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June 13, 2013 at 12:45 PMComments: 0 Faves: 0

On a Scale of 1 to 5, How Screwed Are We?

By Big Dave from SLN More Blogs by This Author

Exterior walls collapsed? Sounds like an EF-3 to me. Walmart's roof gone? Definitely EF-3.

Everyone is used to hearing the classic Fujita scale indicators when referring to tornados. The mild F-0 tornadoes up through gigantic F-5 monsters have caught our attention, whether imaginary, like in the movie Twister, or real, like the fingers of God twirling their paths through the middle of the United States every summer. Hitting the same old stomping grounds, like Moore, OK, or blazing new trails as far east as the nation's capital, as far north as Michigan and Canada, tornado season brings terror and damage.

With our National Weather Service's radar systems only catching tornadoes just-in-time, and the relatively small number of independent scientists observing the phenomenon, there's no easy way to grab a tornado's wind speeds and call it a day.

The Enhanced Fujita Scale

In the early 2000s, the Enhanced Fujita scale was developed as a more accurate set of wind speeds evaluating the force behind a tornado. Slightly changed from the original Fujita scale, you're more likely now to hear "EF-3" than "F-3", or "EF-5" than "F-5". The principle is still the same, though: The higher the number, the crazier the storm.

With this set of wind speeds, there also exists a set of damage indicators that can be used by forensic meteorologists to approximate the wind speeds of a tornado by assessing damage. The full list can be found here at NOAA.

So how can I tell that walls collapsing in a house mean a tornado may have been an EF-3? Simple. For each of the damage indicators, a list of extents of damage and approximate wind speeds are given. For example, this is an excerpt from the table given for a 1-2 family home:

DOD/DescriptionWind Speeds
1: Threshold of Visible Damage 53-80 mph (65 mph avg)
 2: Loss of Roof Covering Material
63-97 mph (79 mph avg)
 ...
 ...
7: Exterior Walls Collapsed
113-153 mph  (132 mph avg)
... ...

By matching up the damage sustained with a set of wind speeds, we can estimate that the storm reached wind speeds consistent with an EF-2 (110-137 mph) or EF-3 (138-167 mph) tornado. Additional data points from other structures in the damage path can be used to fine-tune the estimate.

Hopefully this is something you'll never have to use. But the next time you want to impress friends with your obscure, trivial knowledge, memorize a few tables from the damage indicators list and let them know how that you can tell a run-of-the-mill EF-2 from a monster EF-5.

Source: http://www.spc.noaa.gov/faq/tornado/ef-scale.html

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