This week, the same chart kept showing up on my feed: by late April, every single one of the world’s 50 hottest cities was in India. The India Meteorological Department recorded 47.4°C in Banda, Uttar Pradesh, with five other cities also crossing 46°C and dozens more across northern and central India recording temperatures above 45°C. The Union Health Ministry directed every state to operationalise dedicated heatstroke management units. The National Human Rights Commission urged 21 states to take advance measures, warning that intensifying heatwaves disproportionately affect outdoor workers, the elderly, and the homeless.
Two reactions followed almost immediately.
The first: this is climate apocalypse. Indian cities are becoming uninhabitable.
The second: India has always been hot. We’ve lived through 40°C summers for decades.
Both are wrong, and they are wrong for the same reason. Heat stress is not a temperature.
What heat stress actually is
Heat stress is what happens when the human body, in a specific climate, doing a specific kind of work, in a specific kind of building, can no longer dissipate the heat it produces.
Simply put, the human body cools through four channels: convection (air moving over skin), radiation (giving off heat to cooler surroundings), conduction (touching cooler surfaces), and evaporation (sweat). When ambient air is hotter than core body temperature of around 37°C, the first three reverse and the body starts absorbing heat from its surroundings. Sweat becomes the only channel left. And sweat only works when the air can absorb the moisture.
This is why scientists measure heat stress with three metrics, not raw temperature:
Wet Bulb Globe Temperature (WBGT) – It combines temperature, humidity, wind, and radiant heat. It is the standard for setting safe limits in outdoor labour, sport, and military operations.
Heat Index – It is the “feels-like” temperature, which factors humidity into perceived heat. This is what your weather app shows.
Wet Bulb Temperature (WBT) – It is the most critical. It is the lowest temperature achievable by evaporative cooling. The widely cited 35°C wet bulb threshold is the theoretical survivability limit for a healthy adult in shade with unlimited water. Beyond it, the body cannot cool itself, regardless of acclimatisation.
Two cities can both hit 45°C and have completely different heat stress signatures.
Why Phoenix and Banda are not the same crisis
A 45°C day in Phoenix, Arizona, US — functionally the hottest major city in the developed world, which recorded 70 days above 110°F (43°C) in 2024 alone — and a 45°C day in Banda, Uttar Pradesh, are arithmetically identical. They are not the same event.
Phoenix’s humidity sits at around 10-20 per cent, residential air conditioning penetration is above 90 per cent, the share of outdoor workers is small, the building stock is insulated, the grid is engineered for cooling load, and the population is acclimatised to dry heat. The 45°C day is dangerous, but it arrives in a system designed to absorb it.
In Banda or Lucknow or Nagpur: residential AC penetration is below 10 per cent, much of the workforce is outdoor or in non-air-conditioned indoor settings (construction sites, brick kilns, agricultural fields, street vending, food delivery), housing has high thermal mass that radiates heat through the night, the grid is already strained, and the pre-monsoon weeks bring rising humidity. The same 45°C arrives in a system that has none of the absorptive capacity built around it.
What ‘heat stress’ means in Europe
When Madrid or Paris declares heat stress, they are working from much lower thresholds than the headline numbers suggest. Epidemiological studies place the heat-mortality threshold for Mediterranean cities at around 29.4°C maximum apparent temperature, and for north-continental cities at around 23.3°C. The 2003 European heatwave killed over 14,000 people in France and 7,000 in Germany, over and above what would normally be expected for the month of August, with wet-bulb temperatures during the event reaching between 23°C and 31°C, well below the 35°C survivability ceiling.
These absolute air temperatures would barely register as a heatwave anywhere in Indo-Gangetic India. The reason is the same one running through this whole piece, just inverted. Northern European cities like Amsterdam, Berlin, Paris have residential AC penetration in the low single digits because the building stock is engineered to retain heat against cold winters. When a 35°C day hits the Netherlands, people are baking inside thermally-massive brick homes designed never to let heat escape. European populations also skew older, and physiological acclimatisation is lower.
