Tornadoes usually form when masses of warm, moist air and higher altitude cooler air meet. The cooler air descends, the warm air ascends, and the action creates a vertical funnel of swiftly rotating air. The majority of tornadoes in the world are created in the North American mid-west, when cool air coming over the Rocky Mountains meets the warm air rising from the Great Plains. With these geographical features, North America has what is called "Tornado Alley." Without these geographical features, tornadoes have a more difficult time forming; when they do, they are usually very weak. In fact, in the 1000 years prior to the 20th century, only about 2 dozen tornadoes were recorded in all of Europe.
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| Artist's impression of St. Mary le Bow being destroyed |
Which makes it all the more interesting to learn that a tornado—the earliest known in England and perhaps the biggest ever experienced by that country—hit London in 1091. Once again,
London Bridge fell down. The church of St. Mary le Bow was flattened, and four of its 26-foot-long rafters were driven into the ground with such force that only 4 feet remained showing. Several other churches were damaged or destroyed, as well as 600 houses. Estimates of the force of the tornado seem foolish, but people have tried, and they rate it an F4 on the Fujita Scale (F0-F5), with winds at 200 miles per hour or more. If that is true, then it is truly remarkable that there were only two deaths reported.
A tornado like that hitting London now would be striking a city of more than 8 million, but in 1091 estimates for London's population range from as few as 10,000 to as many as 20,000. In October of 1091, actually, there were even fewer people in London than usual. Thanks to the Anglo-Saxon Chronicle, we know that King William II was up north with his retinue and army fighting King Malcolm Canmore of Scotland (1038-1093). William prevailed, and was out of town when disaster struck. All things considered, it was probably the luckiest military campaign he ever undertook.
