BOSTON -- A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground. The funnel forms from whirling winds, air rising from the ground rushes inward, and the cooling condenses water vapor into air creating the twister's shape. The funnel darkens as it picks up debris.

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The National Weather Service classifies tornadoes in three different ways -- weak, strong and violent. Weak tornadoes account for about 69 percent of all tornadoes. They usually last about 5 to 10 minutes. Strong tornadoes account for about 29 percent of all tornadoes, and they may last up to 20 minutes. Violent tornadoes make up only 2 percent of all tornadoes. They can have wind speeds that exceed 205 mph and sometimes last for more than one hour.

The Fuijita Scale is used to rate the intensity of a tornado by examining the damage caused by the tornado after it has passed over a man-made structure. The scale is named for the late Professor Tetsuya Theodore Fujita, who co-developed the ratings.

The scale ranges from F0, which is designated by some damage to chimneys and trees with winds reaching 40 to 72 mph, to F6, which would have winds reaching up to 379 mph. An F6 tornado has never been recorded.

Each year, about 800 tornadoes are reported nationwide. They are important safety steps that you can take to survive a tornado.

• If you are in a home or small building, go to the basement or an interior room on the lowest level, such as a closet or bathroom. If possible, get under something sturdy and cover your head.
• In a high-rise building, go to a small room in the interior. Avoid windows or rooms with exterior walls.
• In schools, nursing homes, hospitals, factories and shopping centers, go to a predesignated shelter area.
• In a mobile home or car, leave immediately and go to a substantial structure.
• If you are outside and unable to find shelter, lie flat in the nearest ditch, ravine or culvert and cover your head with your hands.