Delhi’s nights are warming rapidly, with the city recording its hottest night in 14 years at 32.4°C.
This signals a sharp decline in post-sunset cooling.
A CSE report links this to urbanisation, loss of green and blue spaces and rising AC use.
It urges cool roofs, climate-sensitive planning and public cooling centres to manage escalating heat stress.
Delhi is no longer cooling down after sunset in summers and warm nights are increasing the risk of prolonged heat stress, according to a new report released by the Centre for Science and Environment (CSE).
The city recorded its warmest night in 14 years on May 25, 2026, according to the report, Making Delhi Heat-Resilient. On this day, the minimum temperature touched 32.4°C despite daytime temperatures remaining above 40°C for much of the month, it noted.
The India Meteorological Department classifies a "warm night" as one occurring when the maximum temperature remains above 40°C and the minimum temperature is 4.5°C-6.4°C above normal. “On May 25, the night-time temperature went up to 32.4°C to qualify it as the warmest on record in 14 years. This was when day-time temperatures have ranged above 40°C across the month,” said Mitashi Singh, programme manager, sustainable habitat, CSE, and lead author of the report.
Further, warmer nights are becoming an increasingly prominent feature of Delhi's summer climate, the report showed. Delhi has been witnessing increases in both annual average maximum and minimum temperatures over the past four decades, resulting in warmer days as well as warmer nights. According to the study, Delhi departed from its normal mean maximum temperature on 42 out of 50 occasions during the summer months between 2015 and 2024.
A key finding of the study is that Delhi's ability to cool down after sunset has weakened significantly. According to CSE's analysis, diurnal cooling — the difference between day and night temperatures — has declined by 9 per cent over the past decade. While the average difference between day and night land surface temperatures was around 12°C during 2001-10, it had fallen to 9.8°C by 2023. The report also noted that Delhi's urban core now cools 3.8°C less than its peri-urban areas, trapping heat round the clock in dense, concrete-dominated neighbourhoods.
Singh warned that warmer nights may pose a greater health risk than daytime heat. “Higher night-time temperature can take a heavier toll on human health. It does not allow the body to cool down after accumulating heat stress through the day. This can lead to more cardiovascular strain, disrupt sleep cycles, and disable tissue repair: In simple terms — lasting physiological damage,” she said.
The consequences are particularly severe for vulnerable groups such as construction workers, street vendors, homeless people, residents of informal settlements, women, children and the elderly. These groups constitute nearly half of Delhi's population, according to the report.
Sareen said these communities are among the worst affected because they have limited means to adapt to rising temperatures. “They toil through the day to earn their daily bread — this becomes brutal when the nights do not provide any respite either. To add to this, if there is a heat-related disruption in work — if they fall sick — it means wage losses,” he said.
The report linked rising nighttime temperatures to rapid urbanisation, shrinking green and blue spaces, dense built-up areas, inadequate ventilation and increasing dependence on air-conditioners. Delhi's green cover has shrunk from 25.36 per cent in 2014 to 14.14 per cent in 2024, while waterbody coverage has declined from 1.25 per cent to 0.99 per cent over the same period.
The report said growing dependence on air-conditioners may be contributing to a feedback loop that intensifies urban heat. CSE noted that AC ownership has tripled over the past decade and that cooling appliances eject waste heat into the surrounding environment, intensifying the urban heat island effect and pushing temperatures higher, particularly at night. The report cited international research showing that neighbourhoods with high air-conditioner penetration can witness night-time temperature increases of 4.4-4.7°C.
To counter rising night-time temperatures, the report recommends a range of cooling measures. These include mandating thermally efficient or "cool" roofs in industrial areas, markets, office complexes and informal settlements, which can reduce indoor temperatures by up to 5°C. It also called for climate-responsive urban planning, greater use of dense-canopy trees and green-blue infrastructure, and the development of public cooling centres in schools, community halls and other public buildings. According to the report, reducing dependence on conventional air-conditioners through passive cooling and non-refrigerant cooling systems can help curb the waste heat that contributes to warmer nights.
A shift from emergency response to active heat management is essential, stressed the authors of the report, arguing that heat can no longer be treated as an occasional disaster. “Addressing heat vulnerability now requires an active heat management approach. Most important is not looking at it as an occasional occurrence but a harsh reality that is going to stay or perhaps even worsen. This has now become vital for cities to make them liveable in climate-risked times,” Sareen said.