Wx Savvy | Tornadoes

What's The Fujita Scale?

The Fujita scale is a subjective and imperfect (kind of like our weather forecasts) rating system. But it is the best we've got (kind of like our weather forecasts). There is no objectively accurate method for measuring a tornado's wind speed, though Doppler radars can help estimate tornadic winds. But way before Doppler radar someone came up with a pretty good idea-the Fujita scale. That somebody was Ted Fujita, a professor of meteorology at the University of Chicago whose analysis and research advanced our understanding about tornadoes and severe storms by leaps and bounds. (You can read more about Ted Fujita by clicking here)

The Fujita scale estimates a tornado's wind speed based upon the damage the tornado inflicted.

There are some problems with this system. For starters, the people surveying the damage area are making assumptions. No one truly knows what caused a roof to be blown off or walls of a house to fall down, because it could have been shoddy craftsmanship.

Also, the scale is based upon damage, but what if no one sees the tornado and the damage is minimal? For example what if a very strong tornado hits a corn field and dissipates before destroying any structures? That tornado would ultimately be ranked much lower than it's actual intensity. Some tornadoes hit corn fields and are never seen thus never reported. For this reason, it is very likely that many more tornadoes occur each year than those that are actually reported. It's kind of like...if a tree falls in a forest and no one is there to hear it, does it make a sound. Ahhh, once again philosophy and meteorology cross paths. But I digress...

In defense of the Fujita scale, even if we develop technology that can accurately measure a tornado's wind speed, I would still want to know the F-scale ranking. After all, it is the destruction that is truly amazing. You'll see the F-scale below where F0 is the least amount of damage and F5 is the worst realistic type of damage.

The Fujita Scale

F-Scale	Intensity Phrase	Wind Speed	Type of Damage
F0	Gale tornado	40-72 mph	Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards.
F1	Moderate tornado	73-112 mph	The lower limit is the beginning of hurricane wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads; attached garages may be destroyed.
F2	Significant tornado	113-157 mph	Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light object missiles generated.
F3	Severe tornado	158-206 mph	Roof and some walls torn off well constructed houses; trains overturned; most trees in forests uprooted
F4	Devastating tornado	207-260 mph	Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated.
F5	Incredible tornado	261-318 mph	Strong frame houses lifted off foundations and carried considerable distances to disintegrate; automobile sized missiles fly through the air in excess of 100 meters; trees debarked; steel reinforced concrete structures badly damaged.
F6	Inconceivable tornado	319-379 mph	These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F4 and F5 wind that would surround the F6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F6 damage. If this level is ever achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies

(Notice that the size of the tornado has nothing to do with its intensity)

About 2% of tornadoes are F4 or F5. However, those F4's & F5’s result in 70% of tornado related deaths. (1)

The fastest winds associated with a tornado are said to be around 318 mph. This was an actual measurement taken using dual Doppler on wheels during an Oklahoma tornado in 1999.

Comparison of Estimated Top Speeds

Tornado	318 mph
Indy Car	225 mph
Porsche 911 Turbo (manual)	189 mph
Speed of Sound (for dry air at STP)	762 mph