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Circular cities have more rain than square cities, and triangular cities have the least rain of all 鈥� a finding from a modelling study that could help urban planners tackle the effects of climate change.

at the University of Texas at Austin and his colleagues decided to investigate the link between the shape of an urban area and its rainfall after noticing that weather data from roughly circular cities such as Dallas and London often show more rain than triangular cities such as Chicago and Los Angeles, but it wasn鈥檛 clear if this was due to their shape or other factors, such as location.

鈥淲e see cities change their shape as they grow, so understanding if there is a feedback between city shape and storminess was a question that needed to be answered,鈥� says Niyogi.

To find out more, Niyogi and his colleagues combined high resolution simulations of air turbulence and a weather forecasting model to simulate rainfall for circular, square and triangular cities with the same area, at both generic coastal and inland locations.

They found that circular cities received 22 per cent more rainfall than triangular cities, and that it is 78 per cent more intense, while square cities saw 8 per cent more than triangular ones. These differences were even more pronounced in coastal locations because the contrast between maritime and urban air tends to be even greater, which drives stronger convection.

鈥淲e typically get rainfall when two different air masses meet each other. A circular city allows air masses coming from all directions to converge at the centre of the city, creating an intense mixing zone and leading to convection and rain. For other city shapes, such as triangles or squares, air masses entering around the corners will meet early and consume energy before they reach the city centre,鈥� says team member at聽the Hong Kong University of Science and Technology.

Large areas of concrete and tarmac make cities particularly vulnerable to heavy rainfall and flooding, and the combination of increasing urbanisation and global warming is expected to exacerbate the risk. 鈥淩ecognising that the design of a city can impact rainfall and flood risk could help urban planners make cities more resilient to the impacts of climate change,鈥� says Niyogi.

at the University of Reading, UK, says the findings are plausible, but thinks it is important we don鈥檛 lose sight of the bigger picture. 鈥淐utting greenhouse gas and particle pollution emissions needs to be the greater priority for towns and cities, along with adapting to the increased severity of heatwaves and flash flooding driven by warming of the climate,鈥� he says.