Urban Heat Islands — Why Cities Are Hotter Than the Countryside

What Is the Urban Heat Island Effect?

An urban heat island (UHI) is a metropolitan area that is significantly warmer than its surrounding rural areas. The temperature difference is most pronounced at night, when cities can be 5-10°C warmer than the nearby countryside.

Why Cities Are Hotter

1. Dark surfaces: Asphalt roads and tar roofs absorb solar radiation and re-radiate it as heat
2. Reduced vegetation: Trees and grass cool air through evapotranspiration; concrete does not
3. Waste heat: Cars, air conditioners, and industry all release heat
4. Canyon effect: Tall buildings trap heat and block wind, reducing cooling
5. Less evaporation: Impermeable surfaces prevent water from pooling and evaporating

The Numbers

Health Impact

UHIs amplify heat waves, increasing heat-related illness and death. The most vulnerable — elderly, children, outdoor workers, and people without air conditioning — are disproportionately affected.

Solutions Being Implemented

• Green roofs and walls: Vegetation cools buildings through shade and evapotranspiration
• Cool roofs: Light-coloured or reflective roofing materials
• Urban tree canopy: Shade from trees can reduce surface temperatures by 10-15°C
• Permeable paving: Allows water to soak in and evaporate, cooling the air
• Urban parks: Large green spaces act as cooling islands within the city

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