Tornadoes

In supercell thunderstorms, the updrafts are particularly strong. If they are strong enough, a vortex of air can develop just like a vortex of water forms in a sink. This precursor to the tornado is called a mesocyclone, and is typically 2 to 6 miles (3 to 10 kilometers) wide. One a mesocyclone forms, there's a roughly 50 percent chance that the storm will escalate into a tornado in around 30 minutes.

A tornado descends from the mesocyclone of a thunderstorm over New Mexico.

Tornadoes and Exploding Houses Have you ever heard that a tornado can make your house explode? This particular myth sounds believable at first. The idea is that tornadoes bring such a drop in atmospheric pressure that the higher pressure inside your home will make it explode unless you open all the windows. Luckily for homeowners, there's no truth to this. Unless you live in a downed spaceship, your house probably has enough venting to avoid explosion. All opening the windows will accomplish is making it a little easier for debris to hit you while the storm is rolling through.

Enhanced Fujita Scale Original Fujita Scale EF0 gale tornado mph Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards mph EF1 moderate tornado mph Lower limit is the beginning of hurricane-force winds. Peels surface off roofs; mobile homes pushed over; moving autos pushed off roads mph EF2 significant tornado mph Roofs torn off frame houses; mobile homes demolished; boxcars pushed over, large trees snapped or uprooted; light-object missiles generated mph EF3 severe tornado mph Severe damage. Roofs and some walls torn off well-constructed homes; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown mph EF4 devastating tornado mph Well-constructed homes leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated mph EF5 incredible tornado over 200 mph Phenomenal damage. Strong frame homes disintegrate or lifted off foundations and carried considerable distance; trees debarked mph

May 21, 2013 Moore, Oklahoma F5 The tornado released more energy than the Hiroshima Atomic bomb

Radar image of the Moore tornado with a debris ball and signature hook echo.

Some facts about Chicago's Significant Tornadoes: There were 92 significant tornadoes in the 8 county Chicago metro area between 1855 and The deadliest tornado occurred on April 21, 1967 during an outbreak of 5 significant tornadoes. A violent F4 tornado formed in Palos Hills in Cook County and traveled through Oak Lawn and the south side of Chicago. 33 people died and 500 people were injured by this 200 yard wide tornado that traveled 16 miles and caused over $50 million in damage. The most recent significant tornadoes occurred on June 7, 2008 over Will and Cook Counties. The only F5 tornado to ever strike the Chicago area was on August This tornado formed near Oswego and passed through Plainfield, Crest Hill, and Joliet. The tornado killed 29, injured 350, and caused $165 million in damage along a 16 mile path.

Monthly Distribution of Significant Tornadoes Most of the tornadoes occurred in the spring, between March and June. The drop off is rather significant during the middle of summer, while late summer and fall is another, yet smaller peak. Surprisingly, there have been as many significant tornadoes in November as in July.

Diurnal Distribution of Significant Tornadoes Most tornadoes occur in the afternoon and evening hours, with a strong peak showing up during the 5:00 PM hour. Tornadoes are extremely rare in the early morning hours. Solar radiation heats the ground, which heats the lower atmosphere. The atmosphere becomes most unstable during the warmest time of day, which is typically in middle to late afternoon. The atmosphere gradually cools and stabilizes after dark, diminishing the threat of severe storms and tornadoes. But tornadoes can occur anytime of day or night.

Decade Distribution of Significant Tornadoes