Salespeople, daily wage labourers, a police patrol on leave, a few foreign tourists and a ragpicker – Delhi’s largest “cooling zone” outside the Jama Masjid metro station was packed. Some people were standing in queues for cold water while others were standing near fans and coolers.
Pressing quietly, autorickshaw driver RK Shukla hesitantly asked if the air cooler could be adjusted: “Same problem at home. When it’s humid, my desert cooler blows hot air.” It was late June when the pre-monsoon heat index – which measures how hot one feels – had left Delhi suffocating. The highest temperature of this month was 53° Celsius.
Monsoon has reached North India, but rain alone does not guarantee thermal comfort. Heat index records from 2021 to 2025 show that Delhi often feels hotter than its air temperature due to high humidity. In July–August, the heat index often exceeded 46–50 °C, creating oppressive conditions. According to a policy brief from the Center for Science and Environment last September, this inconvenience increased the duration and intensity of high power consumption as cooling systems, primarily air conditioners, ran harder and for longer periods.
Also read: Monsoon has arrived in Delhi, but where is the rain? experts tell
As part of its Heat Action Plan (HAP), the Delhi administration has launched temporary cooling zones, mobile vans and cool wards in hospitals. The emergency measures are clearly seasonal, but experts warn that Indian cities must act quickly to adopt informed, targeted and year-round programs to protect against changing heat patterns.
heat reset
There is an alarming increase in hot nights; Increase in relative humidity in the Indo-Gangetic plain; And heat risk increased in dense, urban and economically important districts like Delhi, Mumbai, Ahmedabad, Hyderabad, Bhopal and Bhubaneswar. The Council on Energy, Environment and Water (CEEW) analyzed heat risk in 734 districts last year to uncover these key trends.
Also read: Chance of rain in Delhi from 6 to 11 July; IMD warns of heavy rain as soon as monsoon takes effect
Most recently, on June 24, a study by US-based non-profit Climate Central revealed that since the 1970s, India has seen an increase in dangerously humid summer days from 101 to 141 per year. Globally, the number increased from 10 to 23. The benchmark for such days is the wet-bulb temperature – a measure of how much an object can cool through evaporation – of 25 degrees Celsius or more.
Over five decades, Many such dangerous humid days were seen in Delhi. increased from 96 to 135; Ghaziabad from 99 to 137; Nagpur from 44 to 119; Raipur from 82 to 150; Ahmedabad from 137 to 162; And Jaipur from 55 to 101.
Scientists link these changes to ocean warming. In India, the Arabian Sea and the Bay of Bengal push moisture inland. Due to higher average temperatures induced by factors such as excessive urbanization, the urban heat island effect dominates the desiccation effect (also known as the urban dry island effect) when there is moisture in the atmosphere. This humidity also hinders normal cooling at night. If there is a long period of high humidity without rain and the night temperature remains high, the next day’s maximum temperature is kept high.
A study published in the journal Physics and Chemistry of the Earth, released in March this year, said that night-time heat waves were persistent in 15 smart cities, including Delhi, Agra, Kanpur, Varanasi, Jhansi, Gwalior and Kochi. Analyzing March-June data between 2001 and 2024 for all 100 smart cities in India, the study found that another 16 cities among them – including Aurangabad, Bhopal, Indore, Nagpur, Raipur, Warangal and Puducherry – experienced persistent mixed heat both day and night.
Most cities ignore the full daily dimension of HAP heat risks, focusing mainly on measures such as providing drinking water, raising awareness, improving medical preparedness and adjusting work and school timings, said Kashif Imdad, one of the study’s authors and associate professor at CSJM University in Kanpur.
To combat night heat, Imdad suggests cool roofs, green cover, natural ventilation and reliable electricity; Additionally, a 24-hour response system for cities experiencing intense heat both day and night, health preparedness and passive technologies to cool homes and workplaces.
general solution
It is important to give priority to context-specific solutions over purely theoretical solutions. Rajan Rawal, professor at Ahmedabad’s CEPT University and expert in passive design and urban climate, sheds light on this with the commonly cited solution of cool roofs – surfaces engineered to reflect sunlight and reduce heat.
Since urban heat is not solely related to daytime sunlight, blocking or reflecting sunlight may not significantly improve indoor comfort. This physical comfort depends not only on the temperature of the ceiling surface but also on the temperature and humidity of the air, Rawal writes in an essay co-authored with Radhika Khosla, associate professor at Oxford University’s Smith School of Enterprise and the Environment.
