Till now the scientist are not sure about the formation of tornadoes. It is that they are usually formed in a giant rotating thunderstorm which is known as a supercell. Supercells are formed when there is a collision of cold polar air and warm tropical air. This result in a greater instability which is mostly caused by the rising warm air. A narrow zone of cumulonimbus clouds forms, giving life to the tornadoes.
Then there is Lightning of flashes, heavy rains takes place and then hail begin to fall. This is the easiest recognizable part of the tornado, but when the pressure is dropped within the cloud which takes place due to the increasing wind speeds, it causes the moisture in the air to condense. This action continues and it gives an impression like a funnel is coming downwards from the clouds. When this funnel is clear there is a hissing sound and with a loud roar the tornado is on the earth.
A tornado is a vortex of air that rises into a cloud. The rising action is different to other kinds of thunderstorm winds that blow downwards. All thunderstorms involve rising air, or updrafts. The updrafts create the warm, humid air that fuels thunderstorms. Sometimes, the column of rising air becomes a vortex in the form of a tornado. The wind speed of dangerous torandoes can be around 300 miles per hour, but most often tornado winds are much slower.
Often, tornadoes form on the edge of the updraft. They are next to air that's coming down from the thunderstorm with falling rain or hail. That is the reason that heavy rain or hail can sometimes warn people about a tornado. That is of course not to say that hail or rain means a tornado is on its way! Tornadoes occur more in the United States than anywhere else in the world.