Since tornadoes can destroy any unprotected weather instrument that comes in its path, measuring the actual wind speed inside them is almost impossible. Although engineers and storm chasers have developed instruments capable of withstanding a tornado, the time and logistics involved to deploy such devices before every tornado season are inconceivable.
Due to this, a measurement system, the Enhanced Fujita (EF) Tornado Intensity Scale, was devised to estimate the intensity of a tornado after its impact. The original scale was named after University of Chicago severe storms research scientist Dr. Ted Fujita, who created the scale in 1971. The EF Scale uses revised wind ranges. Here’s a brief description by the Storm Prediction Center about this tornado severity scale:
EF Scale estimated wind speeds: 65 to 85 mph
Typical observations: Light damage—slight damage to siding or gutters, some roof surfaces start to peel, branches break off trees, and shallow-rooted trees are pushed over.
EF Scale estimated wind speeds: 86 to 110 mph
Typical observations: Moderate damage—roof surfaces become severely stripped, mobile homes are badly damaged or overturned, windows and other glass breaks, and exterior doors may come off the hinges.
EF Scale estimated wind speeds: 111 to 135 mph
Typical observations: Significant damage—foundations of frame homes will shift, large trees get uprooted or snapped, roofs of well-constructed houses get torn off, mobile homes are completely destroyed, and cars are lifted off the ground.
EF Scale estimated wind speeds: 136 to 165 mph
Typical observations: Severe damage—large buildings such as shopping malls get severely damaged, heavy cars are lifted off the ground and thrown, entire stories of well-constructed houses get destroyed, trains get overturned, and structures with weak foundations get blown away some distance.
EF Scale estimated wind speeds: 166 to 200 mph
Typical observations: Devastating damage—small missiles are generated, cars get thrown, and whole frame houses and well-constructed houses are leveled.
EF Scale estimated wind speeds: Over 200 mph
Typical observations: Incredible damage—foundations of strong-frame houses are leveled off and swept away, significant structural deformation of high-rise buildings occur, and automobile-sized missiles fly through the air for more than 100m (109 yds).
The EF Scale has been used to rate tornadoes since 2007.