When a tornado hits, its wind speed is often its signature. In Alabama, for example, the 170-mph winds were highlighted as the main focus of the devastating EF4 tornado that killed 23 and caused widespread devastation earlier this month.
And while a tornado's extraordinary wind speed is undoubtedly the overwhelming cause of damage, it's not the only factor responsible for the devastation.
The rapidly fluctuating wind speeds and decrease in atmospheric pressure inside a tornado create strong pressure differences over short distances. The intense pressure decline and fluctuations, therefore, exacerbate the effects of the rapid changes in wind speed, putting more pressure on buildings.
“The impact of a 170-mph wind from a tornado may be more severe than the 170-mph wind of a hurricane because the pressure drop is so dramatic and the pressure inside a tornado can be so low,” said AccuWeather Founder and CEO Dr. Joel N. Myers. "Variations of barometric pressure over such a small distance -- the pressure gradient, as meteorologists refer to it -- is more dramatic in a tornado than in any other phenomenon on Earth.
"Some of the lowest barometric pressure readings on Earth have been measured inside of a tornado -- even lower than is found in a hurricane," Myers said. "And it is more concentrated than a hurricane because a tornado funnel and system is often a mile or less wide, whereas a hurricane can be spread out over many miles."
Retired meteorologist Jim Lushine told the South Florida Sun Sentinel in 2013, “Even though tornadoes and hurricanes might have the same maximum wind speeds, their damage to structures can be considerably different."
The Enhanced Fujita scale for tornado damage that measures wind speed is a set of wind estimates -- not measurements -- based on damage, according to the National Oceanic and Atmospheric Administration (NOAA). It uses three-second gusts estimated at the point of damage based on a judgment of eight levels of damage to 28 indicators. Those estimates vary with height and exposure.
Wind speed, of course, plays the overarching role in the damage created by a tornado; flying debris that can include a car or even a 2,000-pound van has an impact, too. “You have to go through the external vortex and the maximum capacity winds by the time you get to the internal vortex,” Tim Marshall, principal engineer and director of research for Haag Engineering and a tornado expert told AccuWeather. “And houses don’t look good by that time.”
But the pressure gradient and rapid wind fluctuations within a tornado are also a factor. While it is difficult to measure air pressure in tornadoes since most weather instruments can’t survive, according to NOAA's Storm Prediction Center, armored probes have successfully been placed in a few tornadoes.
In June 2003, famed storm chaser the late Tim Samaras recorded a pressure fall of 2.95 inches. This reading showed a drop from 29.95 to 27.00 inches of barometric pressure -- a pressure comparable to what would be found in a Category 4 hurricane. The key difference is that variation from outside the storm of about 30 inches to 27 is concentrated over a much smaller distance in a tornado than most hurricanes and is therefore more destructive. Samaras was killed in 2013 while chasing a twister in Oklahoma.
The current record pressure decrease in a tornado was 5.72 inches, which was recorded in April 2007 by a private storm-chaser. That drop brought the pressure from 30.00 inches to 24.28 inches. "That is lower than any other pressure readings corrected to sea level that has ever been observed on the face of the Earth," Myers said. "The lowest that has ever been observed in a hurricane or typhoon was about 26 inches.
"When a structure is buffeted by a 170-mph wind from a hurricane, it's often over a period of time, certainly a good fraction of an hour and maybe hours long," Myers said. "The duration of that kind of wind speed in a tornado is literally seconds, showing again the possible importance of the extremely low pressure."
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