The Urban Heat Island (UHI) is a meteorological and climatological concept describing the phenomenon where metropolitan areas experience significantly warmer air and surface temperatures than their surrounding rural regions. This temperature difference, which can be up to 10°C in some Indian cities, is often most pronounced at night. The concept's origin dates back to the early 19th century, when English meteorologist Luke Howard documented warmer nighttime temperatures in London in his 1818 book, The Climate of London, attributing it to urban activities.
The UHI effect works primarily through the modification of land surfaces and urban geometry. Urban materials like concrete and asphalt have a high heat capacity and low albedo, causing them to absorb and store large amounts of solar radiation during the day, which is then slowly released as heat at night. This replaces the natural cooling mechanism of evapotranspiration provided by vegetation and soil. Furthermore, tall buildings create an "urban canyon" effect, restricting wind flow and trapping heat near the surface. A secondary contributor is waste heat generated by human activities, such as industrial processes, vehicles, and the exhaust from air conditioning units.
The UHI concept connects directly to critical issues like climate change, which compounds the warming effect, and public health, as it increases the risk of heat-related illnesses and mortality. It also links to the Urban Pollution Island (UPI), as both are intensified by combustion processes and urban form. The UHI effect has become a major focus in India, with a 2024 report noting that 42 major Indian cities have reported increasing UHI effects over the past decade. This recent intensification is driving a sharp surge in residential electricity demand for cooling, with UHI intensity across Indian cities ranging between 2°C and 10°C. Mitigation strategies, such as those under schemes like AMRUT and Smart Cities, now focus on structural changes like enhancing urban vegetation, using reflective materials, and implementing better urban planning.