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The world’s cities are experiencing more heatwaves - 30th January.

London

Since the 1970’s, the number of extremely hot days in a year has increased within the world’s cities, with frequencies peaking in the last five years.

The world’s cities are experiencing more heatwaves and fewer cold spells, according to a study of extreme temperatures in hundreds of urban areas over the past 40 years.  The research found that many cities are seeing fewer extremely windy days than in the 1970s, but have more extremely hot individual days and nights.

Research undertaken by US and Indian universities identified 620 of the world’s urban areas with a population over 250,000 and then chose 217 of these areas that were situated close to an international weather station, with rainfall, wind and temperature records stretching back to 1973.

Four of the five years with the most heatwaves had occurred since 2009 in mainly Africa, East Asia, Europe and North America. However, a few urban areas in East Asia showed significant declining trends. Only 2% of the urban areas experienced significant declines in the frequency of extreme hot days.

The researchers found “extremely windy days declined substantially during the last 40 years with significant declines in about 60% in the urban areas”. The authors of the research defined heatwaves as periods of at least six days where the daily maximum temperature was hotter than 99% of days since 1973.

Urban areas make up a relatively small part of the global land area; but they are the centre of wealth, so damage to urban infrastructure could result in potentially large economic losses. 

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