Why Mumbai’s becoming a giant, urban heat island

Shanties often use materials like tin, asbestos, or plastic for roofing, which absorb and retain heat throughout the day. Without proper insulation or ventilation, these roofs amplify temperatures inside the homes, making slum areas significantly hotter than their neighbouring regions. This intensifies the heat stress on residents, especially during peak summer months. File pic

Rishabh Rathaur, an IT engineer, commutes daily from Powai to the data science company he works at in JB Nagar, Andheri East. The nearly-30-minute motorcycle ride is marked by potholes, traffic, and dust. However, after a long day of troubleshooting IT issues at work, he looks forward to one thing on his way back home - as soon as he crosses the Seepz industrial area, and drives onto Jogeshwari-Vikhroli Link Road during the nighttime, the temperature drops by at least two-three degrees Celsius.

In a city where winters bring only a short respite from the year-round heat - temperatures have surpassed 35°C in mid-January itself - this pocket of cool air during his commute is something that Rathaur can look forward to every day.

"During the winters, the motorcycle ride is actually cold enough to cause a little bit of a shiver," he says. This, of course, only holds true when the road is traffic-free. The Powai resident, who stays near IIM Mumbai, says, "The temperature drops even more when I go towards Aarey Road through Filter Pada [because of the greenery]."

Rajashree Kotharkar and Sree Kumar Kumaraswamy

Many commuters across Mumbai shared similar experiences with this reporter. Those travelling from the greener, more openly spaced Navi Mumbai to the concrete pocket of BKC, and similarly from Mulund to Dadar, also notice temperature changes, usually finding it hotter as they approach a more crowded suburb. These anecdotes highlight the temperature differences across various suburbs, with dense concretisation and a lack of green, open spaces turning many suburbs into urban heat islands.

The link between increased temperatures and dense development is apparent in the daily readings from the India Meteorological Department's (IMD) two stations in Colaba and the other in Santacruz, which record temperatures for the Mumbai City and the Mumbai Suburban districts, respectively. On average, there is a two-degree Celsius difference between these two locations, just 25 km apart.

According to a 2015 study, titled Heat Islands over Mumbai as Revealed by Auto-Recorded Thermograph Data, the temperature differences between Colaba and Santacruz arise from both urbanisation and geography. Colaba, being densely developed with minimal open spaces, retains more heat. The temperature here is also moderated due its proximity to the coast, so there are fewer fluctuations from day to night. In simple terms, Colaba stays hot, day or night. In contrast, Santacruz - which developed much later - has more open spaces, greenery, as well as building height restrictions due to the airport being located there. This results in it getting hotter faster during the daytime due to increased absorption of solar radiation, but it also results in greater nighttime cooling. So, unlike the island city, the suburbs tend to cool down better at night.

An aerial view of South Mumbai's densely packed buildings forming "urban canyons". These narrow spaces between high-rises restrict wind flow, creating pockets where pollutants like PM10 and PM2.5 accumulate, significantly contributing to poor air quality and health concerns for residents. This effect increases urban heat. Pic/File Photo

But these aren't new phenomena. Mumbai's weather began changing back in the '90s, coinciding with a construction boom across the city. Even as climate change makes the planet hotter, metro cities such as Mumbai are bearing the brunt of the impact due to the urban heat island effect (UHIE), where densely developed areas witness higher temperatures due to more buildings, infrastructure and human activities that absorb and retain more heat. With summer temperatures hitting new records every year, Mumbai is particularly vulnerable to extreme heat conditions in the coming future.

The National Disaster Management Authority (NDMA) recently pointed out that Indian cities have become "heat traps" due to unbalanced development, often at the cost of shrinking green cover and water bodies. As more people move into the newly developed areas, it results in greater vehicular use, air conditioning, industrial processes, all of which result in greater emissions, fuelling further heat. A 2024 NDMA report pointed out that UHIE can make cities 2°C to 9°C hotter than surrounding peri-urban and rural areas because the densely concentrated buildings and roadways absorb and store solar radiation.

City-based urban planner and architect, Lubaina Rangwala, who is also associate director at World Resources Institute (WRI), says, "The rapid urbanisation of Mumbai has turned it into a furnace." She explains, "Everywhere you look, we're pouring concrete - buildings, roads, and even public spaces. The problem with concrete is its inability to cool down effectively. It traps heat during the day and releases it at night, so there's no real break from the rising temperatures."

The impact is quite obvious. In what should have been the peak of this winter season, the city experienced its hottest December day in the past 16 years, on December 4, with temperatures soaring over 37°C. Just days later, on December 9, it saw its coldest day since 2015 at 13.7°C. To put the weather anomaly into further perspective, 2024 was also reported as the hottest year since 1901.

