Tornadoes are violently rotating columns of air rising up into a cloud.  A thunderstorm is the first step in the creation of a tornado. A thunderstorm happens when there is moisture in the atmosphere, a lifting force causing air to begin rising, and unstable air that will continue to raise once it starts.  Then, if other conditions are right, the thunderstorm may spin out one or more tornadoes.

St. Louis County average less than one tornado per year, with a total of 37 tornadoes reported between 1950 and 2003. Of these 37, seventeen tornadoes ranked F-0, 10 were ranked F-1, and four were F-2, while six ranked F-3 on the Fujita scale rating the strength of a tornado.

The wind speeds associated with each rating are listed below.

F0: 40-72 MPH	F3: 158-206 MPH
F1: 73-112 MPH	F4: 207-260 MPH
F2: 113-157 MPH	F5: 261-318 MPH

      
Between 1950 and 2003, there was an estimated damage of $6.4 million dollars as a result of tornadoes. Some of the more significant tornado events documented for St. Louis County occurred in August 1969 when a number of tornadoes impacted St. Louis, Lake and Cass Counties. As a result of those tornadoes 88 homes were destroyed, 119 were damaged and 27 farm buildings were destroyed. A tornado in 1976 left a path 400 ft wide and 10 miles long that resulted in $40,000 in timber in downed. Between 1950 and 2003 three deaths have been attributed to Tornadoes.