Tornadoes - Nature's Most Destructive Winds. Facts and Causes

Tornadoes are a fact of life in North America, but a little understanding goes a long way towards reducing the risks.

Many of the pages in this site have suggested several ways of making the most of your home weather station by going a little further in observing those features of the weather beyond what your sensors are telling you.

The theme has been to get a little closer to the weather - take notes, take photos, make videos, and generally upgrade the experience of being able to record the physical aspects of the weather so easily and effectively.

First Known Tornado Photo

The Oldest Known Tornado Photo
Howard, South Dakota, August 1884
Source; NOAA Archives

But there comes a point where you should step back, move aside, or even head rapidly in the opposite direction to the approaching weather, and that, most definitely, is the case with tornadoes.

There's no doubt it would be a huge buzz to see a tornado at close hand, but its not the best sort of exploit for the inexperienced.

But there are ways of reducing the risks which you can follow up in the second half of this article, Watching Tornadoes.

Some Facts About Tornadoes

Meteorologists now know quite a lot about tornadoes and how they form. But there is still a lot to learn.

As far as tornadoes go, the net is full of reports, fact sheets, photos, videos and DVDs, and some of the best of them are listed elsewhere - see More Tornado Resources . One of the most useful sources of information is an article on frequently asked questions about Tornadoes, written by Roger Edwards of the Storm Prediction Centre. Not only does it answer just about every question you may think of, but it contains numerous links to reports, research, myths and forecasting.

Tornadoes are typically a North American phenomenon, with most occurring in the USA, notably in Tornado Alley, a roughly triangular area bounded by Louisiana and Texas in the south, North Dakota in the north, and extending east to Ohio. Tornadoes are always associated with thunderstorms, and as a general rule, but not an infallible one, the bigger and more complex the storm system, the more destructive the tornado.

The larger thunderstorm systems are associated with the interface between warm humid air moving north from the Gulf of Mexico and adjacent seas, and cooler air moving southward. The interplay of these air masses has a lot to do with the type and strength of thunderstorms, and the direction they move in. Other storms and their tornado spin offs form when western dry air interacts with moist air from the Gulf

Tornadoes usually begin to form in the southern states in late winter, typically late February to March. The peak tornado season gradually moves north during spring. May and early June are the most active months in the southern plains, while in the northern plains and upper midwest the peak is in June or July. But if the conditions are right, tornadoes will develop at any time and almost any place.

Overall the peak season on an all state basis is in May/June, with both months experiencing, on average, over 180 tornadoes.

Here are a few tornado facts

  • An important indication of imminent tornado formation is a rotating wall cloud - a small area of cloud below the main cloud base of a thunderstorm, and in a fairly central position.

    Wall Cloud and Lightning

    Supercell with well developed wall cloud, but no tornado. Source; NOAA Library, also published in National Geographic

  • Of the 10,000 US thunderstorms each year, about 1000 will produce tornadoes. 100 to 200 of these will be severe, producing significant damage in populated areas. Ballpark figures suggest that any given square mile in the US can expect a tornado about every thousand years or so, although some areas are more at risk than others. The chance of your house getting hit is much less.
  • The busiest month ever for tornado touchdowns was May 2003 with 516 confirmed.
  • The busiest day was 3rd April 1974, continuing through to the 4th. 147 tornadoes touched down in the US, plus one more in Canada. Thirteen states were affected, 48 were killers, and 318 people died.
  • The deadliest tornado was the "Tri-state" tornado of 18th March 1925, which affected a 219 mile long path through Missouri, Illinois and Indiana. It killed 695 people.
  • The average annual US death toll is about 60, and has been decreasing as warnings become more accurate and emergency plans and reactions have improved.
  • Maximum recorded wind gusts are meaningless - recorders either get broken or just disappear. An estimated gust of 318mph (about 510kph) was derived from a mobile radar unit. Modern forecasting techniques are still improving, but high tornado risks can generally be predicted 12 hours in advance. This doesn't mean a tornado will develop, just that the chances are high.

So how can you gain experience in severe weather observation without putting yourself at risk of injury or death? Follow this link to the Watching Tornadoes.

Supporting resources on tornadoes, including the best of available books, DVDs and posters, can be found at More Tornado Resources, while these pages will give you more information on Waterspouts (and other fast rotating winds), Thunderstorms and other types of Severe Weather.

This link will take you back to the Top, or, when you're ready, here's how to return to the Home page.


But just before you move on...

You may be interested to know that you can find out more about weather and home weather stations by receiving our newsletter ,"Watching Weather". It's published more or less weekly, and apart from tips on how to use your weather station and understand what it's telling you about the weather around you, it also covers many other weather related topics.

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Last update 05/28/2011