Systematic review of UHI research on over 40 Indian cities (2005–2023).

The increased temperature of a city as compared to the surrounding rural settlements due to the presence of impervious surfaces and human activities, which results in the generation of a few or several heat pockets, is known as the UHI effect (Zhang et al., 2009). Luke Howard is the pioneer of urban climatic studies. Many scientists succeeded his study and carried out various experiments, which led to an understanding of the advanced urban environment (Heisler and Brazel, 2010; Stewart, 2019). UHI effect is one of the consequences of anthropogenic activities such as the prevalence of dark, non-reflective surfaces in urban settlements (Islam et al., 2024).

Urbanization and rapid economic development have led to significant changes in the physical environment of cities, which in turn has contributed to urban climate change (Kim and Brown, 2021). It is estimated that in the next four decades, 80% of the global population will live in urban areas (United Nations, 2014; Zhang, 2016; Hoornweg and Pope, 2017). Unbalanced urban expansion between cities due to a significant population influx has resulted in a marked difference in the urban form and services of the core and periphery of cities. Key difficulties include the expansion of slums, poor solid waste management, declining per capita water supply, erratic water quality, insufficient sewage coverage, and decreasing ambient air (Nandi and Gamkhar, 2013; Ghosh and Kansal, 2014). Rapid urbanization alters land use land cover (LULC) of an area which greatly influences the UHI; particularly with reference to reduced vegetation cover (Mohan and Kandya, 2015; Hassan et al., 2021). Hence, LULC change is one of the most effective ways to predict the extent of the effect of urban agglomeration on UHI (Gohain et al., 2023; Das et al., 2024). According to the Intergovernmental Panel on Climate Change (IPCC, 2021), since the 1900s, the global surface temperature has increased by 0.74 °C and will continue to increase under all the current emission scenario up to 1.5 °C. Although the two phenomena are affected by each other, the urban climatic environment is directly and densely affected by global climate change (Kuttler, 2008; Misslin et al., 2016). Extreme heat wave events that are observed in urban areas are the result of this interaction (Basara et al., 2010; Rizvi et al., 2019). The higher temperature in urban areas compared to surrounding sub-urban and rural areas prompts increased energy consumption and health-related issues (Imam and Banerjee, 2016). Human influences have warmed the climate at an unprecedented rate as it has strengthened since Assessment Report 5 (AR5) (IPCC, 2021). Heat stress is perhaps the biggest climate threat to human health (Saeed et al., 2021; Jabbar et al., 2023). UHI has increased heat exposure numerous times particularly in urban areas (Yadav et al., 2023; Boyaj et al., 2024). Urbanization demands enhanced built-up activity in order to ensure residential areas and infrastructure for the rapidly rising population; thus, bringing about changes in the atmospheric conditions (Landsberg, 1981; Bai et al., 2017; Upreti et al., 2024). This leads to detrimental environmental degradation such as reduced precipitation, decreased soil infiltration capacity, increased runoff, decreased water table, and changes in surface albedo, which further imbalance the earth’s radiation (Veena et al., 2020; Kabisch et al., 2023). Emitted greenhouse gases (GHGs) are trapped in the atmosphere as a result of this disruption in the earth’s radiation and the growing number of automobiles and industries in urban regions. This exacerbates the UHI effect in densely populated places (Ramanathan and Feng, 2009; Irfan et al., 2024). Climate change and UHI affect each other by interacting and producing heat waves during extremely hot weather, exposing citizens to heat stress. Studies suggest synergy between climate change and UHI (Santamouris et al., 2015; Singh et al., 2020). The UHI effect poses a risk to human health (Cichowicz and Bochenek, 2024).

To create more resilient and sustainable city environments—cool roofs, urban greening (green roof, community parks, urban forestry), sustainable building designs, reduced impervious surface, innovative technologies (radiative paints, smart windows and BIPVs) and policy driven actions (heat action plan and certifications) are few of the strategies that work together to lower the urban temperature and improve the air quality as well. These are discussed in detail in the later section of the review.

