Tornado Ratings

4.3.2013 Tornado Ratings   Questions or Comments? Contact us at weather@thestormreport.com

Tornado

Example Of A Strong Tornado

How do we determine the power or strength of a tornado? Nowadays, when we hear about tornadoes on the news, they’re described as having a rating of EF-0 to EF-5. The “EF” part stands for “Enhanced Fujita”, for the Enhanced Fujita scale, which has been used since 2006 in the USA. Canada is set to adopt the EF Scale this year.

In 1971, a brilliant scientist named Tetsuya Theodore “Ted” Fujita invented a tornado ranking scale that rated tornadoes based on the damage they caused. His scale was designed to match closely with the Beaufort Wind Scale (long used by mariners), and the Mach speed scale. The Fujita scale was updated in 1973 to account for the length of the path, and the width of the base of each tornado. So, during the 1970s through the mid 2000s we would hear of tornadoes rated as “F-3″ for example. The scale Fujita invented estimated wind speeds of tornadoes based on the amount of damage caused on the ground, what kind of damage, and the strength of the structures that were damaged. Basically, we had to guess at tornado wind speeds because there was no real way to measure them. Originally, Fujita’s scale went from F-0 to F-12, then it was amended for general use, only going up to F-5. He did leave room for an F-6 in the original scale, which he described as an “inconceivable” tornado. This is because no one really knew the limit of their power, (and we still really don’t).

On February 1, 2006, the National Oceanic and Atmospheric Administration switched to the “Enhanced” Fujita Scale. The “EF” is basically the same as the “F” scale in structure, however it was revised to reflect better examinations of tornado damage surveys, so as to align wind speeds more closely with associated storm damage. The new “EF” scale also adds more types of structures and vegetation. It also expands degrees of damage, and better accounts for variables such as differences in construction quality.

EF-5 Tornado Damage

EF-5 Tornado Damage

Both the “F” and “EF” scales are really only a proxy for actual tornado wind speeds, based on damage after the tornado has passed. If you hear anyone talking about a tornado that is happening right now, and they call it an “EF-3″, that’s an incorrect reference. Tornadoes are only classified after the damage has been analyzed by the National Weather Service. While a tornado is happening, you can really only accurately refer to it by size or shape. Once the NWS takes a look at the damage to buildings or ground cover along the path of the tornado, they are the ones who will give it an official “EF” classification. Examples of EF-5 tornadoes would be the Greensburg, Kansas tornado of May 4, 2007 and the Joplin, Missouri tornado of May 22, 2011. To view the damage of an EF-5 tornado, one would think the area had been flattened by a nuclear blast.

Like the original “F” scale, the “EF” scale makes only estimations of the wind speed in a tornado. However, fairly accurate estimations have been made with high-speed film, and mobile radar. Several university research programs have vehicles that carry their own radar. Using two or three of these vehicles, parked at different areas around a tornado, wind speed can be estimated more accurately. Based on this new research, tornado wind speeds have actually been downgraded a little. It was once thought that some tornado wind speeds were well over 350 mph. On May 3, 1999 an F-5 tornado near Bridge Creek, Oklahoma had an estimated wind speed of 302 mph, based on radar observations.

There have been attempts to record tornado wind speeds using “probes”. These are ground-based instrument packs designed to hug the ground during high winds. The probes are packed with cameras and sensors to record close encounters with tornadoes. Engineer Tim Samaras developed a very successful type of probe four years ago. Several “close calls” have been successfully documented, but as you can imagine, it is very difficult and dangerous to get such an instrument pack directly in the path of an oncoming tornado. Right now, the safest and most reliable wind speed measurements are calculated using mobile radar. Readings by different mobile radar units are compared, and an estimate of wind speed is made by measuring the speed of debris flying around the base of the tornado, or rain circulating within the funnel.

Remember, a tornado can only get an “EF” rating by the National Weather Service, after it has occurred. And until someone comes up with an indestructible instrument pack and gets it into the middle of a tornado, we won’t know exactly how fast their wind speeds are. This is why we continue to do research into one of Nature’s most destructive forces.

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