Scientists show that most cities receive more rainfall than surrounding rural areas

Scientists find that more than 60% of the world's cities have more rainfall than surrounding rural areas and cities with taller buildings receive more rainfall than others with shorter buildings due to wind convergence.

Most cities receive more rainfall than surrounding rural areas. NASA
Most cities receive more rainfall than surrounding rural areas. NASA

The effect of urbanisation on temperature is relatively well known: cities tend to be noticeably warmer than the rural areas around them. This is called the urban heat island effect. What is less well known is that the urban heat island has a twin counterpart with equally important consequences: the urban rainfall anomaly, in which the presence of urban development noticeably affects the amount of rainfall in an area.

Conceptual image of urban heat island and precipitation anomalies. Inside Climate News
Conceptual image of urban heat island and precipitation anomalies. Inside Climate News

In a study published in the Proceedings of the National Academy of Sciences, researchers at the University of Texas at Austin looked for evidence of rainfall anomalies in 1,056 cities around the world and found that more than 60% of those cities receive more rainfall than their surrounding rural areas. In some cases, the difference can be significant. For example, the researchers found that Houston, on average, will receive nearly 127 litres/m2 more rainfall per year than its surrounding rural areas. This could have broad implications, the most serious of which is worsening flash flooding in densely built urban areas.

More rainfall in urban areas than in rural areas

Urban rainfall variation is something scientists have known about for several decades, but never on a global scale. Previous studies only looked at certain cities and storm events, said study author Xinxin Sui, a doctoral student in the Cockrell School of Engineering. For this paper, she and other researchers analyzed rainfall datasets from satellites and radar systems, examining daily rainfall anomalies in these 1,056 cities between 2001 and 2020.

The five US cities with the largest positive precipitation anomaly, according to satellite data. These cities will receive more precipitation than the rural areas around them. Credit: Xinxin Su
The five US cities with the largest positive precipitation anomaly, according to satellite data. These cities will receive more precipitation than the rural areas around them. Credit: Xinxin Su

"Overall, we found that more than 60% of these cities around the world have more rainfall (than surrounding rural areas). Then, we compared with different climate zones and found that if the local climate is warmer and wetter, there may be a larger rainfall anomaly compared to cities in colder and drier places," Sui said.

Urban rainfall variation is something scientists have known about for several decades, but never on a global scale. Previous studies only looked at certain cities and storm events, said study author Xinxin Sui, a doctoral student in the Cockrell School of Engineering. For this paper, she and other researchers analyzed rainfall datasets from satellites and radar systems, examining daily rainfall anomalies in these 1,056 cities between 2001 and 2020.

According to a study led by the University of Texas at Austin, the top 10 cities with the largest positive (blue) and negative (red) urban annual precipitation anomalies among the world’s 100 largest cities. Cities in blue will receive more precipitation than their surrounding rural areas, cities in red will receive less. Credit: Xinxin Su, et al.
According to a study led by the University of Texas at Austin, the top 10 cities with the largest positive (blue) and negative (red) urban annual precipitation anomalies among the world’s 100 largest cities. Cities in blue will receive more precipitation than their surrounding rural areas, cities in red will receive less. Credit: Xinxin Su, et al.

Although less common, some urban areas receive less rainfall than their surrounding rural counterparts. This typically occurs in cities situated in valleys and lowlands, where precipitation patterns are controlled by nearby mountains. Cities where this is most pronounced include Seattle, Washington; Kyoto, Japan; and Jakarta, Indonesia.

Other reasons for more precipitation in urban areas: local convergences

There are several reasons why most cities receive more precipitation than their rural neighbours. Co-author Liang Yang, a professor at the Jackson School, said one key factor is the presence of tall buildings, which block or reduce wind speed. This leads to a convergence of air toward the city centre. "Buildings further enhance this convergence by reducing wind speeds, leading to a stronger upward motion of air. This upward motion promotes water vapour condensation and cloud formation, which are critical conditions for rain and precipitation production," Yang said.

The researchers found that population has the highest correlation with urban precipitation anomalies compared to other environmental and urbanisation factors. This is because larger populations typically create denser and taller urban areas, along with more greenhouse gas emissions and therefore more pronounced heat, Niyogi said.

This phenomenon has implications for all cities heading into a climate-change future, Yang said, describing how the increased chances of rainfall in cities combined with the impermeable surfaces that make up their urban environments can be a recipe for flash flooding. "The combination of these two factors means we need to develop innovative ways to prepare for flash flooding," Yang said.

Reference

Niyogi, Dev, Global scale assessment of urban precipitation anomalies, Proceedings of the National Academy of Sciences (2024). DOI: 10.1073/pnas.2311496121.