How in the World Do Scientists Measure a Tornado's Power?
How in the world do scientists measure a tornado's power?
Tornadoes come in many shapes and sizes - from skinny, needle-shaped tornadoes to large, mile-wide, wedge-shaped twisters. Many people assume that the largest tornadoes are the strongest, but that's not always the case. When ranking the strength of a twister, meteorologists evaluate wind speed alone. But determining the wind speed of a churning tornado isn't easy.
In a perfect world, meteorologists could place wind measurement devices in every tornado's path. But that would be dangerous, expensive and, quite frankly, impossible, because tornadoes rarely follow easily accessible roads and highways.
Instead, scientists rely on another indicator to rate a tornado's strength: damage.
Dr. Ted Fujita introduced the Fujita scale in 1971 to rate a tornado's wind speed, based on varying levels of damage it causes.
In 2007, noticing a few flaws in the original methods, engineers and meteorologists developed the Enhanced Fujita scale to replace the prototype Fujita scale. The new scale focuses on more accurate wind assessments based on the construction materials used to build the structures that were damaged. For example, the destruction of a shopping mall requires a higher wind speed than the destruction of a manufactured home.
The EF scale uses 28 damage indicators, including structures and vegetation, with varying levels of degree-of-damage to determine wind speed. Once a tornado has passed, survey teams from the local National Weather Service office visit sites along its path to evaluate any destruction and match it to the EF scale using a software program.
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Once the survey is complete, the National Weather Service generates a report that highlights maximum wind speed based on damage indicators. Then they give it a final rating. Reports also include information on path length and width to give an overall evaluation of the tornado, but these values do not influence the EF scale's rating.
The EF scale has six rankings - EF0 to EF5 - with EF5 being the most powerful.
EF0 and EF1 tornadoes are classified as "weak." EF2 and EF3 tornadoes are classified as "strong," and EF4 and EF5 tornadoes are classified as "violent." Roughly 80 percent of tornadoes are rated as EF0 or EF1. Less than 1 percent are classified as EF4 or EF5.
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The EF scale has one big flaw: The tornado must hit structures in order to get an accurate rating. In most cases in the U.S., tornadoes occur over open fields in the Great Plains. It's very possible that many tornadoes are not rated as highly as their true intensity would justify because they do not hit significant structures.
A few tornadoes have made direct hits on wind measuring devices. In these cases, the wind readings have been used instead of damage indicators alone. On May 19, 2012, for example, a tornado that ripped through a wind farm near Spivey, Kansas, was originally rated an EF1 due to damage to a wind turbine. But a wind measurement device at the top of the turbine recorded a 166 mph wind gust; the tornado was upgraded to an EF3.
The Enhanced Fujita scale isn't perfect, but for now it's the best system we have.
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About the Author
Renny VandewegeFeatured Contributor Mississippi
Meteorologist Renny Vandewege has worked as a television meteorologist at WTOK in Meridian, MS, and KCTV in Kansas City, MO. He's an instructor of broadcast meteorology at Mississippi State University. He covered Hurricane Katrina and Hurricane Rita on television, and as a storm chaser, he has witnessed more than 40 tornadoes.