Kolkata: A toxic combination of climbing midnight temperatures and relentless relative humidity is trapping Kolkata in a worsening cycle of nighttime heat stress, with the city’s low-income populations bearing the brunt of the crisis.The India Meteorological Department (IMD) has forecast that Kolkata’s minimum temperatures — typically recorded past midnight — will rise further in the coming days. Combined with high humidity, this trend is significantly driving up the city’s heat index, making recovery from daytime heat nearly impossible for vulnerable residents.This local crisis reflects findings from a landmark study released recently by the research and advocacy group Climate Trends at the India Heat Summit 2026. Titled Nighttime Thermal Stress in Low and Middle Income Housing in India, the study used high-resolution sensors to monitor indoor conditions between Oct 2025 and April 2026.While the field research focused on baseline data from urban neighbourhoods in Chennai — a coastal city sharing Kolkata’s high-humidity, high-heat profile — the structural revelations directly map onto Kolkata’s dense, low-income settlements like its packed tenement housing and slums.The data exposes a dangerous reality: standard concrete buildings act as thermal sponges. Indoor spaces did not cool down when the sun set; instead, they hit peak temperatures between 8 pm and 9 pm, radiating heat trapped by reinforced cement concrete (RCC) roofs and walls during the day. “Indoor temperatures regularly breached 32°C, with nighttime temperatures rarely dropping below 30°C. Coupled with relative humidity levels consistently hovering above 75%, the indoor environment effectively blocks the human body’s natural ability to cool itself through sweating,” said Aarti Khosla, founder and director, Climate Trends.The study highlights a stark socioeconomic divide: income determines a household’s capacity to cope with heat, not its exposure to it. While high-income residents rely on air conditioning to artificially break the thermal cycle, low-income families are left with only ceiling fans, which merely circulate hot, humid indoor air.“Heat stress has emerged as one of the defining challenges of our times,” said Pralhad Joshi, Union minister for new and renewable energy, speaking at the summit.Compounding this indoor structural trap is an outdoor ecological deficit. A parallel study published this week in Nature Communications revealed that while urban tree cover could halve the intense urban heat island effect, canopy distribution remains deeply inequitable. Densely populated, low-income urban pockets feature the thinnest tree cover globally, depriving the most vulnerable citizens of natural shade and cooling.As Kolkata’s post-midnight temperatures continue their upward trajectory, public health experts warned of widespread sleep disruption, persistent fatigue and rising heat-related ailments. Current urban Heat Action Plans (HAPs) across India trigger emergency responses solely based on daytime, outdoor meteorological thresholds, though Kolkata is yet to have its HAP in place. Experts are now calling for a major policy overhaul to mandate indoor thermal monitoring and subsidise passive cooling retrofits, such as cool-roof coatings, to protect citizens behind closed doors.
