How to Calculate a Tornado's Wind Speed
Meteorologists use two ways to measure a tornado's wind speed: Doppler radar and after-storm damage. The former is more an approach for tornado chasers who haul around a mobile radar and get dangerously close to the funnel cloud. This is not a do-it-yourself approach. The latter, a forensic approach that uses a scale of destruction to judge the wind speed, is simple enough for a layman to use. Note that to use it, you would assess damage on the ground. This requires the tornado to actually touch down, or make ground contact, for you to assess the wind speed.
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Instructions
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Assess the most severe physical damage through which the tornado made touch-down. (Meteorologists have the aircraft to do aerial surveys to determine the path of the tornado.) Note the degree of damage to chimneys, signboards, walls, roofs, foundations and shallow- and deep-rooted trees.
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Look up a Fujita scale. See Resources for a link to the scale. You may want to bring the scale with you to the site to match damage with the scale's descriptions.
For example, if you find trains (not just boxcars) overturned, or you find most trees in a forest uprooted, then the tornado was at least an F3, the level referring to a wind speed range of 158 to 206 mph.
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Differentiate between the quality of construction between different damaged houses. Whether strong or only weak foundations were uprooted differentiates an F4 (207 to 260 mph) from an F5 (261 to 318 mph) tornado. For example, a strong foundation may have rods running into the ground and up into the first floor. A weak foundation might not be anchored into ground.
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Look for any objects that were propelled some distance. Find residents who can tell you how far from their original position they flew. For example, a car thrown 10 feet versus 100 meters would differentiate between an F4 and F5 tornado.
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Tips & Warnings
The Enhanced Fujita Scale is more detailed, and has replaced the Fujita scale for official assessments of tornado wind speed.
Laymen are more impressed with damage than the seasoned surveyor, so keep in mind not to inflate the nature of the damage; otherwise, you could place the tornado into a higher wind speed category than it really was.
