Q&A: You Wanted to Know about Tornadoes, the UK's Population of 100-Year-Olds, the Origins of Our Oceans, an African Border Dispute, England's Nation Status and Alien Invasions. Chris Edwards Has the Answers

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We only seem to hear about tornadoes in the USA and the UK. Do they occur elsewhere? B Lee, Sheffield

Tornadoes have been observed on all of the world's continents except Antarctica. However, certain climates are more prone to tornadoes than others. The UK is surprisingly tornado-afflicted, according to calculations made by the late Dr Tetsuya Fujita while at the University of Chicago. However, the USA gets the lion's share, with hundreds occurring each year, mainly in the flat Midwestern states, indeed, 75 per cent of all tornadoes spring up in Tornado Alley, a region that covers parts of Kansas, Oklahoma, Texas and Nebraska. But that still leaves the 25 per cent that happen elsewhere.

The key trigger for a tornado is a collision between warm and cool airstreams--something that will happen more in temperate climates than in the tropics, where air temperatures are generally more consistent. Once the airstreams have collided, a spinning area of low pressure forms. This then sucks in more warm air from the ground until you get the funnel-shaped vortex of a full tornado.

Outside the USA, tornadoes form across northern Europe and western Russia. They also appear in China and Japan. Northeastern India and Bangladesh are prone to tornadoes, caused by cold winds from the Himalaya hitting the tropical Indian Ocean airstreams. In the Southern Hemisphere, Argentina, Australia and South Africa all experience tornadoes at more or less the same latitudes.

How many centenarians are there in the UK? K Montgomerie, Burnley

According to recent figures produced by the Department of Work and Pensions, there are currently 10,000 people aged 100 or over in the UK.

With life expectancy increasing, the oldest of the old, those aged 85 and above, form the fastest-growing population group in the country. In 1951, a man aged 65 could expect to live another 12 years on average. A woman could expect 16 more years of life, in 2005, more than five years had been added to those expectations, with men beginning to close the mortality gap. By 2051, 65-year-olds of both sexes can expect more than 20 more birthdays, although they might have to work through more of them.

The number of centenarians is expected to rise steadily, reaching 20,000 towards the end of the next decade. And it won't stop there. Future kings and queens of England can expect to be sending a lot more telegrams than does the current queen--the centenarian population is predicted to grow dramatically from 2020 onwards, reaching more than 140,000 by 2055.

Where did the Earth's oceans come from? A Jowell, Doncaster

Right now, scientists don't have a clear answer to this one. However, they're pretty sure that most of the water in the oceans didn't come from comets or meteorites smacking into the planet after it was formed, as many had previously believed. Instead, it looks as though most of the water was present when the Earth formed more than four billion years ago, in spite of the intense heat that would have been present at the time.

Part of the reason for smashing a probe into the comet Tempel-I last July was to work out whether comets could have supplied much of the Earth's water. However, earlier analyses of other comets over the past five years had already cast doubt on the theory that cosmic snowballs delivered the water to a cooling Earth. According to Anita Cochran, an astronomer at the McDonald Observatory in Austin, Texas, and a member of the team analysing the Tempel-I data, scientists believe that as little as ten per cent of the Earth's water came from comets. The evidence mostly comes from isotopes of hydrogen in the cometary ice. Comets tend to be rich in deuterium, which is rare in our oceans.

The water, it seems, was locked up in the rocks that slowly clumped together during the early history of the solar system. However, it probably didn't start off as water--otherwise it would have simply boiled away in the inferno of the early Earth. …