(WHTM) -- With a tornado being confirmed in York County on Monday, June 12 and it being an EF0, some may wonder where the rating comes from and what the ratings mean.
According to the National Weather Service, tornadoes are rated by the Enhanced Fujita System. This is a revised version of the Fujita Scale, which was developed back in 1971 by Dr. Ted Fujita of the University of Chicago.
The first scale was in place from the 1970s until Feb. 1, 2007. This was when the Enhanced Fujita Scale was developed. The National Weather Service says that it was created to better reflect how tornado damage is examined, and so the wind speeds align with the damage closer than the original Fujita Scale did.
Below is The Enhanced Fujita Scale, which is what the National Weather Services uses throughout the state and the country:
| EF SCALE | ||
| EF Rating | 3 Second Gust (mph) | Description |
|---|---|---|
| EF0 | 65-85 | Weak |
| EF1 | 86-110 | Moderate |
| EF2 | 111-135 | Intense |
| EF3 | 136-165 | Severe |
| EF4 | 166-200 | Devastating |
| EF5 | Over 200 | Catastrophic |
The National Weather Service states that the scale is only a set of wind estimates and not measurements that are based on damage.
Damage Indicators
The National Weather Service is the only federal agency to provide official tornado ratings. A tornado is given a rating based on the highest windspeed that occurred within the damaged path. There is a list of 28 indicators that surveyors use to measure the strength of a tornado, which can be seen below:
| NUMBER | DAMAGE INDICATOR | ABBREVIATION |
| 1 | Small barns, farm outbuildings | SBO |
| 2 | One- or two-family residences | FR12 |
| 3 | Single-wide mobile home (MHSW) | MHSW |
| 4 | Double-wide mobile home | MHDW |
| 5 | Apt, condo, townhouse (3 stories or less) | ACT |
| 6 | Motel | M |
| 7 | Masonry apt. or motel | MAM |
| 8 | Small retail bldg. (fast food) | SRB |
| 9 | Small professional (doctor office, branch bank) | SPB |
| 10 | Strip mall | SM |
| 11 | Large shopping mall | LSM |
| 12 | Large, isolated ("big box") retail bldg. | LIRB |
| 13 | Automobile showroom | ASR |
| 14 | Automotive service building | ASB |
| 15 | School - 1-story elementary (interior or exterior halls) | ES |
| 16 | School - jr. or sr. high school | JHSH |
| 17 | Low-rise (1-4 story) bldg. | LRB |
| 18 | Mid-rise (5-20 story) bldg. | MRB |
| 19 | High-rise (over 20 stories) | HRB |
| 20 | Institutional bldg. (hospital, govt. or university) | IB |
| 21 | Metal building system | MBS |
| 22 | Service station canopy | SSC |
| 23 | Warehouse (tilt-up walls or heavy timber) | WHB |
| 24 | Transmission line tower | TLT |
| 25 | Free-standing tower | FST |
| 26 | Free standing pole (light, flag, luminary) | FSP |
| 27 | Tree - hardwood | TH |
| 28 | Tree - softwood | TS |
The NWS says that surveyors will find damage and assign one of the above damage indicators to it. The construction of the structural damage should match the damage indicator it was given, and the observed damage should match one of the degrees of damage used by the scale. Then, the tornado evaluator will make a judgment within the range of upper and lower wind speeds
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 degree of damage.
This is done for several structures before a final EF rating is determined.
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