Tornadoes

A tornado is a violent windstorm characterized by a twisting, funnel-shaped cloud. It is spawned by a thunderstorm (or sometimes as a result of a hurricane) and produced when cool air overrides a layer of warm air, forcing the warm air to rise rapidly. Tornadoes can cause a lot of damage and even deaths. The damage from a tornado is a result of the high wind velocity and wind-blown debris. Tornado season is generally March through August, although tornadoes can occur at any time of year. They tend to occur in the afternoons and evenings: over 80 percent of all tornadoes strike between noon and midnight. From 1950-1995 the total number of tornadoes in Michigan was 722, with an average of 5 deaths and 3,217 injuries (70 a year average) resulting from the storm a year.

Tornadoes are rated on a scale called the Fujita Scale. The storms are labeled with the letter F and then a number 0-6. The Fujita Scale is used to rate the intensity of a tornado by examining the damage caused by the tornado after it has passed over a man-made structure. The table below shows the ratings for a tornado with the Fujita Scale.

The Fujita Scale

F-Scale Number Intensity Phrase Wind Speed Type of Damage Done

F0 Gale tornado 40-72 mph Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards.

F1 Moderate tornado 73-112 mph The lower limit is the beginning of hurricane wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads; attached garages may be destroyed.

F2 Significant tornado 113-157 mph Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light object missiles generated.

F3 Severe tornado 158-206 mph Roof and some walls torn off well constructed houses; trains overturned; most trees in forests uprooted

F4 Devastating tornado 207-260 mph Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated.

F5 Incredible tornado 261-318 mph Strong frame houses lifted off foundations and carried considerable distances to disintegrate; automobile sized missiles fly through the air in excess of 100 meters; trees debarked; steel re-inforced concrete structures badly damaged.

F6 Inconceivable tornado 319-379 mph These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F4 and F5 wind that would surround the F6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F6 damage. If this level is ever achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies

The Fujita Scale is based on damage, not the appearance of the funnel. Storm spotters, storm chasers and other weather observers often try to estimate the intensity of a tornado when they are in the field, basing their judgment on the rotational speed and amount of debris being generated as well as the width. The size of a tornado is not necessarily an indication of its intensity. Large tornadoes can be weak, and small tornadoes can be violent.

Tetsuya Theodore (Ted) Fujita is to thank for the Fujita Scale. Horace Byers of the University of Chicago brought Professor Fujita to the United States in the early 1950's. He proceeded to change the course and the speed of severe storm research like no one else in this century. Shortly after his arrival, he began analyzing single thunderstorms the way larger systems had been studied for decades. He saw them as individual weather systems, which he called mesoscale systems.

He introduced the concept of tornado families, which are made up of individual tornadoes, each with a unique path, but spawned by the same thunderstorm. Prior to this, long damage paths were usually considered to be made by a single tornado. Through analysis of the photographs of the Fargo, North Dakota storm, he introduced concepts of thunderstorm architecture and terms like "wall cloud" and "tail cloud". He saw in these storms things that we take for granted today. But it took a genius to see them for the first time.

In the 1960's, his analysis of the Palm Sunday outbreak of 1965 again changed the course of how we view a tornado outbreak. For the first time, he mapped the entire outbreak in terms of tornado families. From the thousands of aerial photographs of Palm Sunday damage, he concluded that there was indeed something special about certain tornadoes... that they must contain more than one vortex. While multiple vortex tornadoes are well known today, he was the first to identify their existence based on damage patterns.

In the 1970's, he again revolutionized tornado climatology by giving us a system that linked damage and wind speed. Previous to this, all tornadoes were counted as equals. Today, the term "F5" is used casually by the general public, and in movies. Without Ted, one can only guess whether there would be any system at all.

A tornado can develop very quickly. Here is a picture of a tornado forming:

This storm was showing a dramatic visual increase in an opaque precipitation curtain northeast of the updraft, which was verified by a much larger and more intense radar echo. The precipitation curtain extends fully around the back side of the vaulted updraft and the wall cloud is lowering. From this position closer to the updraft, the easterly inflow has increased to 40 to 50 MPH. Not only was there strong inflow, it was becoming progressively stronger as it was "squeezed" into the mesocyclone. The radical increase in inflow, the increase in liquid (and likely frozen) precipitation, and the ominous appearance of the rotating storm led to an inescapable conclusion, that a tornado is about to form.

Tornadoes

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