European heatwaves 'have doubled'

The duration of heatwaves in Western Europe has doubled since 1880, a study has shown.

The authors of the research also discovered that the frequency of extremely hot days has nearly tripled in the past century.

The study shows that many previous assessments of daily summer temperature change underestimated heatwaves in Western Europe by about 30%.

The research appears in the Journal of Geophysical Research - Atmospheres.

The team found that heatwaves lasted an average of three days now, with some lasting up to 13 days. This compares with an average of about 1.5 days in 1880.

Paul Della-Marta, from MeteoSwiss in Zurich, Switzerland, and colleagues analysed daily maximum temperature data from 54 recording stations across Europe.

Forty-six hold records dating back to the 19th Century; others go back to the early 1900s. The data sets come from as far north as Finland, as far south as Spain and as far east as Croatia.

In the past, however, thermometers were not kept in modern Stevenson screens.

These wooden shelters protect thermometers from direct sunlight and indirect radiation coming from the ground, both of which distort temperature readings.

Once the researchers had corrected for these effects, they found a "warm bias" in observations made prior to the introduction of these screens. In other words, temperatures were recorded as being hotter than they really were.

This in turn meant the increase in temperature over time appeared to be smaller than it actually was.
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