Heat
What is a heatwave?
In India a heatwave is a formally defined event, declared by the India Meteorological Department when the maximum temperature at a station clears a zone-dependent floor and departs far enough from that station's own normal. It is a departure from local normal, not an absolute temperature — which is why 41°C can be a heatwave in one city and an ordinary May day in another.
Why the definition is relative
A single national temperature threshold would declare a heatwave in Rajasthan every summer and never declare one in Shillong.
Heat harms people relative to what they are adapted to. A population, a housing stock and a working day built around 45°C summers is not in the same danger at 44°C as a hill town at 32°C. IMD's criteria are built on that: a station has to be hot in absolute terms and unusually hot for itself.
So there are two gates. First a floor by zone — the plains, the coast and the hills each have their own. Then a departure from that station's own normal maximum for the date, or, separately, an absolute temperature high enough to qualify on its own regardless of departure.
IMD declares for a subdivision, not for a point, and requires the criteria to be met at two or more stations. A single hot grid point is not a declared heatwave — it is a day meeting the criteria at a location.
- Normal
- The long-period average maximum temperature for that station on that date. The reference every departure is measured from.
- Departure
- How far above normal the day's maximum ran, in °C. The core of the criteria.
- Subdivision
- IMD's meteorological unit for declarations. Larger than a city, smaller than a state in most cases.
IMD's thresholds
Two ways to qualify. Either the departure route or the absolute route.
Floor — plains
40 °C
India Meteorological Department
Floor — coastal
37 °C
India Meteorological Department
Floor — hills
30 °C
India Meteorological Department
Departure — heatwave
4.5 °C above normal
India Meteorological Department
Departure — severe heatwave
6.5 °C above normal
India Meteorological Department
Absolute — heatwave regardless of departure
45 °C
India Meteorological Department
Absolute — severe regardless of departure
47 °C
India Meteorological Department
There is no legal limit here. Nobody can break a heatwave criterion — it is a definition, not a standard, and this is one of the places on this site where the reading is measured against a threshold that regulates nothing.
How to read a heat figure
Almost every misreading of Indian heat data comes from one of three confusions.
Declared or qualifying? IMD declares for a subdivision on two or more stations. A dataset that counts days meeting the temperature criteria at single grid points — including this site's — is counting a related but different thing, and should say so.
Maximum or apparent temperature? The criteria use dry-bulb maximum. What a body experiences depends on humidity too, and a coastal 38°C at high humidity can be more dangerous than an inland 44°C. The criteria do not capture that.
Station-days or heatwave days? Summing qualifying days across fourteen stations gives station-days, which is not a national count of heatwave days.
What it does not measure
- It cannot tell you about night-time heat, which is the better predictor of mortality. A hot night gives a body no chance to recover.
- It cannot account for humidity, and therefore cannot describe the coastal danger that dry-bulb temperature misses.
- It cannot be compared internationally. Every meteorological agency defines a heatwave differently, and most define it relative to their own climatology.
- It says nothing about exposure — who is outdoors, in what work, in what housing. That is what turns a temperature into a death.
Where this comes from
Every figure above comes from one of these.
- Heat wave — criteria and forecasts India Meteorological Department · The zone floors, departures and absolute thresholds above
- ERA5 reanalysis archive ECMWF, served by Open-Meteo · The thirty-five-year station series this site reads
Reuse freely — CC BY 4.0. Each source above keeps its own terms.