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In 2007, the US started using an Enhanced Fujita Scale in place of the Fujita Scale. From a "civilian's" perspective, the Fujita Scale seemed to measure wind whereas the Enhanced Fujita Scale measures damage. This would suggest that tornadoes based on the EF scale is retroactively labeled rather than labeled while the tornado is taking place. Also, going from saying "F5" to "EF5" is a change.

What is the motivation of using the Enhanced Fujita Scale in place of the Fujita Scale? What are the main differences of the two scales that allowed the Enhanced Fujita Scale to be used today in the US?

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Both the Fujita (F) and Enhanced Fujita (EF) scales measure damage and then use damage as a proxy for wind speed. Wind speed at the low levels is not directly measured unless the tornado is either very close to a doppler radar site or there happens to be a mobile radar sampling it. This provides very few tornadoes with directly sampled wind speed and is the exception rather than the rule.

Where the F and EF scales differ primarly is that in the EF scale construction methods may be considered, not just the damage observed.

Damage is used as a proxy for windspeed by examining damage, construction techniques and methods (EF scale only), materials, and other factors and then determining what threshold wind speed could cause that damage. This means that identical damage can result in different windspeed estimates.

For example, a house that is completely destroyed and leaves behind a clean concrete slab is generally considered EF-5 damage, but some houses with poor construction have been found to not be physically tied to the foundation. A particular case in northern GA, USA had tornado winds blow down a garage door and then lift the house directly off the slab and take it away. This also leaves a clean concrete slab, but the construction methods will cause a lower wind estimate and a weaker tornado determination because less wind was required to cause the damage.

The only other difference between the F and EF scales are that some of the wind speed estimates have been lowered in the EF scale and more descriptive damage indicators are used for wind speed estimates to help standardize those estimates.

In summary, the EF scale differs from the F scale:

  • EF scale can consider construction methods
  • EF scale has more descriptive damage indicators used to estimate wind speed
  • EF scale has reduced wind speeds in each category to better align with engineering estimates of damage
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    $\begingroup$ So, the EF scale takes more factors into consideration from an construction/engineering perspective, but builds off the F scale. Here's a curious question: Would a F5-type tornado with winds over 300 mph primarily operating over an open field not be considered an EF5 tornado? $\endgroup$ – user402 May 6 '14 at 18:27
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    $\begingroup$ @edmastermind29 Don't confuse the scales there. A 300 mph tornado on either scale would be an F5 or EF5 tornado, but only if the damage says so. In the case of a rural field the only damage to be observed may be scoured earth and perhaps asphalt if it happened to cross a road. If the damage observed only supported F3 / EF3 damage estimates, then the tornado would be rated F3 / EF3. If the damage observed supported F5 / EF5 damage, then that is what the tornado would be rated. The actual windspeed is not relevant unless it happened to be accurately sampled. $\endgroup$ – casey May 6 '14 at 18:31
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    $\begingroup$ Not sure if casey was thinking of this when he wrote it, but I sure was... the May 31, 2013 tornado was measured by mobile Doppler at 295 mph... but was basically as you theorized, only doing EF3 damage. It was quickly rated an EF5, but then downgraded to EF3 later after debate. So as you guys suggested, F\EF scale isn't a measure of the actual tornado, but the results of a specific tool. Perhaps in the future we'll have a unified scale, much like hurricanes (which have DVORAK, satellite, recon, etc, but are combined to a best true estimate). Sounds like some work is being done towards such. $\endgroup$ – JeopardyTempest Jul 14 '16 at 7:29

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