The Enhanced Fujita Scale: A Tornado Rating System

T​he Enhanced Fujita Scale, love it or hate it, is our current system for rating the hundreds of tornadoes that occur each year across the United States. To much chagrin, it rates tornadoes solely based on the damage that the tornadoes cause.

Practically speaking, it is nearly impossible to measure the actual wind speed inside tornadoes, as they can destroy just about any unprotected weather instruments placed in their path. While ingenious storm chasers and engineers have devised vehicles and instrument packs that can withstand a tornado, the logistics and time involved to deploy such devices before every tornado would be inconceivable.

This is why a measurement system for estimating the intensity of tornadoes after their impact was devised, a system known today as the Enhanced Fujita Tornado Intensity Scale.

U​nlike the first glance at the scale above, the ratings of tornadoes are actually based on damage indicators and observations.

The Storm Prediction Center has a brief description of the Enhanced Fujita Scale.

EF0 Typical Observations: Light damage. Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over.

EF1 Typical Observations: Moderate damage. Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors; windows and other glass broken.

EF2 Typical Observations: Considerable damage. Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.

EF3 Typical Observations: Severe damage. Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations blown away some distance.

EF4 Typical Observations: Devastating damage. Well-constructed houses and whole frame houses completely leveled; cars thrown and small missiles generated.

EF5 Typical Observations: Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 109 yards; high-rise buildings have significant structural deformation; incredible phenomena will occur.

D​rawbacks and Caveats of the Enhanced Fujita Scale

B​eing that the EF-scale is entirely a damage-based scale, tornadoes must do damage in order to be rated.

The scale also underrates tornadoes in some cases where structures are weak or not hit. H​omes, businesses and other structures must also be built strong enough to support higher ratings. This has tended to limited the number of strong and violent tornado ratings over the years.

A​nother drawback is that the scale only takes damage into effect when rating tornadoes. This means that despite the widespread measurement of tornadoes by radars, and occasionally by observation stations deployed by the National Weather Service, academia and mesoscale observation networks (mesonets), those measurements are always thrown out when rating a tornado. This most famously occurred in 2013's El Reno tornado, which was officially rated EF3 while radars measured wind speeds over 290 mph.

(​MORE: Could A Radar-Indicated EF5 Tornado Occur In The Future?)

A​ final caveat that has been added adjacent to the scale in recent years is rating a known tornado that does no damage. The scenario here occurs either when a tornado is sighted and documented by storm chasers or the public, but when the area is surveyed, no damage can be found. These tornadoes are now given the rating of EF-Unknown or EFU. Another way tornadoes can be rated EFU is when their tracks are found by drones or even high-resolution satellites after the fact, but no damage surveys were needed. This can happen in cornfields, the high-plains or in marshy areas.

H​istory of the Enhanced Fujita Scale

The original Fujita scale is named after Dr. Ted Fujita, a University of Chicago severe storms research scientist who came up with the scale in 1971. Dr. Fujita's original scale, which ranges from F0 to F5, is based upon the type and severity of damage the tornado produced. At that time, there were very few actual measurements of tornado wind speeds that he could relate to the damage, but he used them -- together with a lot of insight -- to devise approximate wind speed ranges for each damage category.

In subsequent years, structural engineers and meteorologists have examined damage from many of those past tornadoes. They use knowledge of the wind forces needed to damage or destroy various buildings and their components to estimate the resultant wind speeds. The engineers and scientists eventually determined that the original F-Scale wind speeds were too high for categories F3 and higher.

In particular, homes are not built so strongly that 250-300 mph winds are needed to destroy them. In some areas, building codes may only require that roofs withstand 70 mph winds. Once the roof is blown off, walls have less support and begin to fail. In other cases, the house frame may not be properly bolted to the foundation. It then begins to slide off the foundation, perhaps partly lifted by strong upward winds in the tornado. By the early 1990s, even Dr. Fujita recognized these shortcomings and recommended adjusting the assessments.

(​MORE: May Typically Is The Peak Month For Tornadoes)

Guidelines in the original scheme lumped together homes, schools, mobile homes, vehicles, and trees in one short description of damage for each F-Scale category. In the enhanced scheme, detailed descriptions are given for examples of damage to 23 types of buildings and five additional objects like trees, towers, and poles. Wind speed estimates are then provided for each structure and type of damage.

The Enhanced Fujita (EF) Scale was devised by a panel of meteorologists and engineers convened by the Wind Science and Engineering Research Center at Texas Tech University. The Weather Channel's severe weather expert Dr. Greg Forbes was on the team of experts who determined the revised wind speed ranges. Since 2007, the EF Scale has been used to rate tornadoes.