New scale to measure tornadoes

November 28, 2007 9:03:52 AM PST
Who can forget the early morning hours of November 28, 1988 when a powerful F4 tornado packing winds of more than 200-mph ripped through central North Carolina?

The twister remained on the ground for more than 80 miles carving a path of destruction from Raleigh to Roanoke Rapids.

In the end, four people were killed, nearly 1000 were left homeless and more than 70 businesses were destroyed.

North Carolina may not be Tornado Alley, but we do average at least a dozen tornadoes a year.

Similar to the Saffir Simpson Scale for hurricanes, tornadoes are measured on the Fujita Scale.

The Fujita Scale rates a storm's intensity from F-0 up to a catastrophic F-5 like the 1999 tornado that high Oklahoma City with wind speeds in excess of 260 mph.

Since the Fujita Scale - - or F-Scale - - was developed in 1971, scientists and engineers have been studying the effects of tornadoes and their powerful winds on different types of structures.

Different types of construction, even different types of trees will react differently when confronted with the exact same wind speed.

That's essentially what the new enhanced Fujita Scale is all about... it includes more variables to determine the strength of the storm. We'll still see values F-1 to F-5 but a lot more will be considered before assigning that F-value.

You'll notice few major changes between the original and new enhanced Fujita Scale, what's most different is the way in which meteorologist will arrive at the F-value.

Jeff Orrock with the National Weather Service has assessed numerous tornado damage sites during his career. He explains the new F-Scale, also known as the E-F Scale. "The intent of the new E-F Scale is to really try and take an engineering look at what's been taken apart...the schools, the houses...how were they constructed, how where maybe some construction things not up to code to go in there and try to assess that."

While the new F-Scale won't result in old storms being re-categorized, it will make it difficult to compare storms of the past with storms of the future. "Another thing the E-F scale is doing, we still have the F1 through F5 categories but the wind speeds associated with these categories is also changing. What that's telling us is that it actually takes less wind than we thought to do severe structural damage."

Orrock plans to use the new scale this year when surveying storm damage even though the National Weather Service won't start officially using the Enhanced Fujita Scale until next year.

Here's some advice on what you should do if a tornado warning is issued for your area.

Go to the basement or to an interior room on the lowest floor, such as a closet or bathroom.

Wrap yourself in overcoats or blankets to protect yourself from flying debris.

Leave cars and mobile homes immediately and go to a substantial structure or designated tornado shelter.

Most deaths occur in cars and mobile homes.

Tornado Watch

This is issued by the National Weather Service when conditions are favorable for the development of tornadoes in and close to the watch area. Their size can vary depending on the weather situation. They are usually issued for a duration of 4 to 8 hours. They normally are issued well in advance of the actual occurrence of severe weather. During the watch, people should review tornado safety rules and be prepared to move a place of safety if threatening weather approaches.

Tornado Warning

This is issued when a tornado is indicated by the WSR-88D radar or sighted by spotters; therefore, people in the affected area should seek safe shelter immediately. They can be issued without a Tornado Watch being already in effect. They are usually issued for a duration of around 30 minutes.


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