Cities experience more rainfall than surrounding areas

The impact of urbanization on temperature is well-documented. Cities tend to be warmer than surrounding rural areas, a phenomenon known as the urban heat island effect. 

However, fewer people are aware that urbanization also influences rainfall patterns, creating what is called the urban precipitation anomaly, where urban development significantly alters the amount of rain an area receives.

Rainfall in cities worldwide 

In a new study published in the journal Proceedings of the National Academy of Sciences, researchers from the University of Texas at Austin examined rainfall patterns in 1,056 cities worldwide and discovered that over 60% of these cities receive more rainfall than their surrounding rural regions. 

In some instances, the difference is substantial. For example, Houston typically gets nearly five inches more rain annually than the surrounding rural areas.

These findings could have serious consequences, particularly with regard to worsening flash flooding in densely urbanized regions.

Urban rainfall anomalies on a global scale

Scientists have known about variations in urban rainfall for decades, but this study is the first to examine the phenomenon on a global scale. Previous research primarily focused on specific cities or isolated storm events, according to study author Xinxin Sui, a PhD student at the Cockrell School of Engineering. 

In this study, Sui and her colleagues analyzed satellite and radar precipitation datasets, examining daily rainfall anomalies in these 1,056 cities from 2001 to 2020.

“In general, we found that over 60% of these global cities have more rainfall (than the surrounding countryside),” Sui said. “Then we compared with different climate zones and found that if the local climate is hotter, if it’s wetter, then it may have a larger rainfall anomaly compared to the cities in cooler and dryer places.”

In addition to Houston, other major cities with significant precipitation anomalies include Ho Chi Minh City in Vietnam, Kuala Lumpur in Malaysia, Lagos in Nigeria, and the Miami-Fort Lauderdale-West Palm Beach metro area in the U.S.

Cities redistribute water like sponges

Study co-author Dev Niyogi, a professor at the Jackson School of Geosciences and the Cockrell School of Engineering, explained that urban environments act like sponges, redistributing rain from one area to another. 

Niyogi compared this to squeezing a sponge: “If you were to pinch one part of the sponge, you would have water coming down more forcefully from one side,” he said. 

“The amount of water you have in the sponge is the same, but because now you have that dynamic sort of squeezing the atmosphere, you have more ability to take the water out from that location.”

Cities located in valleys get less rainfall

Interestingly, while most cities receive more rainfall than rural areas, a few urban regions receive less. This tends to occur in cities located in valleys or lowlands where nearby mountains influence precipitation patterns. 

Cities like Seattle in Washington, Kyoto in Japan, and Jakarta in Indonesia experience this effect.

Factors contributing to urban rainfall0

Several factors contribute to why most cities see increased rainfall. According to study co-author Liang Yang, a professor at the Jackson School, one key factor is the presence of tall buildings, which can block or slow wind speeds, causing air to converge toward the city center. 

“The buildings further enhance this convergence by slowing the winds, resulting in a stronger upward motion of air. This upward motion promotes the condensation of water vapor and cloud formation, which are critical conditions for producing rainfall and precipitation,” Yang explained.

Larger populations drive urban rainfall

The study also found that population size had the strongest correlation with urban rainfall anomalies compared to other factors related to the environment or urbanization. 

Larger populations typically result in denser, taller cities, which generate more greenhouse gas emissions and heat, further intensifying the precipitation anomaly, according to Niyogi.

Further implications of the study

As climate change accelerates, these findings have important implications for urban areas. Yang highlighted how the combination of increased rainfall in cities and the impervious surfaces characteristic of urban environments – such as concrete and asphalt – can create conditions ripe for flash flooding.

“Combining these two factors means we must develop innovative ways to prepare for flash flooding,” Yang said.

In light of the increasing risks, urban planners and policymakers will need to address the growing likelihood of extreme weather events, ensuring that cities are better prepared to handle both excess rainfall and the infrastructure challenges it presents.

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