Cities are magnets for summer rainstorms and now it turns out that they also intensify storms, raising the risk of flash flooding. The larger the city, the more of a deluge it generates, and as climate warms and cities grow, the greater this problem is likely to become.
Herminia Torelló-Sentelles, from the University of Lausanne, Switzerland, and her colleagues studied seven years of rainfall data from eight cities in Europe and the US. The cities varied in size, climate and shape, but all of them were in relatively flat regions and far from large bodies of water. They found that larger cities – such as London and Phoenix – received summer rainfall that was both more intense (up to 11%) and concentrated (up to 15%) than that of their surrounding areas. Meanwhile, smaller cities such as Milan saw rainfall intensify by about 4%.
This extra summer rain is likely to be triggered by a number of factors, including the urban heat island effect, air pollution and city skylines (which act like mini mountain ranges). All of these help to drive warm air rapidly upwards where it condenses into rain clouds. Reporting in the journal Earth’s Future, the researchers say that city planners need to beef up urban drainage systems, put in more flood-resilient infrastructure and prepare for summer rain that pours down.
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