However, despite their small size all thunderstorms are dangerous. They are most likely to happen in the spring and summer months during the afternoon and evening hours but can occur any time of the year and day. All thunderstorms produce lightning which causes extensive damage to property and kills more people each year than tornadoes. Lightning poses a significant threat in Illinois having killed twenty-one people between 1985 and 1997 and injuring dozens more. Most lightning casualties occur in the summer during the afternoon.
Most victims are outdoors under a tree, in an open field, near a vehicle or a building.
Flash floods and floods produced by thunderstorms are the nation's number one killer, claiming nearly 140 lives each year. Most casualties occur when motorists try to drive through flooded roadways and are swept away by strong currents. Floods are Illinois' most destructive and wide-spread natural hazard having caused devastating damages to structures, utility services and crops. Most thunderstorm damage is caused by straight-line winds which can exceed 100 mph. Straight-line winds damage and destroy property often downing utility lines and interrupting services.