The science of tornadoes: how and where do they form?

The intense outbreak of severe thunderstorms in the United States over the past week has spawned at least 38 confirmed tornadoes, with further, unconfirmed reports of nearly twice that number, and the threat isn't over, as more are expected today and possibly tomorrow as well.

The term 'tornado alley' is probably quite familiar to everyone. It's an unofficial name given to a swath of land that includes northern Texas, western Oklahoma and most of Kansas, and is where the majority of tornadoes form in the United States. There's a tornado alley in Canada as well — actually two, depending on how you look at it. One stretches from Edmonton to Winnipeg, and there's another that runs from Windsor to Ottawa. There's also a 'tornado season', considered to be from March through August.

However, tornadoes don't obey geographic borders or time schedules. They form at any time of the year and in any location on the planet, as long as there's the right combination of weather conditions.

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The ingredients list for a tornado is fairly short. You need to mix warm, moist air with cooler, drier air.

In the regions of tornado alleys in the U.S. and in the Canadian Prairies, you get warm, humid air flowing in, close to the ground, usually from the Gulf of Mexico, and you have cooler, drier air coming down off the mountains. Since warmer air rises and cooler air sinks, the cooler mountain air riding over-top of the warmer air from the Gulf sets up powerful updrafts and downdrafts as the two try to switch places. This sets up intense rolling circulations of air.

The rising moist air in these circulations touch off thunderstorms, which form their own internal updrafts and downdrafts, causing the circulation of air to become even more intense. Rolling loops of air underneath and towards the back end of the storm start off roughly parallel to the ground, but friction and the influence of the updrafts and downdrafts cause them to tilt. When the tilted circulation of air touches the ground, it becomes a tornado.

The strength of a tornado is mainly due to how much warm and cool air it can tap into, since that determines how powerful the updrafts and downdrafts are in the storm. However, that's not something that's easy to figure out ahead of time, or even as the tornado is happening. Estimates can be made at the time, but a tornado's strength, based on the Enhanced Fujita Scale, is always determined for certain after the fact, by looking at the amount of damage that it caused and the maximum wind speeds that it reached.

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As for why any specific tornado happens at any particular time and in any particular location, if we knew the answer to that, we would have far fewer injuries and deaths due to them, and possibly even less property damage. The search for the answer to those questions continues and will likely continue for some time.

(Images courtesy: Reuters/Gene Blevins, Wikimedia Commons)

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