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Nature At Its Most Violent
75-Mile-an-Hour Winds Leave Severe Damage in their Path
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How often does a headline like this attract our attention? Tornadoes are one of nature’s most violent phenomena. Tornadoes appear with little or no warning, wreaking havoc on the ground below. Humans and animals in a tornado’s path can be in extreme danger. Meteorologists called "stormchasers" study these storms at close range. It can be very dangerous. The information stormchasers provide has helped the National Weather Service initiate an early warning system designed to limit the loss of life when tornadoes strike.
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Tornadoes
Tornadoes occur when the weather is warm and humid. It is not a coincidence that the same conditions are necessary for thunderstorm development. Thunderstorms spawn tornadoes.
Weather conditions necessary for the development of thunderstorms occur at different times of the year in different areas of the U.S. In the southeastern states, tornadoes occur more frequently from January through March. In the northern tier of states, tornadoes are more frequent in the summer months of June, July and August. This is also true of southern Arizona and New Mexico where summer means monsoon rains arrive in the desert. Texas, Oklahoma, Kansas and eastward to Tennessee, Kentucky and Ohio are sometimes known as Tornado Alley. Tornadoes occur more frequently in these states than any other part of the U.S. and are common between April and June.
Tornadoes are extremely rare in the Northwest. It was not by chance that Dorothy and her little dog Toto started their journey to Oz in Kansas. The month of March averages fewer than 50 tornadoes while May averages around 70. The average number of tornadoes per month in the U.S. jumps to over 150 in April. The least number of tornadoes occurs from July to December. But tornadoes can occur anytime and anywhere.
Categorizing tornadoes by their size, shape and wind speed is difficult. Data is rarely available and eyewitness accounts vary widely. We measure tornado intensity using the Fujita Scale (see "Careers" below for more about the Fujita Scale), which assesses the extent of damage done by the tornado winds. The Fujita Scale ranges from 0 to 6.
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There are two basic types of tornadoes, those with wind speeds at 150 mph or less and those with wind speeds above 150 mph. The first type is most common and has a relatively small vortex (circulation pattern) with a narrow rope-like appearance. The duration of touchdown is short and since wind speeds rarely top 150 mph, damage is limited. There are three types of tornadoes that belong to this first category: waterspouts (tornadoes that form over water); "landspouts," tornadoes that resemble waterspouts in appearance but that form over land; and intense but small whirling wind events called "gustnadoes". All of these form quickly and disappear just as fast. They are category F0, F1 and F3 on the Fujita Scale.
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Stronger tornadoes belong to the second group with winds between 200 and 300 mph. These tornadoes are often broad cones up to a mile wide. They are category F4 and F5 on the Fujita Scale. Extensive damage is done by tornadoes in this group. With winds over 300 mph, Category F6 tornadoes are called "Incredible" and "Inconceivable".
Because tornadoes form quickly and most times die just as rapidly, it is difficult to collect the kind of data and observations necessary to develop a definitive explanation of tornado formation. Today’s meteorologists depend upon scientists from many fields of study to help develop new concepts and equipment to gather the information needed to better understand tornadoes. With this information, the National Weather Service is able to issue Tornado Watches and Warnings in a timely manner. You can learn more about tornadoes in the USAToday weather pages and from the National Weather Service. The Red Cross provides information about tornado safety procedures.
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Classroom Activity
Tornado in a Bottle
Materials Needed:
- Two 2-liter, clear plastic soda bottles
- Water
- Electrical or duct tape
- A washer with a 3/8-inch hole *
- Food coloring *
- Glitter *
- Water
* Optional
Directions:
- Fill one bottle 3/4 full with water. Add a few drops of food coloring or a few pinches of glitter to the water.
- Tape the empty bottle on top of the bottle with the water in it, opening to opening. You may use the washer to attach the bottles together and then tape the necks to together.
- Turn the bottles over so that the one filled with water is on top. Notice that the water flows very slowly, if at all, into the bottom bottle.
