Illinois Tornado Graphs

Jim Angel, state climatologist

This page was originnaly 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 Illinois.

F-scale vs. Fatalities

This graph (above) shows the strong relationship between fatalities and tornado strength with most of the fatalities occurring with F4 and F5 tornadoes.

F-scale vs. Injuries

Like the previous graph, this graph (above) shows the strong relationship between injuries and tornado strength with a majority of the injuries occurring with F4 and F5 tornadoes.

F-scale vs. Month

The monthly distribution of tornadoes with different levels of intensity show many important patterns. The total occurrence of all tornadoes reaches a maximum in April and stays strong throughout the late spring and early summer months. Stronger storms occur in the late spring-early summer because of the frequent mixing of warm air coming from the Gulf of Mexico and cooler air from Canada The mixing causes instability in the atmosphere, which is the type of environment in which strong storms need for development. The number of tornadoes declines in fall and are very low in the winter months arriving at a minimum in February. Each occurrence of the different values of F-scales (i.e. F1, F2, etc.) roughly follow this same pattern of peaking in the late spring-early summer, except for F5's which do not occur very often, so their occurrence is more sporadic.

F-scale vs. Path Length

As the value of the F-scale increases, the path length also increases until it reaches the maximum F-scale value of 4. When a tornado reaches the F-scale rating of 5, the average path length decreases. One reason why the F4 average path length is larger than the F5 length is because there have not been many F5 tornadoes that have occurred in Illinois. Therefore, there is not a large sample to use for comparing them to other tornadoes.

F-scale vs. Path Width

The relationship between F-scale and the tornado path width is positive until it reaches its maximum at F4. It then slightly decreases for F5. This decrease may be due to the small sample size of F5 tornadoes in Illinois.

Fatalities by Hour

The graph of fatalities by hour shows that there is a maximum of fatalities between the hours of 2-6 in the afternoon. Most tornadoes occur during the afternoon, which is the time when solar heating reaches it's maximum amount, causing instability in the atmosphere and causing storms to erupt. The number of fatalities then significantly decreases, remaining very low until it reaches its minimum during the morning hours.

Fatalities by Month

There are three local maximums in the graph of fatalities by month. The maximum of the total values occurs in April, when most tornadoes occur. After April, there numbers decline, but stay high compared to other months due to the fact that most tornadoes occur during the late spring-early summer. The other maximums occur in August and December. This may be attributed to one large tornado causing a number of fatalities, instead of an accumulation of fatalities from small tornadoes producing small numbers of fatalities.

Fatalities by Year

Most years have less than 10 tornado-related fatalities. There are two years with more than 20 fatalities due to tornadoes, 1967 and 1990. These two years both had at least one tornado that caused a significant amount of fatalities instead of an accumulation of fatalities throughout the year. In 1967, there were two tornadoes that cause a large amount of fatalities, one caused 24 deaths and the other 33 deaths. In 1990, a tornado caused 29 fatalities. The amounts of fatalities caused by each one of the tornadoes are more than most yearly amounts of tornado deaths.

Injuries by Hour

Since most tornadoes occur in the late afternoon-early evening, the number of injuries caused by tornadoes does not peak until around this time as well. The morning hours have minimal amounts of injuries until the amount starts to rise around 12 noon. The maximum amount of injuries occurs in the 5 p.m. hour, which is also the peak of the amount of tornadoes per hour.

Injuries by Month

Most injuries during the year also occur during the late spring-early summer. The peak month of injuries is April, vastly outnumbering any other month. There are also peaks in the months of August and December, which emulate the graph of fatalities by month.

Injuries by Year

The amount of injuries by year also emulates it's counterpart of fatalities by year. There are two major peaks in the graph,1967 and 1990. In these two years, there were tornadoes which caused significant amounts of fatalities and injuries. In 1967, there were two tornadoes, one causing 500 injuries the other 450 injuries, which combined caused almost 90% of that year's amount of injuries caused by tornadoes.

Tornadoes by F-scale

The amount of tornadoes by F-scale shows that there is a decreasing amount of tornadoes as the F-scale goes higher. F-0 tornadoes are the most common tornadoes, with F-1 tornadoes having almost the same frequency. F-2 tornadoes become less common, while the biggest drop comes between F-2 and F-3 tornadoes, over 200 fewer tornadoes. F-4 and F-5 tornadoes are thankfully the most rare, because they are the most powerful and need lots of energy to develop, which seldom occurs.

Tornadoes by Hour

Tornadoes usually occur later in the day, rather than early in the morning. The peak of the tornadoes occur in the late afternoon, when the air near the surface is most unstable because of solar heating. This causes the air to rise and form tornado-generating thunderstorms.

Tornadoes by Month

Late spring-early summer is the time when most tornadoes in Illinois occur. This is due to the abundance of both warm and cold air masses, which causes the warm air to rise when the air masses to converge. The rising warm air is very unstable and causes thunderstorms to form. The month with themost tornado occurrences is April.

Tornadoes by Year

The graph of tornadoes by year shows an increase in the amount of tornadoes as the years progressed throughout the century. The peak year of tornado activity was 1974, but the amount of tornadoes have seemed to increased towards the end of the century. This trend may reflect changes in tornado reporting procedures, rather than an actual increase in tornado activity.