The Urban Heat Island (UHI) is a meteorological and climatological concept where a metropolitan area experiences significantly warmer temperatures than its surrounding rural areas. This phenomenon was first detected and measured in the early 1800s by Luke Howard in London, who compared temperatures between the city and nearby villages. The term itself was later named in 1929 by Albert Peppler.
The UHI effect works primarily through the modification of land surfaces. Urbanization replaces natural surfaces like soil and vegetation with materials such as asphalt and concrete, which have a high heat capacity and absorb more solar radiation. This stored heat is then released slowly, often making the temperature difference largest at night, sometimes up to 12°C warmer than rural areas. The lack of trees eliminates the natural cooling process of evapotranspiration and shade. Furthermore, the geometry of tall buildings creates "urban canyons" that trap heat and block wind, while waste heat from human activities like vehicles and air conditioners contributes to the warming.
The UHI effect connects directly to global climate change, as it intensifies the impact of rising baseline temperatures, making cities more vulnerable to heatwaves. In India, this has become a critical issue, with cities like New Delhi and Ahmedabad experiencing a compounded heat crisis in 2024. The UHI effect is mitigated by strategies like Green and Blue Infrastructure (GBI), which includes urban forests and wetlands, and the development of Heat Action Plans, such as the one pioneered in Ahmedabad following the 2010 heatwave.