Mumbai’s nights are getting hotter: What a new study reveals about urban heat and the future of Indian cities

For decades, Mumbai has been known as the city that never sleeps. Increasingly, however, it is also becoming a city that never cools down. A new study published in the Journal of Urban Management by Harekrishna Manna et al. has found that nighttime temperatures across the Mumbai Metropolitan Region (MMR) have risen steadily over the past 25 years, with some densely urbanised parts of the city experiencing temperatures more than 5°C higher than surrounding areas. The research warns that Mumbai’s heat challenge is no longer merely a daytime phenomenon. The city is now accumulating and retaining heat after sunset, creating what scientists call a nighttime Urban Heat Island (UHI) effect—a condition with profound implications for public health, energy consumption, urban planning and climate resilience. 

The study arrives at a critical moment. Indian cities are experiencing more frequent and intense heatwaves, while urban populations continue to expand rapidly. As climate change raises baseline temperatures, the combination of global warming and local urban heat is creating a dangerous double burden for millions of residents. The Mumbai findings offer not only a warning but also a roadmap for cities across India and the Global South.

When the city does not cool after sunset

Heat is usually associated with the hottest part of the afternoon. But nighttime temperatures can be equally important because the body needs cooler conditions after daytime exposure to heat.

When temperatures remain high after sunset, people have less opportunity to recover from heat stress. This can increase the risk of heat exhaustion, cardiovascular illness and sleep disruption, particularly among older people, children and those living in densely built and poorly ventilated neighbourhoods.

The researchers analysed nighttime land surface temperatures across Mumbai from 2000 to 2025 using satellite observations and spatial modelling. The analysis found persistent nighttime warming across built-up areas, particularly in southern and central Mumbai. The urban and surrounding temperature difference reached as high as 5.07°C. The pattern was not uniform across the city.

Vegetated areas and water bodies showed relatively stable thermal conditions during the study period, while built-up areas showed a clear warming trend. Concrete, asphalt and dense clusters of buildings absorb heat during the day and release it gradually after sunset, allowing urban areas to remain warm for longer.

For Mumbai, this is particularly significant. The city has historically benefited from its coastal setting and sea breezes. Yet rapid urbanisation is altering the landscape through which air moves and reducing the influence of natural cooling systems.

What is driving Mumbai’s nighttime heat?

The researchers looked beyond changes in land cover to examine atmospheric, environmental and socioeconomic factors that influence nighttime temperatures.

One of the strongest associations was with nighttime illumination. Satellite measurements showed a strong relationship between artificial light and land surface temperature, with nighttime light intensity recording a correlation coefficient of 0.826 and emerging as the dominant factor across the modelling approaches used in the study.

The study also identified aerosol pollution as a significant contributor. Aerosol Optical Depth (AOD), an indicator of atmospheric particulate pollution, showed a positive relationship with nighttime temperatures. Pollutants trap outgoing radiation and reduce heat dissipation, creating conditions where cities remain warmer for longer periods.

The intensity of built-up areas was also important. The Normalised Difference Built-up Index, or NDBI, showed the influence of roads, pavements and buildings that absorb and retain heat more efficiently than natural surfaces.

As vegetation and open land are replaced by impervious surfaces, the city's capacity for natural cooling is reduced. The findings point towards a broader relationship between urban design and heat. How a city is built, illuminated and developed can influence how much heat it retains after sunset.

Water bodies are part of the city’s cooling system

Among the study's findings, the role of water bodies is particularly relevant to Mumbai.

Lakes, wetlands, rivers and coastal areas showed relatively stable thermal conditions over the 25 year period. Unlike concrete and other built surfaces, water bodies can buffer temperature fluctuations and moderate the surrounding environment.

This places Mumbai's water bodies within a broader understanding of urban climate resilience.

A lake is not only a water storage space. A wetland is not simply an undeveloped patch of land. A river is not only a drainage channel. These spaces can perform several functions at the same time, including supporting biodiversity, regulating water and contributing to local temperature moderation.

The study found that vegetation was also strongly associated with lower nighttime temperatures. Areas with greater vegetation cover benefited from shade and evapotranspiration, while the Normalised Difference Vegetation Index showed a significant negative relationship with nighttime temperatures.

