CCities are magnets for summer rainstorms, and now it turns out they also intensify storms and increase the risk of flash flooding. The larger the city, the greater the flooding, and the warmer the climate and the larger the cities, the greater this problem is likely to become.

Herminia Torelló-Sentelles from the University of Lausanne in Switzerland and her colleagues examined seven years of rainfall data from eight cities in Europe and the United States. The cities varied in size, climate, and shape, but all were located in relatively flat regions and far from large bodies of water. They found that larger cities – such as London and Phoenix – received both more intense (up to 11%) and more concentrated (up to 15%) summer rainfall than surrounding areas. Meanwhile, smaller cities such as Milan saw an increase in rainfall of around 4%.

This extra summer rainfall is likely triggered by a number of factors, including the urban heat island effect, air pollution, and city skylines (which appear like small mountain ranges). All of this helps push warm air upwards quickly, where it condenses into rain clouds. In a report in the journal Earth's Future, researchers say urban planners need to strengthen urban drainage systems, create more resilient infrastructure against flooding and prepare for torrential summer rains.