How Are Tornadoes Rated?
Given that tornado season appears to be in full swing now (a recent tornado killed 11 in Mississippi), I thought it would be a good time to describe how tornadoes are rated. From time to time you might hear the weather reporter talk about an “F0″ or “F3″ tornado, and you might not know what they mean. I’m hear to set you straight!
Tornadoes are measured using the Fujita Scale, which was developed by Ted Fujita of the University of Chicago back in 1971. The scale Fujita developed is used to determine the intensity of a tornado after it has passed through an area and done its damage on human-based structures and vegetation. After a tornado has gone through an area, meteorologists and engineers will do a ground and/or aerial damage survey. If possible, they will also take into account things like eyewitness accounts, ground-swirl patterns, radar tracking and media reports and imagery. After everything has been considered, a rating of F0 to F5 is assigned to the tornado. Here is the criteria that they use to determine the rating:
|
Scale |
Wind Estimate (MPH) | Typical Damage |
|
F0 |
<73 |
Light damage. Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged. |
|
F1 |
73-112 |
Moderate damage. Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off roads. |
|
F2 |
113-157 |
Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground. |
|
F3 |
158-206 |
Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown. |
|
F4 |
207-260 |
Devastating damage. Well-constructed houses leveled; structures with weak foundations blown away some distance; cars thrown and large missiles generated. |
|
F5 |
261-318 |
Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 meters (109 yds); trees debarked; incredible phenomena will occur. |
Keep in mind that the rating cannot be applied until after the tornado has gone through and done its damage…the rating depends entirely on a guess of wind speed, and a measure of the damage done to structures and vegetation.
All of the world uses the Fujita Scale except the United States, which moved to the Enhanced Fujita Scale in 2007. It was revised to reflect better examinations of tornado damage surveys, so as to align wind speeds more closely with associated storm damage. Better standardizing and elucidating what was previously subjective and ambiguous, it also adds more types of structures, vegetation, expands degrees of damage, and better accounts for variables such as differences in construction quality.
The Fujita/Enhanced Fujita Scales are good ways to gauge just how bad a tornado was…not that you have to tell the people who were in them!