On June 4, the National Weather Service said the massive tornado that walloped El Reno, Oklahoma last month was the widest twister ever recorded in the U.S. This may or may not be true (more on that in a minute), but it raises an interesting question: just how big, and how nasty, can a tornado get?
"We don’t really know," tornado researcher Joshua Wurman of the Center for Severe Weather Research told DVICE. "There are certainly extreme limits; it’s unlikely you would get wind speeds greater than the speed of sound, for instance. Whether they can even get close to that, probably not. Nature is always trying to get rid of strong, anomalous things. If there is an extremely strong wind, there are extremely string dissipative forces seeking to get rid of that anomaly."
By mapping damage on either side of the storm’s path, weather spotters determined the El Reno twister spanned 2.6 miles across as it raged over 16.2 miles of ground west of Oklahoma City May 31. With surface winds greater than 200 miles per hour, it was hardly the most powerful tornado ever recorded, however. The record-holder, on May 3, 1999, reached wind speeds of 301 mph. That storm, incidentally, hit Moore, Okla., recently devastated in a powerful tornado a week before the El Reno storm. It’s possible that about 300 mph is just about as powerful as tornadoes get, Wurman said. The 1999 storm caused $1 billion in damage.
"Nobody has ever seen greater than 300 mph, but we've seen 250 mph a lot," he said.
Wurman and his colleagues at the Center for Severe Weather Research travel tornado-prone areas in spring, carting mobile radar systems called Doppler On Wheels. Since 1995, they have studied some 170 tornadoes and thousands of smaller vortices inside supercell thunderstorms, according to a paper in press with the American Meteorological Society journal Weather and Forecasting.
How Tornadoes Work
Tornadoes are defined as violently rotating columns of air dangling from thunderheads and usually visible as funnels, with varying lengths and widths. For a vortex to be classified as a tornado, it must be in simultaneous contact with both the ground and the cloud it stems from, according to NOAA’s Storm Prediction Center.
The U.S. is home to the vast majority of Earth’s tornadoes because of our geography. Put simply, tornadoes are born in the powerful thunderstorms that form when masses of cold, dry air collide with warm, moist air. Flat land in America's midsection enables warm air from the Gulf of Mexico to move far north, where it collides with the westerly winds of the jet stream. "In other places in the world, you don’t have those combinations," Wurman said.
Tornado physics are still not well understood, but scientists believe the storms fall apart when their energy sources are cut off or when they are robbed of their rotation. It's not like there's an equation explaining a certain size at which a storm becomes unstable and dissipates. And what’s more, wind speed, funnel width, and tornado duration don’t necessarily have any bearing on how a tornado is measured according to the Enhanced Fujita scale, which ranks tornadoes between zero and 5. It’s an impact scale, meaning twisters are rated according to the damage they inflict, not necessarily how strong they are, Wurman said. Some broken tree limbs and minor structural damage could equate to an EF-0, while a well-built structure with completely obliterated walls could rank a storm an EF-5. "But if a strong tornado goes through open country, it can’t do that. Let’s say all it hits is a farm shed, or a mobile home, or some wheat," Wurman said. “You can have a strong tornado that’s rated EF-0."
The El Reno tornado, for example, was initially ranked as EF-3 based on damage in the area, but Wurman’s team of storm chasers found EF-5 scale damage, leading the local office of the NWS to upgrade it to the highest ranking.
And what about width? Although the NWS classified the El Reno tornado as the widest ever, Wurman said radar data has indicated wider storms in the past. The 2.6 mile number comes from a damage survey, he said. Weather experts determine the tornado’s center track, and move out from each side to search for the farthest indication of minor damage. That’s determined to be the tornado’s width, although the funnel cloud itself may be much smaller." I had a chaser who works with me who said, 'wow, I guess I was a mile inside that tornado, looking at the tornado.' This chaser was probably sitting in 60-80 mph winds inside this 2.6 mile width," Wurman said. "That width number is influenced very much by how hard one looks."
Storms on Other Worlds
While a nearly 3-mile-wide tornado is a scary thought, it pales in comparison to the vortices that form on other planets. Jupiter's famous Great Red Spot covers an area equivalent to two or three Earths, for instance. Despite its thin atmosphere, even Mars frequently experiences dust storms and tornado-like dust devils. These have actually been a great benefit to the Mars Exploration Rovers Spirit and Opportunity: their solar panels get dirty in dust storms, and sometimes a dust devil comes along and blows them clean.
Earlier this spring, scientists working on the Cassini mission at Saturn spotted a hurricane that spans 1,250 miles, which is about the distance from San Francisco to Denver. Scientists are still not sure how it grew so powerful despite the tiny amount of water vapor available in Saturn's atmosphere.
On Earth, scientists like Wurman are trying to determine how tornadoes form and evolve from powerful thunderstorms. Even with advanced radar, tornadoes are hard to predict and to understand, he said. On the day of the El Reno storm, most storm chasers were in southern Oklahoma, where conditions seemed more ripe for a tornado outbreak, Wurman said. Storm chasers Tim Samaras, Paul Samaras and Carl Young were among the few in the area, and were killed while trying to document the storm.
"Why does a storm like the one that went through El Reno make a big tornado and then stop? Why wasn't it a half an hour earlier, or a half an hour later? We don't know those things," Wurman said. "That’s what we’re trying to study."