How concretising cities are becoming heat islands

The monsoon has currently dissipated intense heat across India’s growing cities, but temperatures are rising and will continue to climb because of the way urban areas are expanding.

With trees, lakes and open spaces replaced by roads, expanses of concrete with closely spaced multi-storeyed buildings - often in violation of zoning and setback laws - Indian cities are turning into “heat islands”, according to an IndiaSpend review of scientific studies in five cities. 

A clear trend is evident: The difference between the daytime maximum and nighttime minimum daily temperatures - the diurnal temperature range (DTR) - is steadily declining. This indicates that concretising cores of cities are retaining heat, even as temperatures rise in formerly cooler outskirts, as they, too, urbanise. A higher range of temperature indicates greater cooling.

* In Delhi, over a decade to 2011, the temperature range declined by more than 2 deg C, one of India’s strongest heat-island effects.

* In Chennai, the morning temperature at the city centre is between 3 to 4.5 degree Celsius higher than its greener fringes.

* In Thiruvananthapuram, when a cool, evening breeze blows, the greener rural areas cool by 3.4 degree Celsius, the city areas by half as much.

* In Guwahati, city areas are warmer by 2.13 degree Celsius than the peripheries during the day and by 2.29 degree Celsius at night.

* In Kochi, a canyon-like effect of buildings funnels heat into the city, creating a “heat island” that makes the centre 4.6 degree Celsius warmer in winter and 3.7 degree Celsius in winter.

Heat islands are created by a combination of design, construction material, and the environment. Closely built buildings form canyons that trap heat reflecting from their walls. Air-conditioning vents, especially in narrow alleys, further warm up buildings and nearby areas.

Trees, shrubs, grass, and soil absorb heat and cool the land, but since these are increasingly absent in Indian urban design, and what existed is being cleared, what’s left is concrete and asphalt, which soak in and intensify the day’s heat, staying hot for many hours at night.

Things are set to worsen, as IndiaSpend reported in March 2016. Kolkata’s tree cover fell from 23.4 percent to 7.3 percent over 20 years, as the built-up area rose 190 percent. By 2030, vegetation will be 3.37 percent of Kolkata’s area. 

Ahmedabad’s tree cover fell from 46 percent to 24 percent over 20 years; the built-up area rose 132 percent.  By 2030, vegetation will be three percent of Ahmedabad’s area. 

Bhopal’s tree cover fell from 66 percent to 22 percent over 22 years. By 2018, it will be 11 percent of the city’s area. Hyderabad’s tree cover fell from 2.71 percent to 1.66 percent over 20 years. By 2024, it will be 1.84 percent of the city’s area.

On an instinctive, tactile level, you can feel the effects of heat islands in cities dissipate and the temperature drop when you pass a rare, green expanse, such as Delhi’s Lodhi Gardens or Jawaharlal Nehru University and Bengaluru’s Indian Institute of Science.

Here are the details of what’s happening in five cities:

Population: 11 million. Area: 1,484 sq km.

As Delhi’s metropolitan population grew by 20 percent between 2001 and 2011, the difference between its maximum and minimum temperatures flattened out, a 2015 paper reported. Wider temperature variations - meaning cooler areas - were evident in urban villages and open areas. Northwest and Southwest Delhi, areas of intense growth, registered the largest fall in temperature variation - between 2.5 to 4 degree celsius.

Population: 4.68 million. Area: 426 sq km.

During the hottest period of the year in late May - called agni natchatiram (star of fire) - the temperature in the commercial complexes and densely populated residential expanses of central and north Chennai registered the most variations; the outskirts were cooler, a 2016 paper said. Compared with observations in 1991 and 2008, heat islands have grown more intense, with a 1.5 to 2 degree Celsius variation with the peripheries in 1991 growing to 2.53 degree Celsius by 2008. Wherever there was vegetation, a cooling effect was evident.

Population: 0.95 million. Area: 216 sq km.

The creation of heat islands in Guwahati indicates that India’s smaller cities, too, have areas of growing heat, as they concretise. A daytime heat-island effect left core city areas up to 2.12 degree Celsius warmer than the outskirts and 2.29 degree Celsius at night, according to a 2014 study, illustrating how heat once absorbed by roads and buildings intensifies. 

Population: 0.61 million. Area: 95 sq km.

Thanks to tall buildings funnelling and focusing heat, the heat-island effect was stronger early morning than late evening, stronger in winter than summer, a 2014 paper said. Heat islands had the greatest impact in what was termed “compact mid-rise zones” close to the city centre, where average building heights range from nine to 24 m. The most intense cooling was apparent in open and sparsely built areas in all seasons. Pre-monsoon rains and overcast skies weakened Kochi’s heat-island effect.

Population: 1.96 million. Area: 215 sq km.

Like Kochi, Kerala’s capital reported a 2.4 degree Celsius higher temperature at the city centre, with areas of densely arranged low-rise (one- to three-storey) and high-rise (three- to eight-storey) buildings the warmest, according to a 2014 paper. The maximum evening temperature drop of 3.4 degree Celsius was reported in rural areas, a degree more than city areas. The city is cooled by a sea wind between 8 and 9 pm, but the wind was blocked in areas with dense buildings, keeping temperatures high.

Variations of these trends were seen in other cities, and it was evident that traditional building material cooled homes better. In Vellore, Tamil Nadu, roofs of thatch had the best cooling effect, a 2015 paper reported.