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Urban Heat Islands

  • Context (IE): Around 80% of the people in India have experienced heat waves in the first 15 days of summer and are disproportionately affected by urban heat stress.
  • Urban heat island is a temporary phenomenon in which certain pockets within a city experience higher heat load than its surroundings.
  • This is due to rising ‘concretisation’ in urban spaces where heat trapped is unable to dissipate easily.

Urban Heat Island - तरुमहिमन्

Credit: Tarumahiman

Factors Contributing to Urban Heat Islands (UHI)

Diagram of a temperature Description automatically generated

Credit: Semantic Scholar

  • Urban deserts: Cities can be thought of as virtual deserts with almost no vegetation and materials that are almost completely impermeable to rain (for e.g. Asphalt). This combination leads to a lack of evapotranspiration, which increases sensible heat.
  • Urban canyons: The tall canyons formed by city buildings trap radiant energy in their walls, blocking wind flow that would have otherwise provided ventilation. This “canyon effect” blocks heat emissions from being released into the atmosphere, contributing to increased temperatures.
  • Humidity effects: Although there is little difference in the amount of water that cities and countryside retain in their atmospheres, the higher urban temperatures effectively lower the relative humidity (since warm air can hold more water than cold air).
  • Urban haze: A haze of air pollution that hangs over many cities can act as a miniature greenhouse layer, preventing outgoing thermal radiation from escaping from urban areas.
  • Anthropogenic heat: Heat from the burning of fossil fuels contributes towards increased urban temperatures.

Impacts of Urban Heat Islands

  • Economic implications: Increased healthcare costs, decreased worker productivity, rise in volatility of prices of food grains, reduced tourism potential.
  • Strain on energy resources due to increased demand for air conditioning and refrigeration, leading to higher greenhouse gas emissions and strain on electrical grids.
    • Studies have shown that UHIs can increase electricity consumption for cooling in urban areas by up to 20% compared to non-urban areas.
  • Adverse health effects: Heat-related illnesses, such as heat exhaustion and heatstroke, become more prevalent during heatwaves.
  • Altered microclimates and ecosystem disruption by altering wind patterns, rainfall distribution, and atmospheric stability, thus affecting urban hydrology and vegetation, leading to reduced evapotranspiration rates, increased stormwater runoff, and decreased water quality in urban streams and rivers.
  • Reduced Air Quality through the phenomenon of thermal inversion, which traps pollutants close to the ground. The combination of high temperatures, stagnant air, and increased emissions from vehicles and industrial activities leads to the formation of ground-level ozone, worsening air quality.

Way Forward

  • Urban greening and heat-resistant infrastructure: Increasing the number of parks, green roofs, and vertical gardens, using cool roofs and pavements, etc.
  • Building Regulations: Implementing building codes and regulations that promote energy-efficient construction, use of heat-reflective materials, and proper insulation can significantly reduce UHI effects.
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