I’ve always wanted a storm shelter since my first fully aware encounter with a tornado. I’ve never lived in a house that contained or allowed one. I have experience building above-ground bunkers that would suffice in a small storm. I don’t want to carve a hole into the house to allow taking shelter without going outside during a storm. Nor do I want to move all the things we would need to be operational after a storm into a non-air-conditioned, high humidity bunker where many of them will be damaged by existing conditions. Of, course, for storms at the upper end of the scale, only an underground shelter is going to be adequate.
Between the relatively high water table and the high incidence of boulders in the area we would have to use, it would be unwise to build an underground shelter. Therefore, we will have to rely on the historical probabilities of such storms happening here in the near future for comfort. That, after all, is one reason we chose this locale.
Considering how we might shelter involves knowing how much damage we would have to plan for. We have a rough understanding of the Enhanced Fujita Scale. But exactly what is involved in assigning each tornado its accurate assessment of strength? Be prepared to know more than you care to know.
The Enhanced Fujita Scale
When the committee met to develop the Enhanced Fujita Scale (see original document) 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:
- Consistent Assessment of Damage
- enhance description of damage with examples and photos
- include not only structures, but also vegetation
- base damage assignment on more than one structure, if available
- develop a PC-based expert system
- develop training materials
- Data Collection
- maintain current tornado database
- surveys should include additional data
- mean and maximum damage path width
- basis for damage assignment
- latitude/longitude of where the path began and ended
- number of hours spent on the damage survey
- names of survey team member(s)
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.
Enhanced F Scale for Tornado Damage
An update to the original F-scale by a team of meteorologists and wind engineers, to be implemented in the U.S. on 1 February 2007.
| FUJITA SCALE | DERIVED EF SCALE | OPERATIONAL EF SCALE |
F Number Fastest 1/4-mile (mph) 3 Second Gust (mph) EF Number 3 Second Gust (mph) EF Number 3 Second Gust (mph)
0 40-72 45-78 0 65-85 0 65-85
1 73-112 79-117 1 86-109 1 86-110
2 113-157 118-161 2 110-137 2 111-135
3 158-207 162-209 3 138-167 3 136-165
4 208-260 210-261 4 168-199 4 166-200
5 261-318 262-317 5 200-234 5 Over 200
*** IMPORTANT NOTE ABOUT ENHANCED F-SCALE WINDS: 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
.
A key point to remember is this: the size of a tornado is not necessarily an indication of its intensity. Large tornadoes can be weak, and small tornadoes can be violent. A good example of a relatively "small" tornado would be the Pampa, Texas tornado of 1995, which can be seen in Tornado Video Classics III. This tornado is pictured on the right side of the slipcase, shown here. Notice the debris in the air. Eyewitnesses to this tornado claim to have seen as many as 6 vehicles in the air at the same time when it passed over a parking lot. Another consideration is the stage in the life cycle of the tornado. A "small" tornado may have been larger, and is at the "shrinking" stage of its life cycle, like the Tracy, Minnesota tornado on one of our posters, and also our logo, seen on the navigation bar to the left if you are using our frames. Large tornadoes can also be strong and small tornadoes can be weak. The Fujita Scale is based on damage, not the appearance of the funnel. Storm spotters, storm chasers and other weather observers often try to estimate the intensity of a tornado when they are in the field, basing their judgement on the rotational speed and amount of debris being generated as well as the width. However, the official estimate is made after the tornado has passed. Personnel from the National Weather Service office that issued the warning survey the site to determine the F-Scale rating. Sometimes they call in experts from out of the area. Aerial surveys are occasionally done after violent tornadoes to determine the exact damage track. Insurance companies may also call in wind engineers to do their own evaluations, but the official rating is set by the NWS. A few of the things they all look for are:
- attachment of the walls and floor to the foundation of the building
- attachment of the roof to the rafters and walls
- whether or not there are steel reinforcing rods in concrete or cinder block walls
- whether there is mortar between the cinder blocks
After the NWS office does the survey, the official rating is recorded, and eventually posted at the SPC site. If it is a killer tornado we also post the rating on our site with the description of the event, and on our "all tornadoes" page. The NWS office may also write up a more extensive report, which may or may not be posted on the web. A good example of such a report would be the one done on the Florida tornadoes of March, 1998.
The Fujita Scale is very subjective, and varies according to how experienced the surveyor is. We have many readers who have tried to do their own "surveys" of tornado damage when storms have occurred in their area. However, the less experienced the surveyor is, the more likely he/she is to be awed by the damage, and the more likely they are to give it a high rating. Brian Smith of the Omaha, Nebraska area NWS office, a former student of Dr. Fujita and an expert frequently brought in to do site surveys, tells of hearing about a tee-shirt with the words "F-3 My Foot" printed on it.
Media hype and inexperience with tornado damage also plays a big part in exaggerated F-Scale claims seen on television or in the paper. A reporter may see a collapsed concrete block home and be very impressed, never noticing that there was no mortar between the blocks. They may be aghast to see a park whose trees have been leveled, but not know that the species had very shallow roots, planted in soil that was soft and soggy from torrential rains, and thus easily toppled. They may see a roof that had been blown a quarter of a mile from its house, and not know that the roof was attached to the house with only a few nails, and when lofted into the air, acted as a "sail." They may see a light post that is bent at a 30 degree angle and think that it must have taken a 600 mph wind to do that, not knowing that a van had been blown into the pole, bending it, then been towed off to help clear the streets. For some of the media, the exaggerations make for a better story than the actual facts. Fortunately, they often make up for this by printing helpful stories about aid available and inspirational human interest stories.
This should provide more information than many people would think necessary. For tech geeks like me, it is only an introduction. Next step for me will be:
A 95 page PDF file explaining the development and makeup of the Enhanced F-scale now is available, both here at SPC and from the Texas Tech server
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