Metropolitan cities such as Delhi, Mumbai, Kolkata, Lucknow, Bengaluru and Chennai with high and compact populations have been extensively studied for UHI status in India (Pandey et al., 2009; Pandey et al., 2012; Mohan et al., 2012; Yadav et al., 2020; Gautam and Singh, 2018; Mohan et al., 2020; Mukherjee and Debnath, 2020; Mishra and Mathew, 2022; Shahfahad et al., 2023; Khan and Chatterjee, 2016; Bhowmick et al., 2023; Halder et al., 2021; Singh et al., 2017; Sarif and Gupta, 2019; Shukla and Jain, 2021; Verma and Garg, 2021; Ramachandra and Kumar, 2010; Sussman et al., 2019; Naidu and Chundeli, 2023, Devadas and Rose, 2009, Jeganathan et al., 2014, Amirtham, 2016; Kesavan et al., 2021, Rajan and Amirtham, 2021; Muthiah et al., 2022). Smaller cities and towns have received less research attention, leaving the UHI pattern and mitigation in such areas underexplored. Formation of heat pockets were concentrated towards the city centre. Temperature differences between the core and suburbs of cities range between 2 °C and 10 °C (Memon et al., 2009; Pandey et al., 2012; Sharmin et al., 2015; Vardoulakis et al., 2013; Tam et al., 2015; Rajan and Amirtham, 2021; Siddiqui et al., 2021; Islam et al., 2024). Diurnal studies revealed that commonly observed differences in temperature in the UHI effect are 10 °C during the night and 3–4 °C during the day (Tam et al., 2015; Azevedo et al., 2016; Mathew et al., 2018). Maximum UHI is recorded within a few hours of the sunset as the built-up structures release their stored heat from the daytime during this hour (Tzavali et al., 2015; Heisler and Brazel, 2015; Lai et al., 2018).

From social and economic viewpoints, it is observed that low-income residents are more likely to dwell in areas with high UHI intensity and are more subjected to heat-related severe problems (Bradford et al., 2015; Debnath et al., 2023). Factors like spatial factors, temporal factors, anthropogenic factors, and geographic factors supports UHI intensities (Deilami et al., 2018). The physical characteristics and layout of urban areas, such as the amount of green space, building density, and land use patterns are referred to as spatial factors (Aijaz, 2016). Temporal factors, on the other hand, are related to variations in UHI intensity due to changes over time, such as seasonal shifts and alterations in daily weather patterns (Siddiqui et al., 2021). Analysis of the diurnal, seasonal and annual changes and trends in land surface temperature and surface UHI intensity (SUHII) have been extensively examined for Indian cities like Lucknow, Pune, Kolkata and Jaipur were studied. These investigations reported an overall maximum average UHI intensity of 7.86 °C in Jaipur. The nighttime mean annual SUHI intensity ranged between 1.34 °C and 2.07 °C (Mathew et al., 2017; Siddiqui et al., 2021). Human activities such as deforestation and emissions of greenhouse gases (GHGs) that contribute to UHI formations are considered anthropogenic factors (Feng et al., 2012; Nica et al., 2019). Not only this, various geographical factors need to be taken into account while studying UHI (Grover and Singh, 2015; Mishra and Mathew, 2022). These factors might include the existence or absence of hilly terrains, the wind flow in the region as well as the presence of water bodies in the vicinity of urban areas (Collier, 2006).

Spatial data is used to study the pattern of urbanization and its development as well as its impact on UHI formation as built-up surfaces directly influence land surface temperature (Morabito et al., 2016; Mukherjee et al., 2017). Temporal data are crucial for monitoring the intensity and variability of UHI as it helps to study the phenomenon for various time scales and hence helps in understanding the full dynamics of temperature patterns and its driving forces (Tan, 2024). This review includes studies based on Remote sensing analyses, observational studies using instrument and fixed station, GIS-based studies, simulations modelling. The goal is to develop an insight into UHI and the methodologies used to study its factors affecting UHI, its impact and mitigations strategies.

There is growing body of literature suggesting that microclimate is directly affected by the exacerbation of global temperatures (Yang and Chen, 2020). UHI mainly increases the urban environmental temperature and air pollution, in addition; it highly impacts the energy consumption, heat mortality and reduced health quality (Liu et al., 2021). To address the adverse effects of UHI, mitigation measures must be implemented.

