New Delhi: From purchasing air conditioners and coolers to consuming copious amounts of lemon soda and coconut water, Delhiites are preparing themselves for a prolonged and scorching summer.
Peak temperatures are not merely crossing the 40-degree-Celsius mark; most cities across India are becoming hotter overall. Moreover, forecasters at the India Meteorological Department (IMD) have predicted above-normal daytime and night-time temperatures throughout May and June. The ambient air temperatures in cities such as Delhi, Mumbai, Kolkata and Bengaluru have seen a marked increase over the past decade.
Data indicates that during the summers of 2001-2010, land surface temperatures dropped during nighttime by 6.2-13.2 degrees Celsius on average from the daytime peak. Between 2014 and 2024, night-time cooling has reduced dramatically to just 1.5-6.2 degrees Celsius on average.
“This essentially means that throughout the season, residents are experiencing warmer average temperatures. Nature’s natural cycle of cooling is now being disrupted,” said a senior official at the IMD who did not wish to be named.
Daily average temperatures have been steadily rising over the past decade, according to the IMD.
“Along with daytime temperatures, we are now recording high night-time temperatures and humidity levels,” IMD Director General M. Mohapatra told ThePrint. “This means that cities are not cooling at the necessary rate, leading to an uncomfortable accumulation of heat throughout the season.”
Most parts of northwest, central and eastern India are also expected to experience higher than normal heatwave days.
Data further reveals that cities are witnessing a rise in minimum temperatures and humidity levels during the summer months, which has intensified the overall discomfort and unease among residents. “Such trends stress the need for climate-resilient interventions. These trends have become the new normal in some cities,” Mohapatra said.
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High night-time temperatures and humidity
The narrowing gap between maximum and minimum temperatures as the season progresses has emerged as the primary reason for prolonged spells of extreme heat. Rising minimum temperatures are worsening the situation.
Mahesh Palawat, Vice President (Meteorology and Climate Change) at Skymet Weather—a private weather forecasting service—explained that after a high heat day, nighttime is when the atmosphere usually cools down.
High daytime temperatures, which circulate closer to the surface, rise to allow the cooler night air to settle. This is how nature cools a region. But this cooling mechanism is being blocked by factors such as dense urbanisation and dwindling green spaces.
“It is simple science. When warm air isn’t allowed to escape, it remains trapped near the surface. The next day, more warm air is added to this, until eventually the seasonal temperature becomes unbearable,” Palawat said.
An analysis by the Centre for Science and Environment (CSE), a Delhi-based environmental think tank, showed that the annual mean land surface air temperature over India during 2023 was 0.65 degrees Celsius above the long-term average.
The report highlighted that the decadal summer-time average ambient temperature has risen by approximately 0.5 degrees Celsius in Mumbai, Bengaluru and Chennai compared to 2001-2010. Kolkata’s decadal average also increased by 0.2 degrees Celsius.
The report also noted that urban centres were not cooling at night.
“When night-time temperatures remain high, people get little chance to recover from daytime heat,” Dr Dileep Mavalankar, former director of the Indian Institute of Public Health in Gandhinagar, told ThePrint.
When nights remain consistently warm, the risk of heat-related mortality also increases, he warned. High relative humidity levels are compounding the summer’s discomfort.
The CSE report indicated that average relative humidity significantly increased over the last 10 summers—between 2014 and 2024—compared to the 2001-2010 average. In Delhi, Mumbai, Kolkata, Hyderabad and Chennai, the decadal summer-time average relative humidity rose by five to ten percent. “This increase has worsened heat stress in warm-humid and moderate climate zones,” the report stated.
Experts say these factors—often overlooked by weather monitoring agencies—reveal the actual impact of heat. Anumita Roychowdhury, executive director, Research and Advocacy at the CSE, told ThePrint that with summers becoming more dangerous, it is imperative that heat be monitored holistically to save lives and reduce damage on the ground.
“Assessing the changing trends in heat, relative humidity and land surface temperature alongside day and night temperatures is necessary to develop a comprehensive heat management plan for urban centres,” Roychowdhury said, adding that the IMD’s heat recordings were not the most accurate representation of heat’s impact on the ground.
The IMD has started issuing advisories that include what the day ‘feels like’.
“The way weather trends are moving these days, temperature readings alone are insufficient. The daytime temperature might be 40 degrees Celsius, but when combined with factors such as high humidity and urban heat islands, it feels significantly more intense,” a senior IMD scientist said.
Urban heat islands
Data from the IMD over 10 years showed that peri-urban areas cool down by 12.2 degrees Celsius, while the city core cools down by only 8.5 degrees Celsius. This indicates that city centres are cooling 3.7 degrees Celsius less than surrounding areas.
In Delhi, a direct correlation is observed between high heat stress and the increase in built-up area, which has risen from 31.4 percent in 2003 to 38.2 percent in 2022.
Granular analysis of Delhi’s temperature trends also revealed that 80 percent of the city’s monitoring stations recorded higher temperatures than the representative station at Safdarjung.
This indicates that a majority of the city is enduring a much harsher season than projected by the Met department.
“Factors such as green cover and open spaces play a significant role in temperature readings. Safdarjung is also an area that is not as densely populated as many other pockets in Delhi. To provide a more accurate picture, the IMD could average readings from multiple observatories. This would give a more transparent reflection of the temperatures experienced by residents,” Palawat said.
(Edited by Radifah Kabir)