“Dorothy lived with her Aunt Em and Uncle Henry on a small farm in Kansas. Their tiny house stood alone on a large, flat prairie. Dorothy had only one friend, her dog Toto. He was a small black dog who loved to jump and play. One day while Dorothy and Toto were playing, they heard the awful sound of a storm. The wind roared and the dust blew smoky circles in the air. Dorothy was frightened. Uncle Henry stopped working and shouted, ‘There’s a cyclone coming, run for the cellar!’ ”
The Wizard of Oz
By: Frank Baum (1939).
Tornados remain a frightening fact of life for those of us in the Midwest. These powerful acts of nature can wreak havoc on everything in its path of destruction. Tornado, twister, funnel, and cyclone: all are synonyms for the awesome force, which has intrigued and puzzled scientists for years.
A tornado is the most violent of all storms. It is a violent rotating column of air extending from within a thundercloud or a developing thundercloud down to ground level. Tornados can vary in diameter from tens of meters to about two kilometers with an average diameter of approximately 50 meters. In the northern hemisphere, winds of the tornado generally blow counterclockwise around a center of extremely low atmospheric pressure. In the majority of tornadoes that occur in the southern hemisphere, winds blow clockwise. The peak wind speeds can range from 75 mph to almost 300 mph. Some of the more powerful tornados can lift cattle, cars, and even mobile homes. A tornado becomes visible when a condensation funnel made of water vapor forms in very low pressures. It also becomes visible when the tornado lifts dust, dirt, and debris upward from the ground.
Most tornados are just small, intense cyclones. The term cyclone comes from the Greek word circle. Sometimes, in very rare occasions, the winds whirl in the direction opposite of a cyclone. For the most part, tornados have damage path less than 500 meters wide, move at less then 35 miles per hour, and last only a few minutes. The very violent tornados may reach up to one mile in diameter, travel at speeds up to 60 miles per hour, and blow for more than an hour.
Many tornados, including the very strong ones develop from a special type of thunderstorm that is known as a supercell. A supercell is a long-lived, rotating thunderstorm that is up to 16 kilometers in diameter that may last several hours, travel hundreds and hundreds of miles, and produce many tornados. Each supercell has the potential to spawn a tornado or a sequence of tornados.
For the most part, scientists generally agree that the first stage in the formation of a tornado 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 has to change with the height in favorable ways for the interaction to occur. This type of interaction causes the updraft to rotate at the middle levels of the atmosphere. The rotating updraft, which is also known as a mesocyclone, stabilizes the thunderstorm and gives it its long-lived supercell characteristics. The next stage is the development of a current of cooler air that moves in a downward direction on the backside of the storm, known as a rear-flank downdraft. The speed of the downdraft increases and the airdrops to the ground rapidly where it fans out at speeds that can go over 100 miles per hour.
In 1971, Theodore Fujita developed a classification system based on the damage done to manmade structures. His Fujita-scale classification system or the F-scale ranks a tornado as weak (F1 and F2), strong (F2 and F3) or violent (F4 and F5). The F-scale is used only in places where manmade structures exist.
The United States has the highest average number of tornados a year in the world with about 800 per year. Most of theses storms occur in a stretch of land known as Tornado Alley, which extends across the Midwestern and Southern states, especially Texas, Oklahoma, Nebraska, Iowa, and Kansas. Australia ranks second in the number of twisters a year. The tornado “prime time” is spring