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Quickfire Science: Oklahoma Tornado

Thu, 23rd May 2013

Pete Skidmore, Elena Teh

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Tragedy struck Oklahoma this week, when a massive tornado at least a mile wide ripped through the town of Moore, leaving at least 24 people dead. Here’s your Quickfire Science on these destructive forces of nature:

 

Pete - A tornado is a rapidly rotating column of air, which descends from the base of a thunderstorm down to the ground.

 

Elena - The most powerful tornadoes often form underneath large rotating thunderstorms, called supercells.

 

Pete - There are several theories about how tornadoes form under a supercell, but one possible way is from wind shear. This is when winds at two different heights blow in different directions or at different speeds.

 

Elena - The clash between these two winds can cause a cylinder of air to rotate around a horizontal axis, which can then be tilted vertically towards the ground by warm air rising within the thunderstorm.

 

Pete - Given the right conditions, this vertical spinning cylinder of air can become a tornado.

 

Elena - Tornadoes occur all over the world, but most commonly form in North America, especially in the central plains of the United States, nicknamed Tornado Alley.

 

Pete - Scientists think they are so frequent here because warm, rising air from the Gulf of Mexico is focussed into thunderstorms by cool, dry air from Canada. Dry winds blowing east from the Rocky Mountains provide the wind shear which drives the rotation of the storms.

 

Elena - Modern methods of detecting tornadoes include using satellite data as well as radar to identify the high wind speeds.

 

Pete - The US National Weather Service’s Storm Prediction Center uses these new techniques, as well as a network of voluntary storm spotters across the country, to provide a tornado warning system to communities at risk

 

Elena - Warnings for Monday’s disaster in Oklahoma were sent out 16 minutes before the twister hit the ground, earlier than the average time of 8 to 10 minutes.

 

Pete - The strength of a tornado is measured from 1 to 5 on the Enhanced Fujita scale, which looks at the amount of damage caused to structures.

 

Elena - The Oklahoma tornado had an Enhanced Fujita score of 5, the most powerful rating, with wind speeds of over 200 miles per hour giving it the power to sweep away strong buildings and overturn cars.

 

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Dr Tetsuya Fujita of the University of Chicago (who also introduced the Enhanced Fujita (EF) scale for rating tornadoes) says —



Both conditions for tornadoes and agricultural growth are found in the same areas for the reasons mentioned above, as is shown on this map.



source Lmnre, Sat, 25th May 2013

Hurricanes are also fed by water in the sense that they are generated by large bodies of upwardly flowing, water-saturated air. The latent heat of condensation released then the water vapour condenses also drives further updraft, pulling in cooler air from across the Earth's surface.

chris, Sat, 25th May 2013

There are a few other interesting questions about the Oklahoma tornado though:
• Why was a phenomenon that is typically about 100 m wide 1.5 km wide? Is it unprecedented?
• Are tornadoes becoming gradually more frequent? More severe?
• What are the factors in the genesis of a tornado that determine its width? Its severity? damocles, Tue, 28th May 2013

I think there is evidence that such storms are becoming more intense; for hurricanes this is certainly the case; a recent study in Nature looked at gravel deposits in Caribbean lagoons as an index of storm surges and showed an increasing trend over time.

This is a transcript of an item I wrote about it in 2007:

"Researchers get wind up about hurricanes - US scientists have persuaded a muddy lagoon in the Caribbean to surrender 5000 years of hurricane history, enabling them to spot some of the key climate conditions that spawn a fearsome storm. Writing in Nature this week, Jeff Donnelly and Jonathan Woodruff, from the Woods Hole Oceanographic Institute, collected sediment cores from the lagoons of several Caribbean islands including one off Puerto Rico called Vieques. Because these lagoons are separated from the sea by a ridge, it takes a big storm, like a hurricane, to push sand and rock grains into them. These grains are then deposited in layers, with each layer corresponding to a different storm from some time back in history. The team were able to use carbon-dating on the mud mixed with the sand to precisely pinpoint the timings of the storms, going back 5000 years. Then, by marrying up this record with other measures of past climate activity, they were able to show that rising sea temperatures, which were previously thought to be the main drivers of hurricane activity, are not the whole
story. In fact, some of the storms they flushed out in the study were much larger than those occurring today even though the sea was cooler then. A major player, it turns out, is El Nino, which is a pool of warm water that periodically moves eastwards across the Pacific. When this happens it seems to disrupt atmospheric circulation over the tropics, causing developing storm systems to stall in the Atlantic. A strong African monsoon, on the other hand, seems to be linked to more severe hurricane activity. "So
working out what El Nino and the African monsoon are going to do in the future is key to working out what the weather has in store for America," says Donnelly... chris, Wed, 29th May 2013

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