- Now, turn the bottles over again. Swirl the bottles in a circular motion, really fast, in one direction. Do not shake the bottles up and down. Turn the bottles over once again so that the empty bottle is on the bottom. The water should now swirl easily into the bottom bottle.
What happened?
As you "swirl" the bottle, the water begins moving in a circular pattern. When the water moves fast enough, it pushes outward against the sides of the bottle and leaves an air hole through the middle of the swirling water. The hole allows air from the bottom bottle to rise up into the top bottle creating a higher air pressure in the top bottle. When the air moves upward, an empty space is created in the bottom bottle (lower air pressure). Water from the top bottle now flows downward easily to fill the space. The combined effects of gravity, air pressure, and water pressure make the water move in a circle and create a vortex, like a tornado!
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Weather Wizard
Cumulonimbus cloud - A cumulus cloud often spreads out in the shape of an anvil extending to great heights
Tornado - An intense, rotating column of air that protrudes from a cumulonimbus cloud in the shape of a funnel or a rope whose circulation is present on the ground
Tornado Warning - A tornado has been reported or is being indicated as possible by Doppler radar; immediate action should be taken
Tornado Watch - Same as a severe thunderstorm watch but tornadoes are also possible in the watch area.
Vortex - A mass of whirling fluid (gas or liquid) forming a cavity in the center toward which things are drawn
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Weather Trivia
What is NOT a clue to a developing tornado?
- Dark, greenish sky
- A loud roar, like a freight train
- Large hail
- Absence of birds, insects
Answer: D - Absence of birds or insects.
Wildlife seems to be mostly unaffected by severe weather. Another important clue to look for is frequent lightning.
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WeatherNet Classroom News
The WeatherNet team is excited about a new camera option available to WeatherNet schools. NetCam brings a whole new perspective to learning using the world outside the classroom. NetCam allows you to pan, zoom and tilt using a control panel on your desktop to find the perfect viewpoint for your daily lessons. Here at WeatherNet, we have been watching the weather, of course;
but right outside our building is a terrific biology lesson -- a conifer with pinecones that are in the process of maturation. What a great way to watch nature in motion. Focus on a cloud or a blossom opening to the spring sunshine or perhaps another exciting weather event. Call the WeatherNet team at 800-544-4429 to find out more about this powerful learning option.
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Careers in Weather
Measuring Tornado Intensity -- The Fujita Scale
We find tornadoes to be extremely interesting weather events. The power of the whirling winds of a tornado is fascinating to watch - from a distance or on TV. But all too often, tornadoes are devastating and deadly in the areas where they touch down. Devastation caused by tornadoes is measured on a scale of 0 to 6 using the Fujita Scale.
Tetsuya Theodore "Ted" Fujita (1920-1998) was born in Japan and received a degree in mechanical engineering. He studied the devastation caused by the atomic bomb in Hiroshima. After moving to the United States, he obtained a Ph.D. in Meteorology.
In 1971, Fujita developed the Fujita Scale, which defines the intensity of tornadoes. The Fujita Scale specifically measures the damage caused by tornadoes and classifies them on a scale of F0 to F6. An F0 tornado causes very little damage. With winds under 73 mph, the damage is usually confined to trees, chimneys, signboards and branches. An F6 tornado with winds over 318 mph is considered "Inconceivable" and one has never been recorded.
There are some problems with the Fujita Scale. Because it measures the damage that is caused by the tornado, it can only be used after a tornado touches down. It takes a highly trained eye to determine the true extent of the damage and often reports are over-exaggerated by the media or well-meaning bystanders. For these reasons and others, the Fujita Scale was slow to achieve wide acceptance.
Another well-respected meteorologist, Dr. Allen Pearson, supported Fujita’s theories and developed his own scale to work in conjunction with the Fujita Scale. Dr. Pearson’s scale measures the length and width of a tornado’s path -- its contact with the ground. The two scales are often used together and known as the Fujita-Pearson Scale.
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