Heat maps of cities, derived from satellite data, typically focus on daytime land surface temperatures and ignore nighttime urban heat island effects, air temperature and humidity. But these parameters, along with wind speed and solar radiation, affect the comfort, health and power demand of people in buildings. He said that even though cool roofs reduce ceiling temperatures, walls, floors and windows have a greater impact on occupant comfort.
According to Rawal, cool roof engineering reduces nighttime temperatures on concrete surfaces, which retain heat. However, it is less effective on tin or cement-sheet roofs because they cool faster at night anyway. Imdad said permanent cool roofs may not be suitable in North Indian cities with near-zero winter temperatures; Instead, removable or seasonal solutions would be better.
Passive designs have limitations. In dense areas, lack of natural ventilation can make homes hot and stuffy. But, Rawal said, ventilation cannot lower the indoor temperature below the outdoor ambient temperature, especially when humidity is high.
He said that traditional architecture with courtyards, perforated screens, high ceilings and ventilators was effective when the surroundings were relatively cool. “But now due to urban sprawl around most areas, ambient temperatures have increased, and even older neighborhoods have lost their natural cooling capacity.”
Instead, Rawal and Khosla propose a passive design approach that uses building materials, construction methods, and room layout with a focus on the three modes of heat transfer – conduction, convection, and radiation. Although the upfront cost may be high, it reduces air conditioning use and the waste heat it produces, reduces urban heat island effects, and improves conditions for those without active cooling.
CEEW’s Vishwas Chitale agreed that with the increasingly humid heat, passive strategies are often inadequate. “We need active cooling solutions. But affordable, innovative solutions, as well as subsidized models and incentives for adoption, remain a challenge.”
sky, ground and trees in between
In cities, the lack of “sky view” makes nights hotter, as dense buildings and tree canopies trap heat rather than allowing it to escape into the sky. Rawal points out that open, raw spaces cool efficiently at night due to their exposure to the sky, but concrete under trees creates islands of heat despite the greenery.
Trees provide shade and evaporation, which cools the environment. In dry regions, heat loss by evapotranspiration is reduced because the air can absorb excess moisture. In humid urban areas, excess moisture trapped near the ground increases heat stress even in the shade.
Based on this, a study from IIT Gandhinagar published in Nature Communications states that the effect of vegetation on heat index varies with climate and morphology, and not just canopy cover.
After analyzing 138 Indian cities, researchers concluded that in cities facing humid heat stress, greenery strategies should simultaneously provide shade, manage moisture and maintain air flow.
going forward
Despite the challenges, Chitale said heat mitigation and governance in Indian cities are becoming more structured and sophisticated. Previously, heat action plans lacked an understanding of the local context, vulnerability assessments and important parameters such as humidity and nighttime temperatures, but the new plans are more comprehensive.
For example, Thane’s 2024 HAP incorporates the IPCC AR5 framework to create a City Heat Index – which defines hazard, risk and vulnerability as key factors in climate risk. The plan incorporates historical and projected climate extremes in dry conditions, humidity and hot nights along with socio-economic factors to support detailed ward-level planning. It also defines triggers, responsibilities and timelines for action, Chitale said.
With this innovative framework, CEEW supports 145 cities in seven states in developing HAPs.
Cities like Cuttack and Sambalpur aim to extend cooling solutions beyond six months, he said, while Chennai is integrating heat action into its master plan.
Bhubaneswar will soon launch a climate-risk dashboard that will map heat stress at the building level, bringing homes, schools, anganwadis, hospitals, health centres, government offices, roads, bus stops, railway stations and bus terminals into one digital view, said Suraj Kumar of Inpact Solutions, which provided technical support for the project.
This digital twin uses satellite imagery, weather data and local sensor feeds to analyze surface and air temperature, humidity and urban heat islands. It is part of the Sustainable Urban Mobility – Air Quality, Climate Action and Access (SUM-ACA) initiative led by GIZ India and the Union Ministry of Housing and Urban Affairs.
Bhubaneswar Commissioner Chanchal Rana said the dashboard will aid long-term strategic planning for urban heat mitigation projects funded by the State Disaster Management Fund. “It is a highly reliable, scalable, open-source tool, with no license fees, providing long-term benefits by avoiding proprietary software,” he said, and if successful, it will be recommended to other urban local bodies in Orissa.