Why the India situation is still genuinely bad
This is where the “India has always been hot” reaction collapses.
The map can read the same temperature anywhere on earth. The body, the building, and the labour market read it differently in every city.
Yes, Indian populations are acclimatised. Yes, our heat metrics should not mirror Europe’s or the US’. None of that changes the trajectory. India is urbanising and industrialising at speed. Manufacturing, construction, and outdoor labour are not residual activities to be designed away, they are the engine of the aspiration to become the world’s third-largest economy.
Heat exposure is being built into the future workforce, not retired from it.
Wet-bulb temperatures in parts of South Asia are approaching physiological limits. The 35°C wet bulb threshold has been crossed for brief periods in pockets of South Asia and the Persian Gulf in recent years. Once it is crossed, no amount of acclimatisation helps. The body cannot cool by evaporation. Death becomes a matter of hours.
Nighttime minimums are rising faster than daytime maximums. The body’s cardiovascular recovery happens at night. Delhi, Konkan, and Odisha are increasingly seeing tropical nights with minimums above 25–28°C, preventing the thermoregulatory reset that healthy bodies depend on. This is invisible on a “hottest cities” map.
The cooling trap is closing. Every new air conditioner cools one room and warms the street outside, drawing on a grid still dominated by coal. The infrastructure response to heat is, in aggregate, accelerating heat. Without grid decarbonisation outpacing cooling demand growth, India is buying cooling with more warming.
Urban heat islands in Indian cities are severe. Dense, low-albedo, tree-poor cityscapes trap and re-radiate heat through the night. The hottest place in an Indian city in May is not the meteorological station, it is the neighbourhoods two to four degrees hotter, with the least cooling infrastructure and the least canopy. While the headline map is averaged, the lived reality is unequal.
Outdoor labour is structural, not optional. A heat advisory that tells people to stay indoors between 11 am and 4 pm assumes a labour market in which they can. With roughly 90 per cent of India’s workforce in informal employment and millions of workers in heat-exposed occupations, “stay indoors” is a protection only for those who already have one.
Also read: Clean energy is not just climate policy—it is economic and strategic security for India
How to read the map
Heat stress thresholds developed in the Global North were calibrated to Global North bodies, infrastructures, and labour patterns. Applying them uncritically to South Asia gets the science wrong in one direction. Dismissing South Asian heat because “people are used to it” gets the science wrong in the other.
Phoenix, on paper, is a city designed around extreme heat, with cooling-centre networks, contractor heat-safety ordinances, and a Chief Heat Officer. Despite that, Maricopa County, of which Phoenix city is a part, recorded 645 heat-related deaths in 2023. That is the system Banda would have to build five times over, against a workforce structure where three-quarters of people work outdoors. The Phoenix comparison is not a “look how much better the West does it” point. It is a “look how much infrastructure they built and people still die” point — and it asks practitioners and policymakers to examine India’s exposure qualitatively, not just quantitatively.
In order to take the India heat map seriously, the response is not better “push notifications” for citizens. It is heat-resilient planning and forward looking housing standards that are being built for the foreseeable 2.5°C temperature rise, enforceable cooling shelter requirements for outdoor workers, ward-level urban tree canopy and cool-roof programmes, shaded walkway investment, grid decarbonisation paced ahead of cooling demand, and real-time heat-health surveillance disaggregated by neighbourhood. Ultimately, it is planning frameworks that treat heat as a chronic condition of the city rather than a seasonal emergency.
This is the conversation the map should start. Not whether India is too hot; India has always been hot, and will only get hotter. The real question is whether a country building cities at breakneck speed can also build them for the climate they will inherit. Growth and resilience are not opposites. But right now, we are designing as if they were.
Supriya Krishnan is an urban climate adaptation specialist and strategy advisor on long-term infrastructure and water systems. She helped set up the interim secretariat of the UN-backed Coalition for Disaster Resilient Infrastructure. She tweets @SupriyaArKr09. Views are personal.
(Edited by Aamaan Alam Khan)