Cities are hot. The fact that urban areas tend to be warmer than their surrounding region — a phenomenon called the heat island effect — is well-known to scientists as well as pretty much anyone who has endured a sweltering summer in a concrete jungle.
“This is everywhere,” said Dev Niyogi, a professor at the University of Texas at Austin and co-author on the paper. “The magnitude of the impact will vary. But just the way we treat urban heat island, we should start treating urban rainfall effect as a feature associated with urbanization.”
The results have profound implications for dense, growing cities, many of which are already struggling to deal with flooding, as they take on the cost of upgrading infrastructure for a hotter world.
Cities are magnets for rain for a few reasons. Skyscrapers tend to slow down incoming storms, allowing them to disgorge precipitation. Auto exhaust and factory pollution can seed clouds that unleash showers. And the heat radiating from concrete and asphalt causes more convection in the atmosphere, leading to rainfall.
“All these factors may contribute to this urban rainfall anomaly,” said Xinxin Sui, a doctoral student at University of Texas at Austin who led the research.
Niyogi likened the effect to popping a big water balloon. “Cities are poking it with the right instability,” he said.
Past research has shown this urban wet island effect exists in a handful of cities. Houston’s sprawling metro area, for instance, is known to have made the rainfall and flooding from Hurricane Harvey worse in 2017.
This new study “reveals for the first time the magnitude of these effects by analyzing cities globally,” said Jorge González-Cruz, a professor at the State University of New York at Albany who is not involved in the study.
For their research, Niyogi, Sui and their colleagues measured precipitation using satellite data from over 1,000 cities between 2001 and 2020. Nearly two-thirds of cities get more rain on average than neighboring rural areas, they found.
The magnitude of this effect nearly doubled over the past two decades, which the researchers attribute in part to increasing urbanization and rising temperatures, since warm air can hold more moisture. “The balloon is not only filled with water, it is now a larger balloon with more water,” Niyogi said.
In the United States, researchers identified Houston, Miami and New Orleans — each of which has experienced devastating flooding from major storms — as among the cities where the urban wet island effect is most acute.
The research team also found the bigger population a city has, the bigger the discrepancy between urban and rural rainfall is. The effect was most pronounced in cities in Africa, which have rapidly grown over this century.
According to Niyogi, the results should be taken seriously by urban planners, who need to take increased rainfall into account to prevent urban flooding and even funnel water back into aquifers to replenish groundwater for farmers in the future.
“We should start thinking about green infrastructure to manage our water resources,” he said. “And there is an opportunity here.”