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.