Chicago-area Tornado Graphs

Jim Angel, state climatologist

This page was originally put together by a student, Brian Dunneback, several years ago. While the figures have not been updated, they should reflect the general features of tornadoes in the Chicago area. The data for Cook, DuPage, Kane, Lake, McHenry, and Will counties were used to create the graphs for the Chicago area.   

Fatalities by Hour

The graph of fatalities by hour shows that there is a peak in the number of fatalities caused by tornadoes in the afternoon hours. This corresponds the peak in tornado activity in the afternoon. The sudden drop at 4 p.m. may be due to the small sample size of tornadoes in the Chicago area leading to erratic statistics.

Fatalities by Month

Most of the fatalities in the Chicago area due to tornadoes are concentrated in two months, April and August. April has the most fatalities, which is common in most other areas of Illinois. Fatalities remain low every other month except August, which could be attributed to the damages of one particular tornado instead of an accumulation of tornadoes.

Fatalities by Year

In the graph of fatalities by year, there are two peak years of fatalities. The remaining years experienced few, if any, fatalities. The two peak years are 1967 and 1990. In these two years there were individual tornadoes that caused major fatalities in the area, which accounted for most of fatalities in those years.

F-scale vs. Fatalities

Fatalities in the Chicago area are mainly due to the more violent tornadoes (tornadoes with the F-scale ratings of 4 and 5). These types of tornadoes account for almost 90% of the fatalities in the Chicago area. The more violent the storm, the more injuries and deaths it causes.

F-scale vs. Injuries

The graphs of injuries caused by tornadoes in the Chicago area closely emulates the graph of F-scale vs. Injuries. The reason for this is that the tornadoes that caused the high amounts of fatalities typically caused high amounts of injuries.

F-scale vs. Month

All six values of the F-scale are shown in the graph F-scale vs. Month. April has the most amount of total tornadoes. June has the most amount of weak (F-0 and F-1) tornadoes, while April has the most amount of severe and violent tornadoes (tornadoes with an F-scale over F-1). August has the only F-5 tornado on record for the Chicago area, which is unusual since most severe tornadoes occur earlier in the year.

F-scale vs. Path Length

Average path length for the Chicago tornadoes peaks at the F-scale value of 3, and then drops to lower levels for F-scale values of 4 and 5.

F-scale vs. Path Width

Tornadoes that have low values of F-scale have a lower path widths than those of higher F-scale values. The size of the tornadoes path width can be directly attributed to its strength. If the tornado is weak (low F-scale values) then it's width will also be small. If the tornado is strong, it will tend to have a larger width.

Injuries by Hour

The amount of injuries peaks in the late afternoon hours, which is when most tornadoes occur. The number of injuries rises about 1 in the afternoon, and then sharply increases. During the 4 o'clock hour, the number of injuries drops rapidly, then rebounds to a maximum during the 5 o'clock afternoon hour. This is probably connected to the similar feature in the distribution of deaths by hour.

Injuries by Month

The amount of injuries per month closely follows the graph of fatalities vs. month. The months that have maximums in fatalities and injuries are April, August, and to a lesser degree in injuries, November. These later two months have individual tornado events that caused great amounts of damage, injuries, and fatalities.

Injuries by Year

The pattern of this graph also emulates the graph of fatalities vs. year. In 1967 and 1990, individual tornado occurrences caused large amounts of injuries because they hit heavily inhabited areas.

Tornadoes by F-scale

The amount of tornadoes decreases as the F-scale increases. Weak tornadoes are the most common. While it is not a smooth progression downward, the amount of tornadoes decreases as the tornado's strength increases.

Tornadoes by Hour

Tornadoes peak in the afternoon, when convectional heating is at a maximum. The amount of tornadoes at any other time is fairly low, when it is compared to the afternoon hours (1-6). The peak hour of tornadoes is at 5 p.m.

Tornadoes by Month

Late spring-early summer, when cold air masses and warm air masses are in constant interaction in the Midwest, is the peak of tornado activity in the year. April, May, and June have the most frequent occurrences of tornadoes. The month with the most frequent occurrences is April.

Tornadoes by Year

The amount of tornadoes per year is very erratic. The years that have maximums in tornado occurrences are 1965, 1972, and 1991.