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why do tornadoes occur where they do

In the U. S. state of Oklahoma, at least 24 people have been killed and scores of others injured by a huge tornado. The twister was nearly a kilometer wide and packed winds of up to 320 kilometers per hour. RFE/RL correspondent Antoine Blua talked to Liz Bentley, a member of Britains Royal Meteorological Society, to get the lowdown on this often devastating natural phenomenon. RFE/RL: What is a tornado? Liz Bentley: Tornadoes are violent rotating columns of air -- quite small-scale -- so they tend to be a few meters across, sometimes 100 meters across. They develop out of supercells or violent thunderstorms that develop in the atmosphere. So the places around the world where tornadoes will develop are where we get these supercells, these huge thunderstorm clouds developing. And for a thunderstorm to develop you need a mixture of warm, moist air and cooler, drier air that interact with each other. And then within the cloud you get this small rotation, and usually what you see on the base of the cloud if you look up at the cloud is a small vortex starting to appear. And as that slowly goes down towards ground level, the tornado develops. And as soon as it touches the ground, we officially call it a tornado.

RFE/RL: In which parts of the world can tornadoes be observed? Bentley: Tornadoes can happen pretty much anywhere in the world, barring probably the polar regions. America is probably four times more likely to see tornadoes than all the European countries put together, but on the scale of smaller countries, parts of the U. K. , Belgium, the Netherland for example, they receive a larger concentration of tornadoes comparative to the size of the country. RFE/RL: Why do the majority of tornadoes occur in the United States, which experiences about 1,200 of them per year? Bentley: Particularly between March and August during the year, we get warm moist air coming in from the Gulf of Mexico and drier, cooler air coming in from the Rockies from the northwest of the U. S. And where they meet, we have something called Tornado Alley, which is an area where tornadoes are more prone to occur across the United States. RFE/RL: The devastation outside Oklahoma City came almost exactly two years after a tornado ripped through a city in Missouri, killing nearly 160 people. Why can tornadoes be so destructive, particularly in the United States?

Bentley: It's really down to the wind strength within the tornado itself. For a small-scale tornado, the wind strength may be about 80 miles per hour (130 kilometers per hour), but wind strength can get up to 200, maybe 300 miles per hour (320 to 515 kilometers per hour) within that small core of vortex. And it's that wind strength that is very destructive. The smaller scale, the lighter winds, may just take down trees and any loose debris and move it around. But once you get wind speeds of over 200 miles per hour (320 kilometers per hour), that's enough to completely [destroy] buildings and structures. The size, the scale of both the thunder clouds and the tornadoes, tend to be much greater in the United States. RFE/RL: To what extent can meteorologists predict tornadoes, their strength, and their route? Bentley: The first thing we want to look at is the general, large-scale structure. So have we got the warm, moist air interacting with the cooler, drier air to develop large-scale thunderstorms? And if so, there's usually a 24-hour prediction to say that tornadoes are likely within this region.

After that, you start to look at radar images, rainfall images to see what the structure is like within developing thunderstorms. So what we are looking for are small hooks of cloud and rainfall. And within that hook is where the tornado is likely to develop. Usually then you get maybe half-an-hour to an hour's warning of possible tornadoes. That's when the sirens will ring out to warn people to get some precautionary action. RFE/RL: Is there any evidence that climate change is making things worse? Bentley: There are certainly more [tornado] sightings nowadays -- in the last decade or so -- than we had maybe 50 years ago. It's very difficult to put this just down to climate change. The fact [is] that we may be seeing more extreme weather because people are more familiar with what a tornado is. There are more people who go looking for tornadoes, so we have had more sightings in recent years. So it could be a combination of our climate changing, but also the fact that people are observing tornadoes on a more frequent basis than we used to do.
Most tornadoes are found in the Great Plains of the central United States an ideal environment for the formation of severe thunderstorms.

In this area, known as Tornado Alley, storms are caused when dry cold air moving south from Canada meets warm moist air traveling north from the Gulf of Mexico. Tornadoes can form at any time of year, but most occur in the spring and summer months along with thunderstorms. May and June are usually the peak months for tornadoes. Which two states have the highest number of tornadoes per year per 10,000 sq miles? Notice that the location with the highest number of thunderstorms does not match the location with the highest number of tornadoes. Why do so many tornadoes form in this part of south central North America? The Great Plains are conducive to the type of thunderstorms (supercells) that spawn tornadoes. It is in this region that cool, dry air in the upper levels of the atmosphere caps warm, humid surface air. This situation leads to a very unstable atmosphere and the development of severe thunderstorms. Look at the map above. The blue lines show the paths of tornadoes produced by storms on April 3rd 4th, 1974. Why are the lines all parallel and pointing in a southwest to north direction?

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