Thunderstorms develop in warm, moist air in advance of east-ward 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. 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.