The EF Scale Explained
I have gotten many questions about this scale, mainly like. When did we go from the F scale to the new EF scale? With the recent tornado outbreaks I thought it was a good time to revisit the scale and what it actually means. Even more importunely to remember that tornadoes don’t get a rating until after the damage has been surveyed. So when I post that a confirmed EF-1 tornado touched down somewhere for example. That means the damage survey has been completed and we know have a damage rating.
History
The Fujita scale for tornadoes was developed by Dr. Ted Fujita from the University of Chicago in 1971.
He is referred to as "Mr. Tornado” because of his long held research into tornadoes and severe weather. He was one of the first to discover what a microburst was when he blamed it for the crash of Eastern Airlines Flight 66 at New York’s Kennedy Airport. These microbursts, which at the time were a controversial topic, are now widely accepted. In fact his microburst research and theories have likely saved millions worldwide since every airport, pilot and aviation expert now knows what and where they form to make flying safer.
His most famous research though was an exhausting survey of the entire April 3-4 1974 Super Outbreak. He had help from some some young graduated student. Who we all know now as Dr. Greg Forbes of The Weather Channel who helped in the survey. Most of what we know and used from his research in 1974 was used to survey all tornadoes across the United States using his Fujita Scale. Which was used from 1971-2007. It was originally a scale that ran from F0-F12 but was later refined to a 6 point scale seen below.
The Change to the Enhanced Fujita Scale February 2007
Due to many problems that arose with the use of the original Fujita scale a new scale was needed. The main problems were that we needed a better estimation of real life wind speeds based on damage to certain structures. For instance damage to a weak building or mobile home is not the same as a brick well built home being leveled. We also needed consistency with rating tornadoes. Too often what constituted a F-3 tornado in one part of the country ended up being a F-5 in other parts. We needed a scale that meant a EF-5 in Maine was the same as a EF-5 in Oklahoma.
So with the help of not only meteorologists but structural engineers the new scale was developed. This scale used more of the real world nature of construction in the U.S. and also what we know about wind speeds in tornadoes. It was meteorology meets engineering. The results were the Enhanced Fujita Scale or EF-Scale.
Here’s a comparison of the old and new scales.
More importantly was the revised way damage surveys are conducted using these damage indicators. This is for consistency across the country when rating tornado damage.
From SPC: The Enhanced F-scale still is a set of wind estimates (not measurements) based on damage. Its uses three-second gusts estimated at the point of damage based on a judgment of 8 levels of damage to the 28 indicators listed below. These estimates vary with height and exposure. Important: The 3 second gust is not the same wind as in standard surface observations. Standard measurements are taken by weather stations in open exposures, using a directly measured, "one minute mile" speed.
Enhanced F Scale Damage Indicators
| NUMBER (Details Linked) | 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 |
Let’s take the earlier 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 8, 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.
