Urban environments are significantly altering weather patterns, with many cities experiencing increased rainfall compared to their surrounding areas. This discovery could reshape urban planning strategies in the future.
Research indicates a phenomenon similar to the urban heat island effect, referred to as the urban rainfall effect. A comprehensive study analyzed satellite data from 2001 to 2020 across 1,056 cities worldwide, revealing that over 60 percent of cities act as “wet islands,” receiving more rainfall annually than their rural counterparts. Notably, cities like Ho Chi Minh City and Sydney recorded an excess of over 100 millimeters of rainfall, while Seattle and Rio de Janeiro emerged as the driest cities.
This breakthrough marks the first global recognition of the impact urban areas have on rainfall dynamics. “We need to view rainfall as a phenomenon interacting with urban spaces,” said a leading researcher.
Urban features contribute to this rainfall variance—heat absorbed by concrete and asphalt generates updrafts favoring cloud formation, while the roughness of urban structures can prolong rainfall duration. Additionally, air pollution may impact cloud formation and precipitation levels. Areas with minimal vegetation often see reduced evaporation, leading to decreased atmospheric moisture.
The extent of these phenomena shifts depending on city size and geographical placement, with larger urban centers often categorized as wet islands. Temperate, tropical, and coastal cities exhibit the most pronounced anomalies, while mountainous areas show less of an impact.
The research indicates that the gap in rainfall between wet islands and their surroundings nearly doubled from an average of 37 to 62 millimeters per year during the study period, attributed to rapid urbanization and rising temperatures from climate change, which augment atmospheric water vapor.
Despite the lack of integration of urban influences in current weather and climate models, experts suggest that this information could guide urban planning. For instance, municipalities prone to flooding could implement strategies to mitigate excessive rain, while areas facing drought could explore options to enhance rainfall.
