How are tornado ratings determined? - WDAM-TV 7-News, Weather, Sports-Hattiesburg, MS

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How are tornado ratings determined?

WDAM First Alert Weather Team WDAM First Alert Weather Team
PINE BELT (WDAM) -

The recent tornadoes across the Pine Belt may have you wondering, how do the National Weather Service know the exact strength of a tornado?

Tornadoes are rated on the Enhanced Fujita scale. This scale is based on what damage has occurred to structures and objects after the tornado has passed. When the NWS conducts a survey, they are looking at how different objects have been damaged, such as trees, building, & cars.  They also take into consideration how old the structure is and how well built it was.  In doing this, they can calculate a wind speed of what it takes to do that type of damage and that is then made into an EF rating!

Then, it is determined if the damage is in a path that displays a pattern that points towards one another, if that is found then it is tornadic. If the damage fans out and away, then it most likely a microburst or straight-line wind damage.

Here are a few examples on what is looked at when determining tornado damage.

Trees:

It doesn’t take much to damage a tree but, we know at what speed it takes to cause specific damage. Debarking a tree generally takes between 123-167mph (EF2-EF3) to occur for hardwoods. While a Pine tree snap in half with 88-113mph (EF1-EF2) winds.

Click for the full graph (Hardwood).

Click for the full graph (Softwood).

Homes:

Houses are a little more complicated since they are built with different materials and can become weaker with age. It only takes winds of 63-97mph (EF0-EF1) to cause minor roof damage, where as a house can be shifted off its foundation with winds between 103-141mph (EF1-EF3). Winds of 165-200mph (EF3-EF5) can wipe a home’s foundation slab clean!

Click for the full graph.

Mobile Homes:

Mobile Homes are the worse place to be in during severe weather due to their weak construction and ability to be tossed or rolled, even when anchored correctly. The roof can be damaged with winds between 61-92mph. The unit can be rolled with winds between 96-128mph (EF1-EF2). Complete destruction happens with winds of only 110-148mph (EF1-EF3)

Click for the full graph.

In doing this, they can calculate a wind speed of what it takes to do that type of damage and that is then made into an EF rating.

Copyright WDAM 2016. All rights reserved. 

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