Illinois State Water Survey - University of Illinois at Urbana-Champaign

State Climatologist Office for Illinois

Tornadoes in Illinois - An Introduction

Dr. Jim Angel, State Climatologist

What is a Tornado?

Thunderstorms develop in warm, moist air in advance of eastward- moving cold fronts. These thunderstorms often produce hail, strong winds, and tornadoes. A tornado is a violent rotating column of air extending from a thunderstorm to the ground. The damage from a tornado is a result of the high wind speeds and wind-blown debris. Tornado season is generally March through May in Illinois, although tornadoes can and do occur at any time of the year. They tend to occur in the afternoons and evenings with 50% occurring between 3 p.m. and 7 p.m. Illinois average 64 tornadoes per year based on the 1998-2007 data.

How do tornadoes form?

Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical. An area of rotation, 2 to 6 miles wide, now extends through much of the storm. Most strong and violent tornadoes from within this area of strong rotation. The newer NWS Doppler radar can detect these areas of rotation, leading to an earlier warning time.

Tornado Facts:
  • Tornadoes can be almost invisible, marked only by swirling debris at the base of the funnel. Some are composed of several mini-funnels.
  • On average, the United States experiences 100,000 thunderstorms per year. About 1,000 tornadoes develop from these storms.
  • The prime tornado season in Illinois is March - May,with 63% occurring during that time.
  • Average number of tornadoes in Illinois is 64 per year, but there were none in 1919 and 1933, and as many as 124 tornadoes in 2006.
  • There are no real time trends in the number of tornadoes. However, a sharp increase after 1954 is due to greater attention to tornadoes by the National Weather Service with better data collection and studies. A second increase appears to have occurred in the mid-1990s as the new NWS Doppler radar systems became operational in Illinois.
  • Illinois has experienced two of the worst tornadoes in the nation's history: the infamous Tri-State tornado of March 18, 1925, with 695 dead, 2000 injured, and $130 million in property damage; the Mattoon tornado of May 26, 1917, with 101 dead, 638 injured, and $55 million in property damage.
  • Damage to buildings is caused by violent winds and debris slamming into the building, not by the low pressure of the tornado as it passes overhead.

Illinois State Water Survey

2204 Griffith Dr
Champaign, IL 61820-7463
217-333-0729
jimangel@illinois.edu

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