Occurrence

Tornadoes in Australia resemble those of the American Midwest, which have been most studied, but are probably smaller, weaker and less common. There are about 700 reports each year in the USA, against ten or so reported annually in Australia. There were about eight per 105 km2 each year in Sydney between 1950 and 1989, which is roughly a fifth of the rate in tornado alley centred in Oklahoma, USA. However, it is notable that a map of the frequency of reported tornadoes in Australia resembles that of population density, so there may be further tornadoes in unpopulated areas.

Tornadoes in Perth and Adelaide occur mostly in winter, whereas they are more common in summer along the east coast of Australia, e.g. November-January in New South Wales. Late afternoon is the most common time, when surface heating is greatest. They also occur on the west coast of New Zealand and in South Africa, and in South America east of the Andes, around 25-35°S.

It seems that the centre of a huge city like Chicago or Tokyo is relatively immune to tornadoes, perhaps because of the roughness of the urban landscape. Tornadoes tend to avoid hills and follow valleys, and intensify on downslopes.

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