[thedoc (OP)]

A mile-wide tornado ripped through Moore, Oklahoma this week. Here's the Quickfire Science on these destructive forces of nature...

Read the whole story on our website by clicking here

  

[Lmnre]

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

The majority of tornadoes occur in agricultural areas. Tornadoes need moisture to feed their parent thunderstorms and the instability associated with spring and summer warming. Crops need moisture to grow and the temperature variation associated with changing seasons.

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

[chris]

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.

[damocles]

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?

[chris]

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...