Heat
Heatwave or extreme heat?
A heatwave is a declared event under IMD's criteria. Extreme heat is a condition — dangerous heat exposure, whether or not any threshold was crossed. The two come apart constantly: humid coastal heat that never reaches 40°C can be more lethal than a dry inland 45°C that qualifies, and most heat deaths happen on days nobody declared.
Why the declared event misses the danger
The criteria measure air temperature. A body does not.
Human beings shed heat by evaporating sweat. That works well in dry air and badly in humid air. At high humidity the same dry-bulb temperature leaves the body unable to cool, which is why a 38°C day at 80 per cent humidity on the coast can be more dangerous than a 45°C day at 15 per cent humidity inland. IMD's heatwave criteria use dry-bulb maximum, so they see the second and not the first.
Night temperature matters as much as day. Physiological recovery happens overnight; a night that stays above about 28°C denies it, and consecutive hot nights compound. IMD handles this under a separate warm-night criterion, which is not what "heatwave" refers to.
And exposure decides the outcome. The same afternoon is a different event for a person in an air-conditioned office and for a construction worker, a street vendor or someone in a tin-roofed room. No temperature threshold contains that information, and it is the largest determinant of who dies.
- Dry-bulb temperature
- Air temperature as a thermometer reads it. What the criteria use.
- Apparent temperature
- What the heat feels like once humidity and wind are accounted for. Closer to physiological risk; not part of the declaration rule.
- Warm night
- A high overnight minimum. Handled separately by IMD and a strong predictor of heat mortality.
Where the definitions sit
One column of this table is a rule. The other is a condition nobody has notified a threshold for.
Heatwave — plains floor
40 °C
IMD, declared event
Heatwave — coastal floor
37 °C
IMD, declared event
Heatwave — departure from normal
4.5 °C
IMD, declared event
Extreme heat — notified threshold
None
No statute sets one
Hottest reading in this site's station set
48.3 °C
ERA5 reanalysis, fourteen stations, 1991 onward
There is no legal limit for heat exposure in India in the way there is for PM2.5 in air or BOD in water. Heat action plans set trigger temperatures for local response, but those are operational thresholds set city by city, not notified standards — and they should not be quoted as though they were.
How to read a heat claim
Ask which of the two the number is about before doing anything else with it.
"Heatwave days" almost always means declared or qualifying days under the criteria. It is a count of a defined event.
"Heat deaths" almost never lines up with those days. Attribution studies consistently find most heat mortality on days below declaration thresholds, because the danger is cumulative and the threshold is not.
A claim that combines the two — "N heatwave days caused M deaths" — is joining a meteorological count to an epidemiological estimate produced by a different method over a different period. Both may be sound; the sentence joining them usually is not.
What this number cannot tell you
- The heatwave count cannot be used as a measure of heat harm. It was not built for that.
- Neither figure captures humidity, night temperature or exposure, which between them decide most outcomes.
- Heat death figures in India are widely accepted to be undercounts; a death certificate rarely names heat.
- Nothing here supports comparing India's heatwave counts with another country's. The definitions are not the same.
Where this comes from
Every figure above is read out of one of these, at the address printed beside it.
- Heat wave criteria and warm-night criteria India Meteorological Department · The declaration rules
- ERA5 reanalysis archive ECMWF, served by Open-Meteo · Temperature, apparent temperature and overnight minima
- Accidental Deaths & Suicides in India National Crime Records Bureau · India's official series for deaths attributed to natural forces, including heat
Reuse freely — CC BY 4.0. The grant covers this page. Each source above keeps its own terms, which is why every one of them is named.
Next
The reading this explains, the explanations next to it, and the one place to stand in it.
environmental deaths One annual table, one cause per death — and the largest killer in it is the one nobody writes about. Stand in it Bridge the Gap Schools are one of the few institutions that can change exposure rather than temperature.