While UHIE is evident on a city-wide scale, it also presents in smaller pockets, often in densely packed slum habitations. In 2020, WRI came out with a report stating that slums in Mumbai are 5-6°C warmer than the neighbouring housing societies. "Many of these areas have tarpaulin or metal roofs and hardly any vegetation. These materials are highly conductive and trap heat, making these neighbourhoods unbearably hot. With no open spaces or greenery to provide shade or cooling, residents have no escape from the heat, even when the temperature is more pleasant otherwise," says Rangwala.

Despite urban heat traps posing a very real risk to the Mumbai population, there has not been much in-depth research on it so far.

Urban climate researcher and academician Rajashree Kotharkar, professor at Visvesvaraya National Institute of Technology, Nagpur, has been studying urban heat islands for decades and is currently researching how to build heat-resilient cities. She says, "Urban heat islands have long been understood to be a result of urbanisation, but the behaviour of these heat islands varies significantly depending on the time of day also."

Kotharkar, who is now studying the heat trap phenomenon in coastal cities, including Mumbai, explains: "In a city like Mumbai, closely packed urban areas - such as the central and southern parts of the city - tend to retain heat during the night due to limited exposure to the sky. These areas experience higher nocturnal heat stress, as they cannot cool down efficiently."

"In contrast, sparse areas, such as those on the outskirts or in city with more open spaces, have a greater potential for radiative cooling, allowing them to cool down faster after sunset," she adds.

However, the pattern shifts during the day. "Sparse areas, which receive more direct solar radiation, heat up more quickly compared to compact areas that might be shaded or obstructed by buildings. This dynamic leads to the formation of multiple microclimates within the city."

Kotharkar explains that in Mumbai, there are both hot and cool islands with varying intensities, especially when comparing different suburbs - such as the island city versus northern suburbs. "While much of the research on UHIE has relied on satellite data to analyse land surface temperatures, I believe that a more detailed ground-level study with on-site stations would reveal even more nuanced findings about these microclimates across urban spaces in Mumbai, as in, how specific areas within Mumbai experience varying levels of heat," she says.

Another factor that poses a major challenge in combating heat in Mumbai is vegetation. "The lack of green areas," explains architect Shivani Singh, "is a critical factor exacerbating the heat island effect in Mumbai. In areas with trees or plants, the process of evapotranspiration can help reduce temperatures, but here, it's virtually non-existent," she says. The Ahmedabad-based architect has done extensive projects in South Mumbai over the years.

The issue, according to Singh, isn't just about the materials used. "It's the way the city is designed - or rather, the way it's expanding without much thought about its environmental impact," she says.

"We have covered every possible inch with high-conductivity materials. Even in areas where greenery could have been preserved, it has been replaced with construction. The city has very little room to breathe. During summers, we observed that many people were walking below metro bridges while the sidewalks were empty because there was no shade. Instead, there were trees below metro pillars for beautification. This shows the misplaced focus on using greenery effectively. Even the plants used for beautification could be chosen to produce more oxygen. Instead, they are merely ornamental, especially around Terminal 2 of Mumbai airport," she says.

Mumbai's iconic skyline recently choked on smog, which was so dense it shrouded even megastructures like the Bandra-Worli Sea Link and towering skyscrapers. Sree Kumar Kumaraswamy, Program Director (Clean air action, sustainable cities and transport) at WRI India, explains, "There is an often-overlooked consequence of the UHIE in cities like Mumbai, where high temperatures exacerbate the formation of harmful pollutants. So, in a way, heat and AQI are somewhat related. As temperatures rise, the chemical reactions between volatile organic compounds and nitrogen oxides in the atmosphere interact more vigorously, forming ozone. This creates more pollution, which leads to health problems, particularly for people with respiratory issues."

On top of that, there is an "urban canyon" effect, too. "In densely built areas like South Mumbai, buildings are often packed closely together with little space in between. This creates urban canyons where the flow of wind is restricted, trapping pollutants like particulate matter (PM10 and PM2.5) between buildings," he says.
Professor Kotharkar, too, agrees that urban heat and air pollution are closely linked. However, she also highlights that it has not been studied extensively.

Building up heat

2°C
Avg temperature difference recorded at IMD stations at the warmer Santacruz and the warmer Colaba

5°C
Rise in temperature in slums compared to other residential areas
'Source: World Resources Institute

9°C
Temperature rise in cities due to the urban heat island effect
'Source: National Disaster Management Authority

17°C
Rise in temperature at the surface of glass buildings
'Source: NEERI