What is the Enhanced Fujita Scale?

Enhanced Fujita Scale (KSLA) Enhanced Fujita Scale (KSLA)

The Enhanced Fujita Scale was developed and implemented in 2007 to help meteorologists to assign ratings to tornados using an increased amount of detail that its predecessor, the Fujita (F) Scale.

The Fujita (F) scale was originally developed by Dr. Tetsuya Theodore Fujita in 1971 to estimate tornado wind speeds based on damage left behind by a tornado.

The tornado rating categories of the EF Scale range from zero to five, with EF0 as having the lowest wind speed and EF5 as having the highest wind speed.

The NWS implemented an Enhanced Fujita (EF) scale to rate tornadoes in a more consistent and accurate manner. The original F scale had limitations, such as a lack of damage indicators, no account for construction quality and variability, and no definitive correlation between damage and wind speed.  These limitations may have led to some tornadoes being rated in an inconsistent manner and, in some cases, an overestimate of tornado wind speeds.

The EF Scale rates tornado categories from zero to five, taking into account more variables than the original F Scale did when assigning a wind speed rating to a tornado. The EF Scale incorporates 28 damage indicators (DIs) such as building type, structures, and trees. For each damage indicator, there are 8 degrees of damage (DOD) ranging from the beginning of visible damage to complete destruction of the damage indicator. The original F Scale did not take these details into account. 

Wind speeds in a tornado vary along its path. The EF Scale rating of a tornado event represents the highest wind speed that occurred during the life cycle of the tornado. 

 

Original F scale                                                          Enhanced F scale

Rating   ¼ mi wind speed          3 second gust speed     Rating   3 second gust speed

            mph                              mph                                          mph

0          40-72                           45-78                           0          65-85

1          73-112                         79-117                         1          86-110

2          113-157                       118-161                       2          110-135

3          158-207                       162-209                       3          136-165

4          208-260                       210-261                       4          166-200

5          261-318                       262-317                       5          > 200

The NWS is the only federal agency with authority to provide "official" tornado EF Scale ratings.  The goal is assign an EF Scale category based on the highest wind speed that occurred within the damage path.  First, trained NWS personnel will identify the appropriate damage indicator (DI) from more than one of the 28 used in rating the damage.  The construction or description of a building should match the DI being considered, and the observed damage should match one of the 8 degrees of damage (DOD) used by the scale.  The tornado evaluator will then make a judgment within the range of upper and lower bound wind speeds, as to whether the wind speed to cause the damage is higher or lower than the expected value for the particular DOD.  This is done for several structures not just one, before a final EF rating is determined.     

An EF Scale rating for a tornado involves the following:

  • Conduct an aerial and ground survey over the entire length of the damage path
  • Location and identification of DIs in the damage path
  • Consider the wind speeds of all DIs and assign an EF Scale category for the highest wind speed consistent with wind speeds from the other DIs examined.
  • Record the basis for assigning an EF scale rating to a tornado event
  • Record other pertinent data related to the tornado event.

An EF Scale rating of an individual building involves the following:

  • Refer to the damage indicator (DI) that matches the type or use of the building being evaluated
  • Carefully observe the damage to the building
  • Identify the degree of damage (DOD) statement that best describes the observed damage to the building                                          
  • Determine if there are circumstances that suggest a wind speed to cause the damage to be higher or lower than the expected value for the DOD.  The value should be within the range of the upper and lower bound wind speeds
  • Select the EF-Scale category that contains the wind speed obtained in the previous step within its range
  • Record both the wind speed estimate and the EF-Scale category

On the web, go to: http://www.spc.noaa.gov/efscale for more information.

Information from the National Weather Service.

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