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According to Olga Zolina of the Meteorologisches Institut, Universitaet Bonn, Bonn, Germany, and colaborators, writing in Geophysical Research Letters, as the world’s climate changes, precipitation patterns are also changing.

Previous studies have found that in Europe the amount of precipitation has been increasing by about 15 – 20% over the past 60 years. It is not clear if this is due to natural cycles or human influence. A new study now shows an increase in the duration of wet periods, potentially affecting the frequency of catastrophic floods.

Zolina et al have used daily rain gauge data from nearly 700 rain gauges in Europe covering the period 1950-2008. Wet periods are defined as consecutive days with significant precipitation (more than 1 mm/day). The researchers find that although the total number of wet days has not changed significantly over the past 60 years, the duration of wet periods has increased. It seems that short rain events have merged into longer rainy spells.

Heavy precipitation events during the past two decades have become much more frequently associated with longer wet spells and intensified compared with the 1950s and 1960s. Heavy rainfall associated with short wet periods has become less intense.

Because flooding depends not only on the amount of rain but also on the duration of rainfall, the changing duration of rainy periods has the potential to alter the frequency and intensity of floods in Europe.

How the observed changes in European rainfall fit into the climate change picture is uncertain. More monitoring and research is required to determine if the changes are natural or otherwise.