In India, summer temperatures can reach up to 50° C, and the average annual global irradiance received by most parts of the country is 5,000 Wh/m²/day or more (Ramachandran and Jain, 2011). Thus, India being a tropical climatic and rapid urbanizing country offers a great potential for implementing environmental and non-environmental measures to counter the UHI effect. Various studies have proposed different mitigation strategies, categorizing them into roof, non-roof, and covered parking strategies (Zhang et al., 2022a, 2022b; Rajagopal et al., 2023; Elhabodi et al., 2023). Broadly, these measures may be categorized into passive cooling, active cooling, and urban planning interventions.

Environmental measures such as increasing urban vegetation by creating green walls, green roofs, green parks and urban forestry is one of the best passive cooling strategies besides utilizing cool materials such as reflective roofs and pavements to minimize surface temperatures (Zhang et al., 2022a, 2022b). Green roofs are partially or fully vegetated roofs and growing medium, installed over waterproof membrane (Khare et al., 2021). Expanding green areas not only ameliorates evaporation capacity, which lowers near-surface air temperatures, but also counteracts the UHI effect, improves poor air quality and regulates temperature (Santamouris and Osmond, 2022), as shown in Figure 3. Reducing impervious surfaces also helps lower near air and surface temperatures, thus, aiding in UHI mitigation. Sustainable roof solutions like green roofs, which involve insulating spaces with a vegetative layer on the rooftop, have proven effective. Studies have demonstrated that installing green roofs can reduce indoor temperatures by upto 5.1 °C when combined with solar thermal shading (Kumar and Kaushik, 2005); depending on insulation, roof design, and other building-specific factors. It contributes to lower energy costs for cooling, improved comfort for building occupants, and a decrease in the overall UHI effect as the vegetation cover over the rooftop reduces the surface temperature and thereby reduces the temperature of the building interior (Figure 5). This happens due to evaporation and increased surface reflection.

Another category of passive strategies are cool pavements and roofs. Compared to traditional materials, these surfaces are made to reflect more sunlight and absorb less heat. Applying high-albedo materials or reflective coatings can drastically lower surface temperatures, which will lessen heat storage and radiative heating of the urban atmosphere. According to studies, depending on local climate conditions and urban morphology, the widespread use of cool surfaces can reduce urban temperatures by 1–3 °C (Larsen, 2015; Zhou et al., 2022).

There are several urban greening initiatives taken by Indian government such as National Urban Greening Policies, development of Green Corridors, and Smart Cities Mission with a motive of enhancing green cover of cities, creating sustainable and resilient urban environment that withstand the challenges of urbanization and climate change (Fatima et al., 2018; Ramaiah and Avtar, 2019; Shah et al., 2021; Das et al., 2022; Aman et al., 2022). These initiatives prove that coordinated community participation and innovative solutions can help in addressing the challenges posed by UHI effect.

Other than this the most effective way is UHI simulation as it helps determine the effect of urbanization on local climate and plan designed targeted mitigation strategies.

Non-environmental measures focus on reducing electricity usage to decrease energy consumption through energy-efficient building designs, promoting economical and fuel-efficient transportation modes, encouraging the use of public transport, and implementing urban planning policies that prioritize non-motorized transport and create more pleasant and healthier urban spaces (Rajagopal et al., 2023). Innovative technologies include reflective and radiative cooling paints, smart materials, building-integrated photovoltaic (BIPV) and thermochromic smart windows (Aburas et al., 2019; Cheela et al., 2021; Elhabodi et al., 2023). Radiative cooling paints keep the surface cooler by reflecting more sunlight and radiating absorbed heat back into the atmosphere. Thus, it decreases the need of air conditioning as it helps in lowering down the indoor temperature (Chen and Lu, 2020; Khare et al., 2021; Cheela et al., 2021). For instance, roof whitening has been shown to reduce energy needs by 8.8%, with a research study indicating a 49% reduction in heat flow from the roof to underlying spaces in different climatic zones of India, such as hot, dry, humid, and warm zones (Arumugam et al., 2015; Rawat and Singh, 2022).

