Key Terms & Concepts — UPSC Mains
Urban Heat Island (UHI)
"A metropolitan area that is significantly warmer than surrounding rural areas due to human activities, dark surfaces, reduced vegetation, and waste heat from buildings and vehicles."
An Urban Heat Island (UHI) is a phenomenon where urban areas experience higher temperatures than nearby rural surroundings — typically 2°C to 5°C higher on average, and up to 10°C higher on calm, clear nights. The primary causes are: 1. Surface albedo change: Dark impervious surfaces (asphalt roads, concrete rooftops, dark buildings) absorb solar radiation instead of reflecting it. In contrast, vegetation reflects more sunlight and cools through evapotranspiration. 2. Reduced evapotranspiration: Urban surfaces have little moisture and no plants — unlike rural areas where evaporating water cools the air. 3. Waste heat emissions: Air conditioners, vehicles, factories, and data centres generate heat that raises ambient temperature. Air conditioners are doubly problematic — they cool interiors but heat the outdoor air. 4. Urban geometry (street canyon effect): Tall buildings trap longwave radiation, preventing heat from radiating out at night. Heat is absorbed by building facades and re-emitted slowly. 5. Reduced wind speed: Urban structures reduce wind flow, preventing convective cooling. UHI consequences: increased heatwave mortality (urban poor and elderly most affected); higher energy demand for cooling (creating a feedback loop); increased air pollution (higher temperatures accelerate ozone and particulate formation); altered precipitation patterns (UHI can trigger localised convective storms). Mitigation strategies: green roofs and walls, urban forests, cool pavements (high-albedo materials), cool roofs (white/reflective rooftops), permeable pavements, urban water bodies, and building orientation for natural ventilation. India context: Delhi, Mumbai, Bengaluru, Chennai, and Hyderabad have measured UHI effects of 3–6°C. Bengaluru's tech-corridor expansion drove significant UHI intensification as green cover shrank. Delhi's summer heat is worsened by both UHI and the loo (dry hot winds from the northwest).
Frequently tested in UPSC GS3 (Environment, Geography, Disaster Management) and GS1 (Human Geography). Prelims: definition, causes, mitigation measures, and which cities show strong UHI effects. Mains: UHI as a factor amplifying heatwave mortality in India's megacities; urban planning and climate adaptation; intersection of UHI with the SDG goal of Sustainable Cities (SDG 11). Also relevant in essays on urbanisation and climate justice — UHI disproportionately affects informal settlements and outdoor workers.
- 1 UHI: urban areas 2–5°C warmer than rural surroundings; up to 10°C warmer at night
- 2 Primary causes: dark surfaces, reduced vegetation, waste heat, urban geometry, reduced wind
- 3 Air conditioners cool indoors but heat outdoors — a UHI feedback loop
- 4 Street canyon effect: buildings trap longwave radiation, slowing nocturnal cooling
- 5 Mitigation: green roofs, urban forests, cool pavements, reflective rooftops, permeable surfaces
- 6 India cities with strong UHI: Delhi, Mumbai, Bengaluru, Chennai (+3–6°C)
- 7 UHI amplifies heatwave mortality and increases energy demand in a positive feedback cycle
- 8 Loo winds (NW India) + UHI together make Delhi heatwaves more severe than regional average
A study of Bengaluru found that tech-corridor areas where tree cover was replaced by concrete and glass had temperatures 4–5°C higher than parts of the city that retained their tree canopy — a textbook Urban Heat Island effect demonstrating how rapid urbanisation without green infrastructure amplifies heat stress.
