Measuring the Strength of a Tornado

The strength or force of a tornado is measured on the Fujita-Pearson Tornado Scale. 

According to the National Weather Service, F6 is the highest category of tornado, with winds capable of 261-318 mph. Homes may be lifted from their foundations and carried considerable distances in this type of tornado. Six of the top ten “killer tornadoes” in the United States were rated F5. 

Oklahoma has had five F5 tornadoes since 1950, the year the National Weather Service started rating them. While an F5 tornado is ominous and generally causes the most severe damage, the other lesser classified tornadoes should not be dealt with lightly. An F4 is completely capable of flattening cars and hurling cattle and an F1 can overturn or push a mobile home off its foundation. 
  

The Fujita – Pearson Scale 
  

F Scale Number
 Intensity Phrase
Wind Speed 
Type of Damage
F0 
Gale tornado
40-72 mph
Minor damage  to chimneys, trees and or tree branches, and street signs
F1
Moderate tornado
73-112 mph
This is the beginning of hurricane wind speed:
 loss of roofs, mobile homes dislodged from secure bases,  vehicles moved from roadways, attached garages may be destroyed
F2 
Significant tornado
113-157 mph
Considerable property damage: roofs torn off frame houses, mobile homes demolished, mature trees snapped or uprooted
F3
Severe tornado
158-206 mph 
Roof and  walls torn off well constructed houses, large non-permanent structures overturned, most trees uprooted
F4 
Devastating tornado 
207-260 mph
Well-constructed houses leveled, vehicles of all sizes air lifted, and large objects become airborne with destructive force
F5 
Incredible tornado
261-318 mph 
Strong frame houses lifted off  foundations and carried considerable distances, then dropped, automobile sized objects fly through  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. 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

The Fujita-Pearson Scale, designed by Professor Fujita and Allen Pearson, Director of the National Severe Storm Forecast Center in 1971, is used to rate the intensity of a tornado. It is measured in both path length and width. 

The important thing to remember is that the size of a tornado is not necessarily a good indication of its intensity. A small tornado can be extremely violent, while a large tornado may be quite weak. 

Identified by the Fujita-Pearson Scale, a tornado must cause actual structural damage to receive an F rating. As a consequence, a devastatingly violent tornado may rate only as a F1 if it touches down where there are no structures present.   

The National Oceanic & Atmospheric Adminstration divides tornadoes into different categories based on the wind speed, time they are on the ground and whether there is human life lost. 
  

The National Oceanic & Atmospheric Administration divides tornadoes into these categories 
  

 
Weak
Strong
Violent
Of all tornadoes 
69%
29%
2%
Tornado deaths 
5%
25%
70%
Time
1-10+ mins.
20+ minutes
may exceed 1 hour
Wind speeds
110 mph
110-205 mph
greater than 205 mph