Tornadoes are very frequent in United States. They occur mostly during the spring and summer seasons. Nearly 800 tornadoes occur in different parts of U.S per year. Tornadoes also occur in many other parts of the world, but they are most frequent in U.S.
Tornado is a violently rotating air that extends from the thunderstorm to the ground. They can cause a lot of damage to lives and property. The most powerful tornadoes move with the speed of 250mph (miles per hour) or even more. These destructive forces come in different shapes and size but the most common shape of a tornado is much like a funnel i.e. narrow from the bottom and wide from above.
Tornadoes form in an unusually violent thunderstorm. When it gets the favorable conditions i.e. the air gets unusually warm and humid in lower atmosphere, cooler and dry in upper atmosphere, quick change in speed and direction of the air and fall in temperature as the height increases, cause tornadoes.
The sun heats up the on the ground which starts moving upward and the cold air above blocks it. So the air starts moving in circular direction. Meanwhile more warm air from the ground continues to rise. As a result warm air becomes stronger than the cold air. So the cold air present on the top starts sinking and warm air starts rising upward in a fast circular direction. This causes tornadoes. More is the amount of warm air, more is the taller, wider and intense is the tornado formation.
All conditions are necessary for the formation of the tornado. If one condition is very strong and the other is weak then weak tornadoes can occur but if the conditions are strong then it forms strong and intense tornadoes.
Weak tornadoes occur more frequently or we can say 69% of all tornadoes are weak tornadoes, 29% of them are strong tornadoes and 2% are violent tornadoes. It can occur anytime and at any place, but they occur most in United States. Tornadoes are almost transparent until they pick up debris and dust particles with them.95% of the tornadoes rotate in counter-clockwise direction. Water spouts are also weak tornadoes that form above warm water.