Together, these findings point to the value of what is increasingly described as blue and green infrastructure: water bodies, vegetation, wetlands, rivers, coastal areas and connected green spaces that provide environmental functions alongside the services expected from conventional urban infrastructure.

For Mumbai, where land is scarce and property values are high, protecting these spaces can be difficult. Yet losing them can also reduce the city's ability to moderate the effects of rising temperatures.

Nature remains the most effective cooling infrastructure

If artificial surfaces and human activity are driving heat accumulation, the study also identifies what works best to counter it. Vegetation emerged as one of the strongest cooling factors across the Mumbai Metropolitan Region. Areas with higher vegetation cover consistently recorded lower temperatures, owing to evapotranspiration and shading effects. The Normalised Difference Vegetation Index (NDVI) showed a significant negative relationship with nighttime temperatures, demonstrating that green spaces continue to cool cities long after sunset.

Equally important are water bodies. The study found that lakes, wetlands, rivers and coastal zones displayed remarkable thermal stability over the 25-year period. Unlike built-up surfaces, water bodies buffer temperature fluctuations and help moderate surrounding thermal conditions.

Wind speed also played a crucial role. Areas with better airflow experienced lower nighttime temperatures because moving air enhances heat dispersal. This finding highlights the importance of preserving urban ventilation corridors, an aspect often neglected in high-density development.

These findings reinforce a growing body of global evidence showing that urban forests, wetlands, green corridors and blue-green infrastructure are not aesthetic luxuries. They are essential climate adaptation assets.

For Mumbai, where developable land is scarce and property values are among the highest in the country, protecting and expanding ecological infrastructure may prove politically challenging. Yet the study suggests that failure to do so will impose far greater economic and social costs in the future.

From heat mapping to heat governance

Perhaps the most significant policy implication of the research is that urban heat cannot be addressed through isolated interventions. It requires a systemic approach that integrates climate science into urban governance.

The study's advanced modelling framework revealed substantial spatial variation in the drivers of heat. Using Geographically Weighted Regression (GWR), the researchers demonstrated that factors influencing temperature differ significantly across neighbourhoods. The GWR model achieved an exceptional explanatory power, outperforming both conventional regression and machine-learning approaches.

This finding carries an important lesson for policymakers. There is no universal solution to urban heat. A neighbourhood dominated by industrial activity may require different interventions than a dense informal settlement or a commercial business district.

The authors recommend a portfolio of measures that can be integrated into urban planning and climate action frameworks. Urban greening should be prioritised through the creation of parks, green corridors, street tree networks, rooftop gardens and vertical vegetation systems. These interventions not only reduce temperatures but also improve air quality, biodiversity and public well-being.

Mumbai should also accelerate the deployment of cool roofs, reflective pavements and high-albedo construction materials. These technologies reduce heat absorption and can significantly lower surface temperatures in densely built environments. The city’s Development Plan 2034 and Mumbai Climate Action Plan already recognise the value of such measures, but implementation remains limited.

Another recommendation concerns lighting. Since nighttime illumination emerged as the strongest predictor of heat accumulation, municipalities should adopt smart lighting systems that optimise energy use while maintaining public safety. Adaptive LED systems, motion-sensitive lighting and energy-efficient urban design could reduce both heat emissions and electricity consumption.

Equally important is the protection of ventilation pathways. New developments should be evaluated not only for density and floor-area ratios but also for their impacts on urban airflow. Maintaining wind corridors can significantly improve nighttime cooling and reduce thermal stress.

The study also advocates for real-time heat monitoring using satellite imagery, remote sensing and GIS-based decision support systems. Continuous monitoring would allow city authorities to identify emerging hotspots, target interventions and assess policy effectiveness.

Most importantly, heat governance must address equity. Informal settlements, which often have limited green cover, poor housing materials and inadequate access to cooling, face the greatest risks. Community cooling centres, heat-resilient housing programmes, shaded public spaces and targeted adaptation measures should become central components of urban climate policy.

Mumbai's experience is increasingly becoming the experience of urban India. As cities expand and climate change intensifies, the challenge is no longer simply managing growth. It is managing heat. The new study provides compelling evidence that the solutions are already known: more vegetation, better urban design, cleaner air, smarter lighting and stronger climate governance. The question is whether policymakers will act before hot nights become the defining feature of life in India's cities.