Tornados

Dr. T. Theodore Fujita first introduced The Fujita Scale in the SMRP Research Paper, Number 91, published in February 1971 and titled, "Proposed Characterization of Tornadoes and Hurricanes by Area and Intensity". Fujita revealed in the abstract his dreams and intentions of the F-Scale. He wanted something that categorized each tornado by intensity and area. The scale was divided into six categories:

Dr. Fujita's goals in his research in developing the F-Scale were

     * categorize each tornado by its intensity and its area
     * estimate a wind speed associated with the damage caused by the tornado

Dr. Fujita and his staff showed the value of the scale's application by surveying every tornado from the Super Outbreak of April 3-4, 1974.  The F-Scale then became the mainstay to define every tornado that has occurred in the United States. The F-Scale also became the heart of the tornado database that contains a record of every tornado in the United States since 1950.

The United States today averages 1200 tornadoes a year. The number of  tornadoes increased dramatically in the 1990s as the modernized  National Weather Service installed the Doppler Radar network. The  National Weather Service modernization also began the Warning  Coordination Meteorologist program increasing partnerships with media  and Emergency Management across the United States. This program also  initiated the training of storm spotters across the County Warning  Area of each Weather Forecast Office. With more people trained to  relay information on storm activity to the Weather Forecast Office and  improved communication and digital technology, more tornadoes could be reported.

While the Super Outbreak of tornadoes was the spring board for the F- Scale, it was the Jarrell, TX tornado of May 27, 1997 and the Oklahoma City/Moore, OK tornado of May 3, 1999 that brought to the forefront the problem that maybe the wind estimates were too high in the F- Scale. Meteorologists, Emergency Managers and Engineers convened on Moore, OK to study the weaknesses in the structures destroyed by the tornado of May 3, 1999. The findings can be found in the document FEMA 342, Building Assessment Report, Midwest Tornadoes of May 3, 1999, Observations, Recommendations and Technical Guidance. Engineers claim that many homes are rated to withstand winds to 100 mph. Therefore, the question was raised that if a tornado has over 200 mph winds, how can the structure reveal this estimate when much of it is gone?
 

The Fujita Scale

The Fujita Scale is a well known scale that uses damage caused by a tornado and relates the damage to the fastest 1/4-mile wind at the 
height of a damaged structure. Fujita's scale was designed to connect smoothly the Beaufort Scale (B) with the speed of sound atmospheric scale, or Mach speed (M). Fujita explains explicitly that "F-scale winds are estimated from structural and/or tree damage, the estimated wind speed applies to the height of the apparent damage above the ground."

This graphic shows the relationship between the three scales.

Over the years, the F-Scale has revealed the following weaknesses:

For example, if a tornado knocks down the walls of an area of homes. If it is determined that the walls collapsed, then the damage assigned is F3. If it is a brick home, then that lowers the damage to F2. Then, according to the table since it was a brick structure, then you +1 making the rating F3.

The Enhanced Fujita Scale

When the committee met to develop the Enhanced Fujita Scale one point was made very clear: it must continue to  support and maintain the original tornado database.; In other word, there must be some conformity to that of the F-Scale that is listed in the database. Other ideas were agreed to including:

When using the EF-Scale to determine the tornado's EF-rating, begin with the 28 Damage Indicators. Each one of these indicators have a  description of the typical construction for that category of indicator. Then, the next step is to find the Degree of Damage (DOD).

Each DOD in each category is given and expected estimate of wind speed, a lower bound of wind speed and an upper bound of wind speed.

Let's take this example, a tornado moves through a neighborhood  and walls are knocked down of an area of homes. Here the Damage indicator would be #2, One or Two Family Residences (FR12). The typical construction for this fits being a brick veneer siding home.

The DOD would be a 9, most walls collapsed in bottom floor. Thus, the estimated winds would be 127 - 178 mph with the expected wind speed of 152 mph. Now, taking this number to the EF-Scale, the damage would be rated EF-3 with winds between 136 - 165 mph.

Source: http://www.spc.noaa.gov/efscale/