May 1, 2006 The Fujita Scale rates tornadoes based on the damage inflicted upon buildings, so accurate rating requires knowing how resistant buildings were in the first place. Starting February 2007, the National Weather Service will use a new rating system, called the Enhanced Fujita Scale. The EF Scale will draw on more damage indicators, to make ratings more consistent nationwide and avoid overestimating wind speeds.
NORMAN, Okla.--Until now, the damage of an F5 tornado in Oklahoma was rated the same as if it struck Ohio. But now the National Weather Service is updating a standard tornado scale to reflect consistency in reporting storms.
On May 3, 1999, Mark Hay was watching a tornado at his office. As the twister tore through, Hay and eight of his co-workers ran for cover in a small closet of their dentist office.
"Figured this was gonna be the last day on earth for me," he says.
The storm Hay and his colleagues witnessed was an F5 on the Fujita scale, the scale used to measure tornadoes. All tornados that occur in the United States are given a number from 0 to 5 depending on the damage they do.
The old Fujita scale used to be based on what a tornado did to a well-built house, but building materials vary in different parts of the country. Now the National Weather Service is introducing an enhanced version of that scale. Meteorologists and wind engineers from all over the country came up with the new scale that rates tornado damage and estimates wind speed.
Joe Schaefer, director of the National Oceanic and Atmospheric Administration's Storm Prediction Center in Norman, Okla., says, "We have a slate of 28 different things we can look at the damage in ... It varies from where you are."
The Enhanced Fujita (EF) tornado damage scale takes into account things like damage to a sided house compared to a brick house or to a three-story building vs. a shopping mall. With the new scale, any wind speed over 200 mph is considered an F5 tornado whereas with the original scale, an F5 tornado had estimated wind speeds between 261 mph and 318 mph.
Schaefer says the EF will make reporting of tornadoes more consistent. And because wind speeds for each F level are lower, home builders can increase safety factors, making it safer for all of us. He hopes builders will see that and start building houses to meet the standards, giving more people like Hay a chance to survive a devastating tornado.
Meteorologists expect the EF scale to be fully implemented by February 2007.
BACKGROUND: NOAA's National Weather Service will implement an Enhanced Fujita Scale (EF) to rate the severity of tornadoes. It will replace the original Fujita scale, which was originally developed for wooden structures. The enhanced scale will still run from zero to five. However, ranges in wind speed will be more accurate with the improved rating scale, and it will also take into account the type of structure. For instance, winds moving at 100 MPH would do different damage to a house than it would to a concrete building or a strip mall.
THE FUJITA SCALE: The F Scale was developed in 1971 by Theodore Fujita to rate tornadoes and estimate their wind speed based on the damage they cause. But the original scale's limitations may have led to inconsistent ratings, including possible over-estimates of wind speeds. The new EF scale incorporates more damage indicators and degrees of damage to provide a more detailed analysis and better correlation between damage and wind speed.
ABOUT TORNADOES: A tornado begins with a thunderstorm cloud, which can build up a lot of energy. If this energy creates a particularly strong updraft of air, it will form a vortex, much like how a whirlpool forms in a bathtub that is draining. The air is pulled toward the center in a spiral, forming a tornado under the thundercloud. Wind speeds can reach 200 to 300 MPH, and if the dangling vortex touches ground, the combination of the whirling wind's speed, the updraft, and pressure differences can cause severe damage. The path of a tornado is determined by the path of the parent thundercloud, but it will often appear to hop (called a "jumper"). This occurs when the vortex is disturbed, causing it to collapse momentarily and reform.
The American Meteorological Society contributed to the information contained in the TV portion of this report.