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

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 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.