Ask Ellen: How are EF ratings for tornadoes determined?
PORTAGE, Mich. (WOOD) — The tornado that touched down in the Portage area on May 7 has been rated as a strong EF2. Meteorologists at the National Weather Service Grand Rapids reached the rating after conducting a survey the day following the tornado.
So how are tornado ratings decided? The answer is in the damage. Meteorologists measuring and assessing the damage path look for key damage indicators.
NWS confirms 4 tornadoes in Southwest Michigan Tuesday
Each item on the “Enhanced Fujita Scale Damage Indicator” list has been studied rigorously in laboratories to see how they behave under different intensities of wind. This research is then used by survey crews to be sure they are assigning the correct wind speed to the damage they are seeing.
Buildings destroyed, trees down after tornadoes in SW MI
Below is an example. If a tornado were to hit a small barn, a surveyor could first confirm the “small barn” matches the typical construction listed. Then, the surveyor could match the damage with a Degree of Damage, or DOD, number. This number can be used to find the correct wind speed on the corresponding graph.
Surveyors can use this approach to account for poorly made construction, too. This ensures a tornado isn’t labeled to have been stronger than it was due to shoddy workmanship of the buildings hit.
Inside woodtv.com: Southwest Michigan tornadoes
How is the Enhanced Fujita scale different from the “F” scale?
Radar data can be helpful when looking at a tornado, but it is not what official ratings are based on. Radar can measure wind speed inside a storm or tornado, but the only way to determine if wind of a certain speed hit the ground is by looking at the damage done.
Damage assessments are most accurate when a twister churns through a variety of materials like trees, cars, and houses. Some tornadoes that touch down in fields are never rated, because the damaged items left behind, like a soybean field, can’t fill in enough details.
The tornado that ripped through the south side of Portage, Michigan damaged 60 buildings. The track of the Portage tornado allowed meteorologists with the National Weather Service a clear view of the winds that likely hit the surface. A look at the chart above shows a clear linkage between the damage reported in Portage and the EF2 rating that was assigned.
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