Published Apr 08, 2026 | 7:15 AM ⚊ Updated Apr 08, 2026 | 7:15 AM
Hot and humid weather (iStock)
Synopsis: A new study tracking temperature data found that South India now faces the most intense round-the-clock heat stress in the country. Not just hot days, hot nights too, simultaneously, with no window of recovery in between. The study’s authors describe this as “diurnal persistence”, heat that does not release after dark but carries through the full 24-hour cycle.
Every summer, millions of people across South India wait for the same thing: Sunset. The logic is simple; the afternoon sun is brutal, but the night brings relief. Sleep. Recovery. A chance for the body to reset before the next day’s heat arrives.
That logic is breaking down.
A new study tracking temperature data across 100 Indian smart cities over 24 years has found that South India now faces the most intense round-the-clock heat stress in the country. Not just hot days, hot nights too, simultaneously, with no window of recovery in between.
The research, which analysed daily temperature records from 2001 to 2024, identifies a specific and alarming pattern in the 40 smart cities of the southern peninsular region: compound heatwaves, in which both daytime and nighttime temperatures remain elevated simultaneously, are more frequent here than anywhere else in India.
The human body is not designed to withstand heat without pause. On a hot day, the cardiovascular system works overtime to cool itself, pumping blood to the skin to trigger sweating and strain the heart. The night is when that burden lifts.
When nights stay hot, that recovery never comes.
The study’s authors describe this as “diurnal persistence”, heat that does not release after dark but carries through the full 24-hour cycle. In South India, they found an average of 3.4 such compound days per pre-monsoon season, the highest of any region studied.
The researchers are direct about what this means for human health.
“Elevated nighttime temperatures limit physiological recovery, disrupt sleep and increase cardiovascular and all-cause mortality, even in the absence of extreme daytime heat,” they write.
The risk falls hardest on those who cannot escape it: Outdoor workers, the elderly, people in densely packed urban neighbourhoods with little ventilation, and those without access to air conditioning.
There is a deeply counterintuitive finding buried in the data, one with direct implications for how governments respond.
Across the southern peninsular region, the raw count of heatwave days is actually declining. On paper, the situation looks like it is improving. But the researchers found that this apparent stability masks a more dangerous reality: The cumulative heat burden, the total thermal stress accumulated across all heatwave days, is rising in several cities regardless.
Fewer events, but more damaging ones. And the damage is happening at night.
The authors put it plainly: “Urban heatwave risk in India is increasingly shaped by diurnal persistence and cumulative thermal burden rather than daytime extremes alone.”
The geography and climate of South India create conditions that are particularly hostile to nocturnal cooling.
High pre-monsoon humidity, the thick, airless quality that makes Chennai and Kochi feel so suffocating in April and May, physically prevents heat from escaping into the atmosphere after dark. In drier climates, the ground and buildings radiate heat upward, and temperatures fall quickly after sunset. In humid conditions, that process is suppressed.
Layered on top is the urban heat island effect: dense concrete, tar roads, and the concentrated heat output of millions of people and machines in cities like Bengaluru, Hyderabad, and Chennai absorb solar energy during the day and release it slowly through the night.
“Nighttime persistent and compound regimes are likely reinforced by elevated pre-monsoon humidity, which suppresses nocturnal radiative cooling, particularly in coastal and humid inland cities,” the researchers write.
Weak ventilation in the weeks before the monsoon compounds the problem further, leaving little wind movement to carry heat away from densely built neighbourhoods.
India’s heat action plans, the official government frameworks that trigger warnings, open cooling centres, and advise the public, are almost entirely built around daytime temperature thresholds. When the mercury crosses a certain point in the afternoon, the system responds.
The study argues this framework is missing the point.
“Current Heat Action Plans, which are largely daytime-centric, likely underestimate risk. Effective mitigation requires strategies specifically targeting nocturnal cooling and compound stress,” the authors wrote.
They call for a fundamental shift: incorporating minimum nighttime temperatures into early warning systems, providing nighttime cooling shelters, and redesigning cities to improve nocturnal ventilation through green cover, cool roofs, and building materials that reflect rather than absorb heat.
The study is careful to frame this not as a future risk but as a present reality.
“The transition from episodic daytime extremes to chronic, diurnally persistent heat stress represents a fundamental shift in India’s climate risk profile,” the authors conclude.
For South India, where humidity, dense urbanisation, and pre-monsoon weather patterns create the most compound-prone conditions in the country, that transition is furthest along.
The researchers studied cities under the Smart Cities Mission, a government programme specifically designed to improve urban infrastructure and resilience. The implication is pointed: the cities most equipped, in theory, to respond are the ones the data says are most exposed.
Whether the response matches the scale of the problem is a question the data alone cannot answer.
(Edited by Muhammed Fazil.)
