It is well-established that urban areas are warmer than surrounding rural areas due to the urban heat island effect. However, fewer people are aware of its counterpart, the urban precipitation anomaly, which has equally important consequences. This phenomenon implies that urban development has a considerable impact on the amount of rainfall in a given area.
In this article we are going to tell you Why it rains more in cities than in rural areas.
Why it rains more in cities than in rural areas
A new study published Monday in the journal Proceedings of the National Academy of Sciences, which received funding from NASA, was conducted by researchers at the University of Texas at Austin. They investigated rainfall anomalies in 1.056 cities around the world and found that more than 60% of these urban areas experience more rainfall compared to their rural surroundings.
In some cases, the disparity can be quite pronounced. For example, The study found that Houston typically receives nearly 130 inches more rain per year than nearby rural regions. This finding could have important implications, especially with regard to the increasing severity of flash floods in densely populated urban environments.
Scientists have known about urban rainfall variation for many decades, but never before at a global level. According to study author Xinxin Sui, a doctoral student in the Cockrell School of Engineering, previous research focused only on specific cities and storms. In this paper, she and her fellow researchers investigated rainfall data sets from satellites and radar systems, analyzing daily precipitation anomalies in 1.056 cities from 2001 to 2020.
Results of the investigation
Sui said: "In total, We found that more than 60% of the world's cities experience higher rainfall than their surrounding rural areas. Furthermore, we analyzed several climatic zones and observed that in warmer and wetter local climates, there tends to be a more significant precipitation anomaly compared to cities located in colder and drier regions.
In addition to Houston, other major cities exhibiting significant rainfall anomalies include Ho Chi Minh City in Vietnam, Kuala Lumpur in Malaysia, Lagos in Nigeria, and the Miami-Fort Lauderdale-West Palm Beach metropolitan area in the U.S. “The research offers a compelling examination of a phenomenon that has previously been recognized and documented at local scales in several urban areas: the effects of urbanization on rainfall and the development of a distinctive urban climate, using data from over a thousand locations around the world.
"This study improves our understanding of the phenomenon by evaluating it from a broader perspective, allowing us to discern patterns and variations that might not be evident in localized studies," said María del Carmen Casas Castillo, a professor at the Universitat Politècnica de Catalunya (UPC-ESEIAAT).
Speaking to SMC Spain, the expert stressed that it is essential to pay attention to the details and unique characteristics of each city when it comes to specific applications in urban environments, particularly in the design and management of drainage and sewerage systems aimed at flood prevention. This understanding is vital to understanding the phenomenon in specific local contexts. Tall buildings play a critical role in generating rainfall.
Role of buildings
Dev Niyogi, an author on the study and a professor at the Jackson School of Geosciences and the Cockrell School of Engineering, clarified that urban environments often redistribute rainfall from one location to another, similar to how a sponge is squeezed: “When you squeeze a specific section of the sponge, the water will come out more forcefully from that side. The total amount of water inside the sponge remains unchanged, but because of a dynamic compression of the atmosphere, there is a greater capacity to draw water from that area.” Although it is less common, Certain urban regions experience lower rainfall compared to their rural surroundings..
This phenomenon usually occurs in cities located in valleys and low-lying areas, where rainfall patterns are influenced by nearby mountains. Notable examples of such cities include Seattle (USA), Kyoto (Japan) and Jakarta (Indonesia).
Multiple factors contribute to the phenomenon where urban areas experience higher rainfall compared to their rural counterparts. According to co-author Liang Yang, a professor at the Jackson School (USA), one important reason is the existence of tall buildings that impede or slow down the wind speed. As a result, this creates a convergence of air directed towards the city center.
Yang said: "The convergence is further amplified by buildings that slow down the winds, generating a more vigorous upward flow of air. This upward airflow encourages water vapor condensation and cloud development, which are essential for creating rain and precipitation. According to the researchers, the strongest correlation with urban precipitation anomalies is found in population size compared to other factors related to the environment and urbanization.
Niyogi explains that larger populations generally lead to the development of denser and taller urban landscapes, which in turn contributes to increasing greenhouse gas emissions and heat levels. Yang points out that this trend poses significant challenges for cities in a future affected by climate change. He explains that the likelihood of increased precipitation in urban areas, coupled with the impervious surfaces characteristic of these environments, creates conditions conducive to flash flooding.
