Tornado

April in Illinois sees more tornados than any other month, although the peak season will last through June.  Tornados form mostly in the late afternoon or early evening, after the sun has had time to heat the atmosphere.  About 54 occur yearly in Illinois, with more than half of them coming during the next three months as the jet stream centers over our latitudes. 

Tornados develop over several hours.  First there is a thunderstorm where a warm, moist air mass is located in front of an eastward moving cold front.  The cold air pushes up and over the warm air creating instability in the air mass.  Warm air pushes upward and cools as it reaches higher altitudes.  As the air cools, moisture carried aloft in the warm air condenses to form water droplets that fall back toward the warmer air.  This area, where air continues to rise and fall, is called a convection cell.  With small amounts of air and water, clouds will form; with large amounts, a thunderstorm may develop. 

Wind at varying heights, moving in different directions and speeds, can start a rotating column of air.  As warm air is drawn into the bottom of the column, rotation speed increases, becoming faster toward the higher regions.  If the air in the rotating column near the ground is very cold, it will spread away from the storm and a tornado will not form.  Once a tornado is spawned, it may persist for only a few seconds or for several hours.  Tornados move about 30 miles per hour and generally cover less than six miles.  There are an average of 100,000 thunderstorms occurring in the U.S. annually, spawning about 1,000 tornados.  Damage from these storms is caused by the high winds as well as flying debris.

A variety of scientific instruments are used to detect when and where tornados will form including satellites, weather balloons, radar and computer modeling.  The first time radar detected a tornado was in 1953 in Champaign, IL, while running tests on a new radar site.  This led to a push to create a nationwide network of radar sites for tracking dangerous weather related events.  In the 1990s, a new radar system was created that was able to track the Doppler Effect which shows a change in the frequency of a sound wave as the source of the sound is moving in relation to the radar site.  Doppler Radar can detect areas of rotating air within a thunderstorm, and rotational speed, providing more precise information for forecasters trying to determine if a tornado will form.

Being prepared is the key to staying safe and reducing the risks from these natural events.  Pay attention to weather reports, warnings, and watches.  At home, know where safe shelter is located.  Whether home or away, tune to NOAA Weather Radio or follow instructions from local officials.  If your community has sirens, become familiar with the warning tones.  When outside, do not trust bridges – you are safer in a low, flat location.  Be aware of flying debris, and use your arms to protect your head and neck.  At home, move to a basement or small interior room away from windows, doors, and outside walls.  Remember to include pets in your plans.  You may consider storing emergency supplies including water, non-perishable foods, medical supplies and medication.

There have been numerous studies and statistical analyses done on tornado events.  In the past 70 years, there has been no real increases in number of tornados occurring annually.  Due to better data collection techniques, historical records show sharp increases in the number of recorded tornados occurring twice, first in 1954, after the new countrywide radar network was activated and in the mid-1990s after Doppler Radar was put into general use.  More information is available at the Illinois State Climatologist’s website including maps of previous events for each county at IL Tornado Mapping and plots of tornado related statistics at IL Tornado Plots.