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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.

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 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 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: Top 10 Worst Tornadoes | 10 Worst Tornado Outbreaks)

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.

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

EF-0

Original Fujita Scale estimated wind speeds: less than 73 mph.

Enhanced Fujita Scale estimated wind speeds: 65 to 85 mph.

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

EF-1

Original Fujita Scale estimated wind speeds: 73 to 112 mph.

Enhanced Fujita Scale estimated wind speeds: 86 to 110 mph.

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

EF-2

Original Fujita Scale estimated wind speeds: 113 to 157 mph.

Enhanced Fujita Scale estimated wind speeds: 111 to 135 mph.

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.

EF-3

Original Fujita Scale estimated wind speeds: 158 to 206 mph.

Enhanced Fujita Scale estimated wind speeds: 136 to 165 mph.

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.

EF-4

Original Fujita Scale estimated wind speeds: 207 to 260 mph.

Enhanced Fujita Scale estimated wind speeds: 166 to 200 mph.

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

EF-5

Original Fujita Scale estimated wind speeds: 261 to 318 mph.

Enhanced Fujita Scale estimated wind speeds: Over 200 mph.

Typical Observations: Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 m (109 yd); high-rise buildings have significant structural deformation; incredible phenomena will occur.

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