The worst tornado outbreak on record occurred during an 18-hour period on April3-4, 1974. The storms ravaged the land from Alabama to Michigan, killinghundreds and leaving thousands homeless. The tornadoes occurred when asupercharged jet stream intercepted a very unstable, warm-humid air mass. Manyof the storms were as big as they come rating an EF4 or EF5 on the EnhancedFugita Scale. The devastating Xenia, Ohio, and Brandenburg, Kent., tornadoeswere part of this storm. Those two towns were literally blown to bits.Tornadoes are the meanest storms nature can muster. Fortunately, these violentstorms are also the smallest. Your average size tornado is about 200 yardsacross at the base; however, some of the F5 monsters grow to be a mile indiameter. That is still much smaller than the eye wall of an intense hurricanewhich can be 50 to 75 miles across. Then there are the massive extratropicalcyclones (winter storms) that can be affecting hundreds of thousands of squaremiles at any given time.
Tornadoes occur when strong thunderstorms start to rotate. The rotation comesfrom wind shear. Changing wind speed and direction in both the horizontal andvertical planes can cause an overturning motion in the atmosphere. If the aircould be dyed, one would see huge rotating cylinders along the horizontalplane. When this spin is entrained into a developing thunderstorm, the axis ofthe rotation is lifted and becomes vertical. In turn the storm cloud itselfstarts to rotate. When this spin tightens in one portion of the storm, atornado often occurs.
What goes on within the thunderstorm just before a tornado can be likened to afigure skater doing a spin. The rotation of the skater increases as the armsand legs are drawn inward, which is due to the conservation of angularmomentum.
Story by AccuWeather.com Senior Meteorologist John Kocet.
