Heat and smog cause most cities to receive more rain than surrounding areas.

Some cities get more rain than the surrounding areas.

Paul Brown / Alamy

Urban environments affect the weather, with many cities around the world receiving more rain than surrounding areas, and this discovery could have implications on the way cities are built in the future.

“Just like the urban heat island effect, it also has an impact on rainfall in cities.” Dev Niyogi At the University of Texas at Austin.

He and his colleagues looked at satellite data on rainfall in 1,056 cities and nearby rural areas across a range of climate regions from 2001 to 2020. They found that more than 60 percent of cities are “wet islands,” meaning they receive more rainfall than their surrounding areas, while several others are “dry islands,” showing the opposite pattern. For example, Ho Chi Minh City and Sydney are among the rainiest, each receiving more than 100 millimeters more rainfall per year than their surrounding areas. Seattle and Rio de Janeiro are among the 10 driest cities.

It has long been known that individual cities affect rainfall, but this is the first study to show that this is a global pattern, Niyogi said. “We need to consider rainfall and cities as interacting,” he said.

Cities can increase or decrease rainfall in several ways: Heat absorbed by asphalt and buildings causes updrafts of air, helping rain clouds form; the “roughness” of buildings slows down weather systems, allowing rain to last longer in urban areas; and air pollution seeds clouds., However, they can also reduce precipitation by cooling the air, and paved surfaces with little vegetation can reduce evaporation, reducing moisture in the air.

The impact of these factors varies depending on the size and location of a city. The researchers found, for example, that larger, more populous cities are more likely to be wet islands. Cities in temperate, tropical and coastal regions tend to have the largest outliers, while cities in mountainous regions are generally less affected.

The researchers also found that the difference in average precipitation between the wettest islands and their surrounding areas nearly doubled during the study period, increasing annual precipitation from an average of 37 to 62 millimeters, while anomalies in dry areas remained unchanged. Niyogi says this is because rapid urbanization and rising temperatures due to climate change have combined to increase the total amount of water vapour in the air.

Current weather and climate models don't explicitly consider how cities affect rain, but Niyogi says urban planners might be able to take into account how their decisions affect rainfall. For example, wetter cities that are more vulnerable to flooding could take steps to limit flooding, while drier cities might build in ways that increase rainfall.