Tornadoes

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1 Tornadoes are the most violent storms on Earth. Winds spiraling into them usually exceed 100 mph and can reach speeds of 300 mph. In the USA, an average of 1,000 tornadoes spin up beneath thunderstorms each year, and these typically kill about 60 people. Tornadoes and the threat of tornadoes are a key part of the USA's spring weather because spring brings favorable tornado conditions. But tornadoes can occur any time of the year, during the day and at night. The National Weather Service defines a tornado as "a violently rotating column of air in contact with the ground and pendant from a thunderstorm." In other words, a thunderstorm is the first step in the creation of a tornado. Then, if other conditions are right, the thunderstorm might spin out one or more tornadoes. The three key conditions required for thunderstorms to form are: • Moisture in the lower to mid levels of the atmosphere. • Unstable air. That is, air that will continue rising once it begins rising from near the ground. • A lifting force. Something is needed to cause the air to begin rising. The most common lifting force is heating of air near the ground. As the air warms it becomes lighter and begins rising. Advancing masses of cool air, which force warm air upward, also trigger thunderstorms. When all the conditions are present, humid air will rise high into the sky and cool and condense into towering clouds, forming thunderstorms. This air rising into a thunderstorm is called an updraft. Tornadoes form in within a thunderstorm's updraft. The strongest tornadoes are often near the edge of the updraft, not far from where air is descending in a downdraft caused by the thunderstorms with falling rain or hail. This is why a burst of heavy rain or hail sometimes announces a tornado's arrival. Tornadoes are commonly associated with the nation's heartland – in a 10-state area stretching from Texas to Nebraska that also includes Colorado, Iowa, Illinois, Indiana, Missouri and Arkansas, known as Tornado Alley. But, they are not limited to this region. Tornadoes have occurred in all 50 U.S. states and are, in fact, more common in Florida than they are in Oklahoma. Florida tornadoes are generally weak — for tornadoes — with winds around 100 mph. Tornadoes that have hit Oklahoma, on the other hand, are some of the most violent on record. A tornado that struck Oklahoma City and its southern suburbs in 1999 had winds of nearly 320 just above the ground. Tornadoes are ranked by the damage they do using the six-tiered Fujita Scale. F0 and F1 tornadoes on the scale are considered "weak" and cause minimal to moderate damage with winds from 40-112 mph. F2 and F3 tornadoes are considered strong, packing winds of 113-206 mph that can cause major to severe damage. Violent tornadoes are those classified F4 and F5 with winds exceeding 206 mph. Damage is extreme to catastrophic.  Most weak tornadoes last 10 minutes or less, traveling short distances. Violent tornadoes have been known to last for hours and a few have traveled more than 100 miles. 


 * 2“Tornadoes form from severe thunderstorms. They are very destructive because they have a high energy density. They also don't last very long. This makes it hard to learn about them. Since scientists don't know much about tornadoes, it's also hard to forecast them. And because little is known about tornadoes, there are many myths about them which aren't true.

Tornadoes can form anywhere in the world. About 75% of them happen in the United States. Most of these form in the central U. S. in a place called tornado alley. There are also some other interesting facts about tornadoes. **

**People who are interested in tornadoes sometimes become spotters for their community. Sometimes people actually chase tornadoes! After a tornado passes by, scientists try and figure out how strong it was by using the Enhanced Fujita Tornado Scale.”**

Tornado winds can reach 300 miles per hour. How do they work? In order to form a tornado, you need three very different types of air to come together in a particular way: 1. Near the ground, there's a layer of warm, humid air and strong south winds. In the upper atmosphere, you'll find colder air and strong west or southwest winds. The air near the surface is much less dense than the cold, dry air aloft. This condition is called //instability.// It means that if the warm, moist air can be given an initial push to move upwards, the air will keep on rising, sending moisture and energy to form a tornado's parent thunderstorm. 2. The second ingredient is a change in wind speed and direction with height, called wind shear. This is linked to the eventual development of rotation from which a tornado may form. 3. The last thing you need is a layer of hot, dry air between the upper and lower layers. This middle layer acts as a cap and allows the warm air underneath to get even warmer and make the atmosphere even less stable. When a storm system high in the atmosphere moves east and begins to lift the layers, it begins to build severe thunderstorms that spawn tornadoes. As it lifts it removes the cap, setting the stage for explosive thunderstorms to develop as strong updrafts form. If the rising air encounters wind shear, it may cause the updraft to begin rotating, and a tornado is born.” **
 *  3 “A tornado is a violently rotating column of air that descends from a thunderstorm. No other weather phenomenon can match the fury and destructive power of tornadoes. They can destroy large buildings, lift 20-ton railroad cars from their tracks, and drive straw and blades of grass into trees and telephone poles.

Works Cited 1__Tornadoes are Earth's most violent storms.__ 2009. USA Today. 23 February 2009.  2__Tornadoes.__ 2008. Windows to the Universe. 9 February 2009. < http://www.windows.ucar.edu/tour/link=/earth/Atmosphere/tornado.html&edu=high >.
 * // 4 "In an average year, 800 tornadoes are reported nationwide, resulting in 80 deaths and over 1,500 injuries." //**

3__Severe Weather and Natural Disasters__ 2009. Scholastic. 10 February 2009. [|<http://teacher.scholastic.com/activities/wwatch/tornadoes/]>. 4__The Unusual Suspect.__ 2006. VorTex Unraveling the Secrets. 6 February 2009 //** **<http://www.nssl.noaa.gov/noaastory/booklo.htm>l**