Heavy rainfall on the increase

The work, published this week in the International Journal of Climatology, updates and extends previous studies by Dr Osborn and colleagues, using five-times as many rain gauges and looking at measurements over a longer time period.

Their classification took into account the typical differences in rainfall between summer and winter and across different regions of the country. In parts of East Anglia, for example, heavy rain meant at least 20mm falling on a single summer day, while in winter, 10mm in a day was sufficient to reach the heaviest category. For some locations in the north-west Highlands of Scotland, rain or snow falls of at least 30mm in summer and even 60mm in winter were the minimum required to count towards the heaviest category.

This new, more extensive study, using up-to-date records, supports the existence of a long-term increase in winter precipitation intensity that is very widespread across the UK. In the late 1960s, about seven per cent of the UK’s winter precipitation came from heavy rain or snow events, while in the last 10 years that figure has been about 12 per cent.

Until the late 1990s, most areas of the UK had seen a decreasing contribution of extreme rainfall during the summer. The updated measurements indicate that this trend towards lighter summer rainfall reversed during the last decade, but it is too early to tell whether this new trend will continue into the future.

“So far it is not clear what causes these trends and variations. In the next stage of our study, we will be looking at possible physical mechanisms and whether man-made global warming is contributing,” explained Dr Maraun.