estimated for the larger, inconceivable storms. The duration of a tornado's life is usually only a couple of minutes, but some have been known to last for multiple hours. Even though the storms are short-lived, their highly concentrated nature makes them intensely violent and dangerous storms. The list of damage that can be caused by these storms is endless. They are so diverse and unpredictable that damage can range from a broken tree branch to a house being torn from its foundation!
How do tornadoes form? [Top]
All tornadoes usually follow a very uniform and predictable formation cycle, unlike their erratic paths. Tornadoes can first be spotted by the formation of a strong whirlwind of dust from the ground surface. A short time after the dust cloud appears, a funnel-like cloud grows from the storm cloud above it. The funnel then becomes more organized and descends from the cloud, often times making contact with the earth's surface. It is not certain how tornadoes form, but meteorologists do know that they result from great instability in the atmosphere and are often associated with thunderstorms or cold fronts. There is really no long term warning issued for a tornado, but weather forecasts include tornado alerts when the conditions arise. Tornadoes can occur ahead of warm fronts and even behind cold fronts. Little is known of their origins, but the existance of a strong updraft (generated by a severe thunderstorm), and the conservation of rotational momentum are clues to how they are formed.
How are tornadoes classified? [Top]
Tornadoes are classified on something called the Fujita-Pearson scale. This scale incorporates maximum wind speed, and damage done in order to classify the tornado. Length of the tornado's path as well as the width of the funnel are sometimes used in addition to speed and damage, but they are not nearly as considerable. The Fujita scale classifications are as follows:
| Classification | Max Wind Speed (MPH) | Damage | Frequency |
| F0 | 40-72 | Some damage to chimneys and any pole-like protrusion; weakly rooted trees uprooted; sign boards damaged. | 29% |
| F1 | 73-112 | Slight damage to roofs; windows broken; trees uprooted; automobiles and trailors overturned. | 40% |
| F2 | 113-157 | Roofs torn off houses; houses with weak foundations demolished; trees uprooted; light object missles generated; automobiles blown off highways. | 24% |
| F3 | 158-206 | Roofs and some walls torn off of houses with strong foundations; forests leveled; automobiles lifted off the ground; rural buildings demolished. | 6% |
| F4 | 207-260 | Well constructed houses leveled; large missles generated; trees debarked; automobiles lifted and thrown over long distances; large flying debris. | 2% |
| F5 | 261-318 | Houses with strong foundations lifted completely off of foundations, thrown over long distances to disintegrate; trees completely debarked; automobile sized missles generated; concrete and steel structures badly damaged. | Less than 1% |
| F6 | 319-over | Damage has never been conceived | 0% |
When and where do most tornadoes occur? [Top]
Tornado season takes place most often in the spring. Most tornadoes have been known to form in the late afternoon or early evening. In the United States, tornadoes are most common in the Great Plains. Because there are so many tornadoes in this area, it is often called Tornado Alley. The greatest occurance of tornadoes is assumed to be in North America (especially in the Mississippi Valley), but other countries have just as many or more. Italy, New Zealand, and the United Kingdom exceed, or at least challenge the incidence rate of the United States. As far as statistics are concerned, Australia ranks second to the United States in the number of tornadoes per year. The United States is most notable for severe tornadoes of the F4 and F5 rankings. Most tropic tornadoes are extremely weak and often begin as waterspouts. Tornadoes occur as far south as parts of South America, and as far north as Stockholm, Sweden and Saint Petersburg, Russia.