What is a tornado?             Tornadoes are massive columns of rapidly rotating air that form during severe thunderstorms. They are characterized by strong winds and are sometimes accompanied by hailstorms and flooding. Wind speeds in a tornado can range anywhere from 40 miles per hour to over three hundred miles per hour. Tornadoes are classified based on their wind speeds and are ranked from F0 (slowest speeds) to F5 (highest speeds). This classification system was created in the 1970s by Dr. Theodore Fujita, a scientist working at the University of Chicago. Dr. Fujita based his classification on the amount of damage done to an area by the tornado; the more damage done by the tornado, the highber its classification and therefore its wind speeds. This is not without problems; basing the strength of a tornado solely on the damage done could mean that a tornado that should be ranked as an F5 ends up with a much lower raking while one that should be ranked as an F3 gets a much higher ranking.. The most well-known area where tornadoes form is a region of the American Midwest known as Tornado Alley; this region extends from northern Texas up through Oklahoma and Nebraska, with extensions into Colorado, Iowa and Minnesota. http://www.oar.noaa.gov/spotlite/archive/spot_climaology.html However, tornadoes are not exclusive to this area; while the most tornadoes are reported here, tornadoes can form anywhere in the US and in the world. For example, in May of 1985 27 tornadoes touched down in Pennsylvania and Ohio, causing several million dollars worth of damage and injuring many people. The most frequent times of tornado formation vary by region; generally it occurs during the spring and summer months, although tornadoes can form at any time during the year. http://www.nssl.noaa.gov/edu/safety/guideimg/pic12.jpg