The Silent Threat: Night-time Heatwaves Reshaping Urban Heat Risks in India

Indian cities are facing a new dimension of extreme heat, where the respite of cooler nights is vanishing. A groundbreaking study published in the journal Physics and Chemistry of the Earth reveals that heat risks are no longer confined to scorching days but are increasingly driven by persistent high night-time temperatures and compound events—intense daytime heat followed by sweltering nights. This shift demands a reevaluation of how we understand and combat urban heat stress, particularly in the context of rapidly growing smart cities across the country.

The research, spanning data from 2001 to 2024, analyzed 100 smart cities using daily ERA5-Land temperature records from the European Centre for Medium-Range Weather Forecasts. By developing a novel Diurnal Persistence Index (DPI), the authors quantified the frequency, cumulative intensity, and mean intensity of daytime heatwaves (DHW), nighttime heatwaves (NHW), and compound heatwaves (CHW). Heatwaves were defined as temperatures exceeding the city-specific 88th percentile for at least three consecutive days, highlighting a diurnal reorganization where nighttime recovery is diminishing.

Researchers Behind the Study: Academic Insights from Kanpur and Bengaluru

Lead author Kashif Imdad, an Associate Professor of Geography at Pandit Prithi Nath (PPN) Post Graduate College in Kanpur, collaborated with PhD scholar Anubha Yadav and researcher Archana Chaudhary from Vikramajit Singh Sanatan Dharma College, also in Kanpur. Rayees Ahmed from the prestigious Indian Institute of Science (IISc) in Bengaluru contributed expertise in environmental modeling. Their work underscores the role of Indian higher education institutions in addressing pressing climate challenges through interdisciplinary research.

Imdad emphasized the oversight in current policies: "Most state and district plans, including the Uttar Pradesh State Plan, focus almost entirely on daytime heatwaves. We have not identified night-time and compound heatwaves as a problem yet." This study from affiliated colleges and IISc highlights how regional academic centers are pivotal in generating data-driven solutions for national issues.

Key Findings: Which Cities Face the Greatest Night-time Heat Burden?

The analysis categorized cities into plains, plateaus, coastal areas, and hills, revealing distinct patterns. In the Indo-Gangetic Plains, daytime heatwaves are stabilizing or declining, but NHW and CHW are surging due to urban heat retention. Southern peninsular cities still dominate with DHW, while coastal and hill cities see more NHW.

Kruskal-Wallis tests confirmed significant differences in intensity regimes (p < 0.001), with the DPI underscoring sustained nocturnal stress in urban settings. For instance, Varanasi's intense NHW means residents endure prolonged exposure without the body's natural cooling cycle, amplifying cumulative thermal load.

Health Risks Amplified: Why Night-time Heat is Deadlier Than Daytime Scorches

During the day, people seek shade or air-conditioned spaces, but nights offer no escape in affected cities. The human body relies on nocturnal cooling to recover from diurnal heat stress; without it, core temperatures remain elevated, leading to heat stroke, cardiovascular strain, and exacerbation of non-communicable diseases (NCDs) like diabetes and hypertension.

India undercounts heat-related deaths significantly. While official figures report thousands annually, studies estimate up to 1,116 excess deaths per year from 2008-2019, with a 55% rise between 2000-2021. In 2024, scientists documented over 700 heat deaths, far exceeding government tallies. NHW compound this by disrupting sleep, impairing cognitive function, and increasing mental health issues, particularly among vulnerable groups: the elderly, laborers, slum dwellers, and those with pre-existing conditions.

Urban Heat Island (UHI) effect—where concrete and asphalt trap heat—intensifies this, with cities like Delhi seeing nighttime temperatures 8.3°C warmer than rural areas.

Photo by Shantanu Kumar on Unsplash

Supporting Evidence from Prior Research: A Consistent Trend

This study builds on earlier work. A 2024 Centre for Science and Environment (CSE) analysis of 247 cities using MODIS satellite data (2001-2024) found night-time cooling has plummeted, with UHI peaking nocturnally due to concretization and green cover loss. IIT Bhubaneswar reported urbanization caused 60% more night-time warming in 140+ cities compared to rural areas.

CEEW's 2025 study noted very warm nights rose faster than days (2012-2022), affecting 70% of districts with 5+ extra warm summer nights. These converging findings from Indian institutes like IITs, IISc, and CSE affirm the urgency.

Case Study: Varanasi's Intense Night-time Heat Challenge

Varanasi, topping the NHW intensity list, exemplifies the crisis. Its dense urban fabric along the Ganges traps heat, with ghats and narrow lanes limiting airflow. Residents, including pilgrims and laborers, face heightened risks during summer festivals when outdoor activities peak at night. Local HAPs must incorporate night shelters near ghats and cool roofing for heritage-compliant buildings.

Similar patterns in Delhi and Ahmedabad show UHI adding 4-6°C at night, correlating with excess mortality spikes during compound events.

Mitigation Strategies: From Cool Roofs to Green Corridors

Effective solutions target UHI reduction:

• Cool Roofs: Reflective coatings lower indoor temperatures by 5-10°C; removable versions suit seasonal climates like Uttar Pradesh.RMI India guide
• Urban Greenery: Tree canopies and green roofs cool via evapotranspiration; Chennai's green loss doubled UHI.
• Ventilation Corridors: Preserve wind paths in planning to enhance nighttime airflow.
• Night Cooling Centers: Community shelters with fans and hydration stations.

Trials in Ahmedabad slums show cool roofs cut heat stress by 1.2°C indoors.

Revamping Heat Action Plans: Policy Shifts Needed Now

India's HAPs, like Delhi's 2024-25 plan, mention night conditions but lack specifics. Recommendations include:

• Hot night early warnings via apps/SMS.
• Reliable power for fans/AC during peaks.
• Cumulative exposure alerts over daily max.
• Equity focus: Slum retrofits, worker schedules.

NDMA guidelines define heatwaves but must integrate DPI-like metrics. States like Uttar Pradesh can lead with Kanpur's research.NDMA Heat Wave

Photo by albin tr on Unsplash

Future Outlook: Projections and Research Imperatives

Under 2°C warming, concurrent day-night heatwaves could rise sixfold. CMIP6 models predict intensified NHW exposure. Indian universities must expand modeling, with IISc and IITs at forefront. Interdisciplinary programs in climate geography, public health, and urban planning are crucial for resilient cities.

By 2050, coastal humid heat may push tolerance limits, shrinking fisher workdays.

Stakeholder Perspectives: From Academics to Communities

Imdad warns of winter risks from permanent cool roofs in variable climates. CEEW stresses job losses (35M by 2030) and 4.5% GDP hit. Labor unions call for night shifts adjustments; health ministries for NCD surveillance during NHW.

Actionable: Municipalities audit UHI, fund green infra; residents plant trees, use reflective paints.