Determining just how strong a tornado’s winds were, like the ones that came through last week, is not as easy as it might seem. First, the winds aren’t measured like the winds you hear about on the weather report every day---those are from a wind gage in real time from area observations. Tornado winds are estimated, based on the damage they caused. This task was first tackled in 1971 by Dr. Ted Fujita whose scale of F-0 to F-5 tornadoes looked like this:
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Interestingly, Fujita actually drew out his original scale to determine winds along the Beaufort wind scale used in nautical measurements starting with “calm” all the way to winds as strong as the speed of sound! Notice the Mach 1 in this scale going to 738mph!
Given that actual wind speeds don’t go to Mach 1, the Fujita scale only went to F-5 rather than F-12! A couple of tornadoes in the past have actually been classified F-6 originally, but were reclassified later as “only” F-5.
The scale served the weather community well for almost four decades, but there were inherent problems. For instance, different types of construction were not considered, the damage done was a subjective opinion and easily biased, and some winds could be easily overestimated. And what if there were no structures for the tornado to destroy--just a lot of fields? How do you assign a rating?
In came the ENHANCED Fujita scale at the hand of engineers. In 2007, the newer scale brought in 28 parameters of just what was damaged. Were destroyed trees hardwood or softwood? Are the affected homes newer construction, older homes, made of brick, made of wood? You can see below the different categories.
Those blue numbers by each are linked to even more information about observing damage done. You can follow the link here and click on a few of those blue numbers to see more and that is where the Degree of Damage is also taken into consideration! Even then, things can get tricky and experience matters! I asked Jeff Evans, Meteorologist in Charge here at the Houston National Weather Service, about determining wind speeds. He gave me an example of looking at an affected home: For instance, a single family home could look completely destroyed, but we can tell the garage door failed and the roof went. Without the roof, the walls collapsed. That would cause us to slide down the wind estimate, versus a well built home that was just leveled from the strength of the winds. There is some subjectivity, but the EF scale really helps balance that out a bit.
Taking into consideration how much damage was done to different structures and the surrounding environment, the meteorologists can assign different numbers and eventually come up with an EF rating and a wind determination. So now you have insight into just what goes into surveying a tornado damaged area and really drilling down to how strong those winds are or aren’t!
Frank
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