Jackson and Franklin Counties in southern Illinois suffered some of the most concentrated damage. Despite the fact there was a continuous damage track, it is possible the Tri-state Tornado could have been a series of tornadoes instead of one single tornado, but we may never know for sure. Tornadoes are classified according to the strength of their wind speeds. Since tornadoes very seldom pass over wind equipment, their wind speeds are estimated by examining damage patterns. The most commonly used scale of tornado intensity was devised by Theodore Fujita.
Breaks branches off trees.
Some damage to chimneys. 73 to 112 MPH Moderate damage. Peels surface off roofs. Attached garages may be destroyed. Mobile homes may be overturned.