Smart materials such as thermochromic material changes its color according to the temperature which in turn reduces energy required for heating or cooling purpose (Aburas et al., 2019; Santamouris and Yun, 2020; Zhang et al., 2022a, 2022b). For Kolkata metropolitan region, thermochromic materials had been an excellent heat mitigation technology (Khan et al., 2022). Similar results were obtained in the study conducted for different climatic zones which documented for the energy saving of 15 to 35% varying for hot-dry to tropical climatic conditions (Rawat and Singh, 2022).

BIPVs are solar panels integrated into windows, facades and roofs which reduces the dependency of external non-renewable energy source of power generation; by generating on site energy generation (Skandalos et al., 2022; Elhabodi et al., 2023). According to the literature available, BIPVs installation experiments has been conducted in India for both moderate and cold climatic regions—Bangalore, and Srinagar respectively, to test the energy consumption differences between the conventional and BIPVs installed system (Agrawal and Tiwari, 2011). Similar studies have been conducted in different climatic zones of India such as Mount Abu, Jodhpur, Mumbai, New Delhi, Kolkata, and Bhopal (Ranjan et al., 2008; Rai et al., 2021). The Indian government has recognized the PV solar energy capabilities as the future energy security of India and thus targeted the goal 100 GW till 2022 which is progressing significantly to reach its target in the coming years (Hairat and Ghosh, 2017).

Though these mitigation techniques have quantifiable advantages; there are several obstacles to their practical deployment. In addition to reducing the UHI impact, the majority of environmental and non-environmental interventions also reduce energy consumption, air and water pollution, and greenhouse gas emissions (Akbari, 2011; Santamouris, 2014; Aram et al., 2019; Lai et al., 2019). Besides offering thermal insulation in both summer and winter, they help prolong the roof’s lifespan by shielding it from inclement weather, UV rays, and temperature drops (Khare et al., 2021; Sharma et al., 2021). Due to their high maintenance and life cycle costs, all these advantages have drawbacks (Akbari and Matthews, 2012; Seifhashemi et al., 2018). Another difficulty is choosing a roof type based on the climatic zones (Khare et al., 2021).

Heat action plan included both environmental and non-environmental measures to combat the UHI effect and mitigating the health impact of extreme heat, implemented in Ahmedabad, Gujarat. It was South Asia’s first Heat Action Plan (Nastar, 2020). This plan helped in improving urban resilience to heat waves and reducing heat related mortality (Hess et al., 2018; Golechha et al., 2021). As cities grapple with issues like UHI effect, air pollution and rapid urbanization, the initiatives like urban greening and green corridors comes into picture to create more sustainable and liveable areas. Tree plantation drives, green roofs and walls, urban parks and gardens, urban farming are a part of urban greening measures taken by the Indian Government (Figure 6).

Increasing infrastructure in cities demands sustainable building designs. To ensure the design sustainability, assessments like Green Rating for Integrated Habitat Assessment (GRIHA) and Leadership in Energy and Environmental Design (LEED) have come into operation. These are national rating systems for green buildings developed by The Energy and Resources Institute (TERI) and U.S. Green Building Council (USGBC) respectively. These are adopted by different bodies in India, Ministry of New and Renewable Energy (MNRE) and Indian Green Building Council (IGBC) correspondingly (Arya and Sharma, 2022). Both target on the factors like energy conservation, waste reduction, health and well-being, and environmental protection (Bahale and Schuetze, 2023); hence, encouraging innovative sustainable practices and technology. Key features and common benefits of GRIHA and LEED certification system is depicted (Figure 7).

India’s efforts to reduce UHI are still in the nascent state (Sharma et al., 2021). Government is providing rebates and incentives under GRIHA, LEED, IGBC rated green buildings (USGBC, 2014; TERI, 2020; Arya and Sharma, 2022; Kadiwal, 2025). Government initiatives have successfully raised awareness and started significant legal and infrastructure improvements, but institutional, financial, and technological constraints still prevent them from reaching their full potential (Khare et al., 2021; Sharma et al., 2021). More integrated policies, funding sources, capacity training, and scaled initiatives that correspond with regional contexts are needed to address these constraints (Seifhashemi et al., 2018; Kadiwal, 2025).

