The urban heat island (UHI) effect is the phenomenon whereby cities are significantly warmer than surrounding rural areas — typically 2–5 °C warmer on average, but the difference can exceed 10 °C on calm, clear nights. The effect was first documented by Luke Howard in London in 1818 and is now one of the best-understood aspects of urban climatology.
Several factors create UHIs: dark surfaces (asphalt, roofs) absorb more solar radiation than vegetation; thermal mass of concrete and brick stores heat during the day and releases it at night; reduced evapotranspiration (less vegetation means less cooling by evaporation); waste heat from vehicles, air conditioning, and industry; and urban canyon geometry (tall buildings trap radiation and block wind). The net effect is that cities have higher minimum temperatures (warmer nights), earlier spring, later autumn, and different precipitation patterns (5–25 % more rainfall downwind of cities).
The UHI amplifies health risks during heatwaves — nighttime temperatures that remain above 25 °C prevent the body from recovering, increasing mortality among vulnerable populations. In Madrid, the UHI intensity can reach 6–8 °C on summer nights between the centre and surrounding countryside. Mitigation strategies include green roofs, urban forests, cool (reflective) pavements, water features, and improved building design. Urban heat island monitoring is becoming a standard component of city climate services, with networks of sensors providing real-time temperature maps to guide public health responses.