What is the ‘Urban Heat Island Effect’?

The air above the urban cities is hotter than the air above surrounding rural areas. This phenomenon is called the ‘urban heat island’ effect. Rapid and unplanned urbanisation of cities and concomitant reduction in vegetation results in increased rise in temperature compared to non-urban areas.

With heavily built-up areas and concrete structures, most cities in India and in the world are warmer than surrounding non-urban areas due to the urban heat island effect. For instance, Delhi is 4-12°C warmer due to the urban heat island effect.

Sources: The Hindu and The Hindu

Primary Reasons for formation of Urban Heat Islands

• Densely constructed structures.
• Extensive traffic.
• High population density.
• Decrease in water bodies and natural vegetation.

The population of urban India is estimated to reach 600 million by the year 2030. Current statistics stand at 450 million. This alarming estimated rate of migration of almost 25 people per minute into our cities means the need for more construction, more traffic and subsequently, more paved areas and less tree cover. This uncontrolled expansion will only lead to further overheating, leading to extremely unhealthy living conditions for city dwellers.

Relation with agriculture and irrigation – Agriculture and irrigation are two dominant drivers of urban heat islands (UHIs) in India. In addition, significant presence of atmospheric aerosols—mainly pollutants—over urban areas can influence UHI. Moisture from irrigation canals and agriculture fields plays a key role in keeping rural areas surrounding cities much cooler than urban areas, thus, cities becoming heat islands. When the surrounding non-urban areas have no agriculture during summer, cities are relatively cooler during daytime. However, if the non-urban areas are under irrigated agriculture, cities are warmer than surroundings. This means UHI effect during day time is mainly driven by agriculture and irrigation. However, in the night-time, cities are significantly warmer than surroundings in both winter and summer seasons. Night-time urban heat is mainly driven by the amount of heat stored in buildings and other impervious surfaces.

The affect of wind – High temperature reduced the wind speed, which, in turn, decreased the ‘heat and pollutant flushing capacity’ of the region.

Source: Down to Earth

Solutions

• There is a need to focus on urban development that is smart, eco-friendly and sustainable to mitigate heat.
• Planting more and more trees would help to keep the temperature of urban areas in check.
• A major contributor to the urban heat island effect is the extent of impermeable paving in our cities including roads, side walks and other paved areas which absorb and radiate midday heat into the atmosphere during cooler evenings. If this paving was replaced by soil or grass the radiation will be much less. As an added benefit, rain water percolation into the ground would also increase.
• Building materials that absorb less heat and are sustainable that can reduce the amount of heating caused by stored heat must be used.

Sources: Down to Earth and Down to Earth