In conclusion, to properly address the issue of UHI, both environmental and non-environmental measures must be put into place. These can significantly reduce the urban temperature and improve overall urban climate resilience. As described above, the implementation of environmental initiatives underscores the importance of a multifaceted approach to urban planning and energy management in combating the UHI effect and fostering sustainable urban development. Heat Action Plan (HAP), urban greening and green corridors benefit the environment by decreasing greenhouse gas emissions and the urban carbon footprint.

The systematic review on UHIs (UHIs) in this manuscript offers a comprehensive examination of the factors driving UHI formation, the wide-ranging impacts, and potential mitigation strategies. The main causes are identified as anthropogenic factors, including impervious surfaces, densely populated areas, and decreased vegetation cover. In addition to raising local temperatures, these factors also increase energy use, air pollution, and health hazards, which disproportionately impact vulnerable groups.

A critical review of this manuscript reveals UHI in various geographic areas of the world; however, it falls short in evaluating the socio-economic aspects of UHI impacts, especially in low-income urban areas with limited adaptation capacity. Furthermore, a lot of mitigation techniques-like cool pavements, urban forestry, and green roofs—are discussed in the manuscript. Still, it requires a careful assessment of their long-term feasibility or cost-effectiveness in light of climate change.

The review correctly highlights the necessity of integrated urban planning, to ensure functions for social equity and economic viability along with environmental protection to create sustainable, resilient, and inclusive cities; but it frequently addresses individual solutions rather than advocating for cross-sectoral policies that integrate social justice and climate resilience.

In addition to taking local cultural, economic, and environmental contexts into consideration, future research should place a high priority on participatory approaches that involve communities in the co-development of adaptive solutions.

This map (Figure 8) illustrates the UHI Intensity (UHII) across India based on the Köppen climate classification. The climate zones that are being taken into account are Tropical (Am, Aw), Arid (BSh, BSk, BWh, Bwk), Temperate (Cfa, Cfc, Csa, Csb, Csc, Cwa, Cwc), Cold (Dfa, Dsa, Dsd, Dwa, Dwb, Dwd), Polar (EF). Red circles indicate that the UHI is positive, but yellow circles indicate that Urban Cool Island has been reported at the location. The size of circles indicates the magnitude of UHII ranging from 0 to 10 °C. BSh (Arid Climate) experienced the highest UHII with magnitude of 8–10 followed by Csc (Temperate Climate) having magnitude of 4–6; however, in the north eastern region, low UHII magnitude reported ranging from 0 to 2. Unexpectedly, two cities (Indore and Cuttack) of tropical climate reported UCI. This figure was created using QGIS software.

A compilation of the review carried out in the present study has been tabulated in Supplementary Table S2.

In the present study, the extant literature on the UHI phenomenon in over 40 Indian cities spanning from 2005 to 2023 was systematically reviewed, with an emphasis on the methodologies employed, the specific variables and indices investigated. It also focussed on the spatio-temporal factors that contribute to the UHI effect. This study endeavours to compile the existing body of knowledge on the UHI effect, providing a comprehensive guide for researchers to gain an overview of the UHI phenomenon in India and its dynamics. The study analysed that the primary drivers of increased urban temperatures are unplanned urban agglomeration, decreased vegetation cover, elevated energy consumption, anthropogenic heat emissions, and extensive built-up areas. This review also elucidates the connection between the UHI effect and its consequent impacts on heat-related illnesses and deteriorating air quality. The present study revealed that satellite data are extensively used due to better spatial and temporal resolution; and have advantages over ground-based studies in terms of time and human efforts. Consequently, it is recommended to incorporate satellite data analysis along with field observations for better decision making. Field observations offer ground-truth data that can validate and supplement the insights derived from satellite pictures, addressing potential limitations such as resolution restrictions and temporal breaches. This suggestion is made to increase the accuracy and comprehensiveness of UHI investigations. The role of simulation models in UHI has also been studied to underscore their importance. This study also reviewed both environmental and non-environmental measures implemented to mitigate the UHI effect for a holistic understanding of the approaches that can be employed to reduce urban heat. It is necessary to evaluate these measures for developing integrated and sustainable solutions that address the multifaceted nature of the UHI phenomenon. The study delineates the scope of future research, as outlined in the following points: