Atmosphere And Weather

How Tornadoes Form



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Spring is here and the threat of tornados in "tornado alley" already has begun with storm damage in a nearby town. Each year, when clouds form and storms begin, area residents watch television weather reports with trepidation and ready themselves to take cover if necessary.

A 2008 tornado swept cars off a busy highway south of Interstate 44, destroying homes and businesses along that road and a parallel highway several miles away. Two nearby communities recently have been devastated by tornados, and my own town has experienced the wrath of a tornado in past years. According to National Weather Service statistics, 123 people died in tornadoes in 2008. There were 1,390 tornadoes in the first seven months of the year.

Tornado watchers are on the lookout, hoping to see the beginnings of the dangerous formations and warn residents early that they may need to take cover. According to an article by the U.S. Department of Commerce National Oceanic and Atmospheric Administration National Weather Service, there are about 800 tornados in the United States each year, causing more than 1,500 injuries and 80 deaths.

A tornado is a violently rotating column of air extending from a thunderstorm to the ground. Tornadoes can have a wind speed of up to 250 miles per hour, and sometimes more. The path of damage caused by a single tornado can be more than a mile wide. There are several elements to the formation of a tornado. First, thunderstorms usually form in warm, moist air before hitting cold fronts moving eastward. The storms can produce large hail, strong winds, and tornados. The storms may affect several states.

Spring and summer tornados often occur in the central states when there are strong frontal systems that move to the east. Tornadoes sometimes develop along what is called a "dryline," where warm, moist air from the east is separated from hot, dry air to the west. When the dryline moves to the east, it may cause thunderstorms that produce tornadoes. Other situations also can result in tornados, including thunderstorms that form when air near the ground flows up, such in mountainous terrain. Tornadoes also can form when tropical storms and hurricanes hit land, and usually are ahead of the storm center path.

The basic formation of a tornado occurs when a change in wind direction and an increase in wind speed result in an unseen horizontal spinning effect. A severe thunderstorm usually occurs when winds change speed and direction with height. As the air rises within the thunderstorm, updraft moves the horizontal rotating air to vertical rotating air. The rotation, which may be from two to six miles wide, then can extend throughout most of the storm. Warm moist air shoots upward and meets colder, dryer air. Since the warm moist air is lighter than cold dry air, an updraft occurs.

Depending on the type of tornado that forms, a tornado may be accompanied by little or no rain, or hail and damaging straight-line winds. Some tornadoes appear to extend only part way to the ground, and signs of debris may be seen below the funnel. Other tornadoes can't be seen as well because of rain or low-hanging clouds.

Weak tornadoes last from one to 10 minutes or more, with winds of less than 110 miles per hour. Most tornadoes fall into the weak category. Strong tornadoes, which account for about 30 percent of tornadoes, may last as long as 20 minutes and have winds ranging from 110 to 205 miles per hour. Violent tornadoes account for 70 percent of tornado deaths, but only 2 percent of tornadoes. Those tornadoes can last longer than an hour.

Although the explanation for how tornadoes form can include much more technical and scientific information about wind direction, cloud formation and the various types of tornado, the basic formation is caused by wind speed creating a spinning effect that makes the cloud travel at a high speed, capable of picking up objects and carrying them for miles.

Tornados can hit almost anywhere, contrary to the myth that they occur only in certain areas of the United States and other countries. When the weatherman gives a tornado warning, take it to heart.

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