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A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes are capable of tremendous destruction with wind speeds of 250 mph or more. Damage paths can be in excess of one mile wide and 50 miles long. Once a tornado in Broken Bow, Oklahoma, carried a motel sign 30 miles.
What causes tornadoes?
Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes. Tornadoes occasionally accompany tropical storms and hurricanes that move over land. Tornadoes are most common to the right and ahead of the path of the storm center as it comes onshore.
Although tornadoes occur in many parts of the world, these destructive forces of nature are found most frequently in the United States east of the Rocky Mountains during the spring and summer months. In an average year, 800 tornadoes are reported nationwide, resulting in 80 deaths and over 1,500 injuries.
The meteorological factors that drive tornadoes make them more likely at some times than at others. They occur more often in late afternoon, when thunderstorms are common, and are more prevalent in spring and summer. However, tornadoes can and do form at any time of the day and year.
Tornadoes’ distinctive funnel clouds are actually transparent. They become visible when water droplets pulled from a storm’s moist air condense or when dust and debris are taken up. Funnels typically grow about 660 feet (200 meters) wide.
Waterspouts are weak tornadoes that form over warm water.
Waterspouts are most common along the Gulf Coast and southeastern states. In the western United States, they occur with cold late fall or late winter storms, during a time when you least expect tornado development.
Waterspouts occasionally move inland becoming tornadoes causing damage and injuries.
How Do Tornadoes Form?
Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere.
Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical.
An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.
Tornadoes Take Many Shapes and Sizes
| 69% of all tornadoes
5% of tornado deaths
Lifetime 1-10 minutes
Wind less than 110 mph
| 29% of all tornadoes
30% of all tornado deaths
May last 20 minutes or longer
Winds 110-205 mph
|Only 2% of all tornadoes
70% of all tornado deaths
Lifetime can exceed 1 hour
Wind Speed more than 205 mph
Tornadoes have been reported in Great Britain, India, Argentina, but most tornadoes occur in the United States. In India, though Tornado is not a common feature but parts of extreme eastern India have experienced twisters. One of the most disastrous Tornadoes that India experienced was in 1902 before India, Pakistan, and Bangladesh separation. It was reported in the New York Times that more than 400 people were killed.
Check this link to read the news item in The New York Times
Have a look at this slide show that contains rare pictures of Tornado.
Source : The National Geographic