A ” tornado begins in a severe thunderstorm called a supercell. A supercell can last longer than a regular thunderstorm. The same property that keeps the storm going also produces most tornadoes. The wind coming into the storm starts to swirl and forms a funnel. The air in the funnel spins faster and faster and creates a very low pressure area which sucks more air (and possibly objects) into it.
It takes cold dry polar air meeting warm moist tropical air with an atmosphere needs to be very unstable to form a tornado type storm. This type of storm is more common in the United States from midwest down through Texan area called Tornado Alley.
Most tornadoes form in May, but April has the record for the more severe and damaging ones.
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Make your own tornado http://dsc.discovery.com/tv/storm-chasers/handbook/handbook.html