Scientists already knew that all the pavement and rooftops in cities tend to create heat islands, areas that are hotter than their surroundings. That has implications for human health and public policy. 

But now, new research has revealed another way that cities influence the weather: an "urban rainfall effect," the Washington Post reported.

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What's happening?

According to a study published in the Proceedings of the National Academy of Sciences, most cities get more rain than the areas surrounding them.

"This is everywhere," said study co-author Dev Niyogi, a professor at the University of Texas at Austin, per the Post. "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."

There are several factors playing into this effect. Skyscrapers tend to catch storm systems so they linger over cities and dump more water. Fumes from cars and factories seed clouds that then cause more rain. And of course, the heat islands themselves can stir up the atmosphere over cities and cause more rain too.

"All these factors may contribute to this urban rainfall anomaly," said doctoral student Xinxin Sui, who led the study, per the Post.

Not only are cities often wetter, per the report, but Sui's team found that the effect has been increasing over the past 20 years as the world has gotten hotter.

Past research has shown greater rainfall in individual cities, but the new study examines urban areas worldwide and shows the scale of the effect.

To reach these conclusions, Sui's team used satellite data from more than 1,000 cities, starting in 2001 and ending in 2020, the Post revealed. Almost two-thirds of the cities examined got more rainfall than the surrounding average, and the effect nearly doubled in magnitude over the 20 years of the study. In the U.S., Houston, Miami, and New Orleans were among the most affected cities.

Why does extra rainfall matter?

Cities already struggle with flooding due to the nature of the landscape. The ground is less absorbent due to being paved in most places, and while storm drains and sewer systems exist to handle the runoff, they are often no match for severe weather.

Now, this new study reveals that city planners aren't just contending with normal rainfall for the region; the city itself is attracting more water. That can lead to dangerous conditions for a city's many residents, especially when a major storm hits.

What can be done about the urban rainfall effect?

Luckily, the existence of this research could give urban planners an advantage, as they'll now know to plan for even more water.

"We should start thinking about green infrastructure to manage our water resources," said Niyogi, per the Post. "And there is an opportunity here."

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