Urban heat islands

Source: By Anjali Marar, Sonal Gupta: The Indian Express

Several parts of the country are reeling under heat wave conditions. Cities, especially, are a lot hotter than rural areas. This is due to a phenomenon called an “urban heat island”.

On 15 May 2022, for instance, two areas in Delhi recorded temperatures close to 50 degrees Celsius. Mungeshpur recorded a temperature of 49.2 degrees Celsius, while at Najafgarh, it was 49.1 degrees. The maximum temperature in Gurgaon, meanwhile, soared to 48.1 degrees Celsius, while in Noida, it was 47.1 degrees. Temperatures around these cities, however, were not as high.

So, what are urban heat islands, and why are they caused? Are there ways to mitigate them?

What is an urban heat island?

An urban heat island is a local and temporary phenomenon experienced when certain pockets within a city experience higher heat load than surrounding or neighbouring areas on the same day.

For example, a greener locality like Pashan in Pune often records cooler temperatures than urban areas like Shivajinagar, Chinchwad or Magarpatta.

The variations are mainly due to heat remaining trapped within locations that often resemble concrete jungles. The temperature variation can range between 3 to 5 degrees Celsius.

Why are cities hotter than rural areas?

Rural areas have relatively larger green cover in the form of plantations, farmlands, forests and trees as compared to urban spaces. This green cover plays a major role in regulating heat in its surroundings.

Transpiration is a natural way of heat regulation. This is the scientific process of roots absorbing water from the soil, storing it in the leaves and stems of plants, before processing it and releasing it in the form of water vapour.

On the contrary, urban areas lack sufficient green cover or gardens and are often developed with highrise buildings, roads, parking spaces, pavements and transit routes for public transport. As a result, heat regulation is either completely absent or man-made.

Physics teaches us that black or any dark coloured object absorbs all wavelengths of light and converts them to heat, while white reflects it.

Cities usually have buildings constructed with glass, bricks, cement and concrete — all of which are dark-coloured materials, meaning they attract and absorb higher heat content.

Thus forms temporary islands within cities where the heat remains trapped. These are not the typical islands around water bodies, but urban heat islands that record higher day temperatures than other localities.