Tornado is usually formed when the cold air meets the warm air. Then because of different pressure level, usually a funnel in the cloud is formed producing a tornado. But I think you would better understand it by reading on this link. It's a cool and concise way of defining.
http://library.thinkq...rnadoinfo/formed.html
Just for added information, I found this great article showing aftermath pics and a video on Joplin on the time of the tornado. We all know lots lost their homes. Hopefully they have secured homeowners insurance for that. Media URL: http://www.insuranceproviders.com/joplin-tornado-damage-photos/
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