Britain:Tornadoes are best known for their frequency and spectacular ferocity on the Great Plains of the United States.
That part of North America is often the meeting zone of a mass of warm, moist air advancing from the Gulf of Mexico and a cold blast moving down from the northwest and Canada; it therefore provides a favourite breeding ground, and 20 or more tornadoes in a single day is not uncommon in "Tornado Alley", the flat landscape stretching through the states of Texas, Kansas and Oklahoma.
Next to this part of the US, large tornadoes are most common in the plains just east of the Andes in South America, and in the eastern regions of the Indian sub-continent. But they can, and often do, happen virtually anywhere in the would outside the polar regions, occurring in Britain and Ireland more frequently than is often realised.
Every year about 50 to 100 tornadoes are reported in England. Among the more spectacular was the twister which carved a path up to 500 metres wide and 12km in length through the centre of Birmingham on the afternoon of July 28th, 2005. Also well remembered is the Selsey tornado of January 1998 which passed very close to the house of the astronomer, Patrick Moore, near the Isle of Wight, and caused severe damage to his prized back-garden observatory.
Here in Ireland they are comparatively rare, with perhaps 10 or 20 being reported every year.
Earlier this year, a tornado on July 31st between the villages of Glengoole and Ballysloe in Tipperary damaged at least three houses, destroyed two cars and left a newly-married couple temporarily homeless. And, on August 17th, 2001, a tornado appeared on Dollymount strand in Dublin, and moved quickly northwards towards Raheny before disappearing as rapidly as it had formed. No injuries or major damage were reported, but Dubliners, naturally enough, were worried lest this be portent of worse things to come.
But as it happens, there is no evidence that tornadoes are becoming more frequent over Britain or Ireland. The essential requirement for tornado formation is the presence of strong persistent updraughts in the atmosphere, and plentiful supplies of moisture in the first few thousand feet above the ground.
It so happens that these very same conditions encourage the development of thunderstorms, hail and heavy showers, so all these phenomena often coincide.
Sometimes in these thundery conditions the rising air may be forced to "turn" because of variations in the strength and direction of the wind with height, a phenomenon known as "vertical wind shear". Occasionally this turning motion rapidly accelerates to result in a tornado.
But why some thunderclouds spawn these devastating twisters, while other produce nothing more dramatic than a shower of hail, is one of the few remaining mysteries of meteorology.
A tendency for more frequent and more powerful tornadoes would not be inconsistent with the gradual rise in average temperature that has been a feature of our climate in recent decades, but there is no direct evidence that this is happening.