Urban Heat Island Effect

The urban heat-island effect is the tendency of a city or densely built-up area to remain warmer than its surrounding rural landscape. It results from changes in land cover, urban geometry and human activity that increase heat absorption, reduce natural cooling and delay the release of stored heat, especially after sunset.

Replacement of natural surfaces

Urbanisation replaces vegetation and moist soil with concrete, asphalt and other impervious surfaces. These materials commonly absorb and store substantial solar energy, while the loss of vegetation reduces shade and evapotranspiration, through which water normally uses heat while evaporating.

• Dark roofs and paved surfaces generally reflect less solar radiation than lighter surfaces and therefore become hotter.
• Impervious surfaces rapidly drain rainwater, leaving less moisture available for evaporative cooling.

Heat trapping by urban form

Closely spaced, high-rise buildings create urban canyons. Their walls and roads exchange radiation repeatedly, store heat during the day and release it slowly at night; a restricted view of the sky also limits night-time radiative cooling.

• Dense construction can obstruct wind flow and reduce the removal of warm air.
• A low sky-view factor limits the amount of long-wave radiation that can escape directly towards the open sky.

Human-generated heat

Transport, industries, buildings and energy use release anthropogenic heat directly into the urban atmosphere. Air conditioners cool indoor spaces by transferring heat outdoors and also consume energy, thereby adding to local heat loads.

• Dense populations concentrate vehicles, machinery, lighting and other heat-producing activities in a small area.
• The combined effect of stored heat and human-generated heat often makes the heat island particularly evident at night.

Conditions that intensify the effect

The intensity varies with season, weather and city design. Clear, calm conditions favour heat accumulation, whereas strong winds and cloud cover can reduce the temperature contrast between urban and rural areas.

• Sparse tree cover and limited open water weaken local cooling.
• Compact development, extensive paving and poor ventilation strengthen heat retention.