Wetter winters give Britons the rainy-day blues

Forget deep and crisp and even. Think soggy. Britain's winter rainfall, predicted to increase with global warming, has indeed become markedly heavier in the past 40 years, a new study reveals.

Forget deep and crisp and even. Think soggy. Britain's winter rainfall, predicted to increase with global warming, has indeed become markedly heavier in the past 40 years, a new study reveals.

The prospect of wet rather than white Christmases is revealed in a detailed analysis of more than two million individual weather records, which show the number of days with heavy rain – defined as more than 15mm (nearly two-thirds of an inch) – has increased throughout the country by about one-third. In some regions, such as western Scotland, the increases are more than 50 per cent and in some individual places they are substantially higher.

Even more significantly, the number of "multi-day" periods of heavy rain, when rain falls continuously for five days at a time, has gone up from about three per winter in the mid-1980s to about five now. These extended periods cause river banks to burst and lead to the sort of devastating flooding seen last year. Meanwhile, the number of days of heavy rain in summer has shown a corresponding decrease.

The changes, which are consistent with computer models of global warming, are revealed in the study by Timothy Osborn and Mike Hulme, scientists at the Climatic Research Unit of the University of East Anglia. Their paper will be presented to a meeting on "Flood Risk in a Changing Climate" today at the Royal Society in London.

The researchers did a detailed statistical analysis of daily rainfall records from 146 weather stations across the United Kingdom since 1961, in a number-crunching exercise involving nearly two and a half million pieces of data. The analysis shows the increase in winter rainfall is detectable over 90 per cent of Britain and is highly unusual in the context of rainfall patterns for the 20th century as a whole – which means it is unlikely to be a random climatic variation.

The increase is even heavier than the super computer models of climate change predict it should be.

"There is a clear pattern, and there has been a rather large increase in the intensity of winter rainfall over the period," Dr Osborn said. "The potential for flooding will be greater. We would have expected an increase on the basis of climate change, but the observed increase is actually rather bigger than we would have expected to occur by now."

The increase has possibly been caused not only by man-made climate change but also by a shift in the North Atlantic Oscillation, the alternation of high and low pressure between the areas of Iceland and the Azores, which in recent years has brought stronger westerly winds to Britain. But this change itself may be due to global warming. The unprecedented rains of last winter, the seemingly endless downpours that caused flood chaos across the country, were the heaviest since the 18th century and perhaps even longer. Official Meteorological Office figures show that the year from 1 April 2000 to 31 March 2001 was the wettest since records began in either 1766 or 1727 (depending on which records you use). In either case, the record was comfortably broken.

Last spring Philip Eden, the weather forecaster for BBC Radio 5 Live and a meteorologist respected by the Met Office, calculated that it may have been the wettest winter since Tudor times – or even before.

Throughout England and Wales as a whole the aggregate total for the 12 months was 52.61in (133.63cm), compared with a long-running average of 36.2in (91.95cm) – an excess over the average of 45 per cent.

Mr Eden's statistical analysis indicated this was so far in excess of the average that its "return period" – the frequency with which it could reasonably be expected to recur – was 500 to 750 years.