Heat
How does India define a heatwave?
A station qualifies if its maximum temperature reaches the floor for its zone — 40°C in the plains, 37°C on the coast, 30°C in the hills — and then either departs from its own normal by 4.5°C or more, or reaches 45°C outright. Severe is a departure of 6.5°C or more, or 47°C outright. IMD declares for a subdivision when the criteria are met at two or more stations.
The rule, gate by gate
Three gates in order. A station that fails the first never reaches the second.
Gate one — the floor. The station's maximum must reach the base for its zone. This is what stops a cool region qualifying for a heatwave at a temperature nobody there would notice, and it is why hill stations have their own much lower base.
Gate two — the departure. The maximum must run at least 4.5°C above that station's own normal for the date. A departure of 6.5°C or more is a severe heatwave. The normal is a long-period average, so the comparison is against the station's own climatology and not against any other place.
Gate three — the absolute override. Regardless of departure, a maximum of 45°C is a heatwave and 47°C is a severe heatwave. This exists because a station whose normal is already very high could otherwise reach a genuinely dangerous temperature without ever clearing the departure bar.
- Zone base
- 40°C plains, 37°C coastal, 30°C hills. The first gate.
- Long-period average
- The reference "normal" for a station and date, computed over a standard multi-decade window.
- Absolute override
- 45°C and 47°C. Qualifies regardless of how far from normal the day was.
Every number in the rule
All seven, as IMD publishes them.
Plains base
40 °C
India Meteorological Department
Coastal base
37 °C
India Meteorological Department
Hills base
30 °C
India Meteorological Department
Heatwave departure
4.5 °C
India Meteorological Department
Severe heatwave departure
6.5 °C
India Meteorological Department
Heatwave, absolute
45 °C
India Meteorological Department
Severe heatwave, absolute
47 °C
India Meteorological Department
The coastal base of 37°C is the one most often left out when the criteria are quoted, and leaving it out systematically undercounts heat events on the coast — where humidity already makes the same dry-bulb temperature more dangerous.
Applying the rule to a dataset
The criteria are precise. Applying them to gridded data introduces choices the criteria do not make for you.
IMD applies these to observing stations and declares for a subdivision on two or more of them. A dataset built on reanalysis grid points is applying station criteria to something that is not a station, and the honest description is "days meeting the criteria at this location".
The normal you compare against is a choice. A different baseline period gives a different departure for the same day, and therefore a different count of qualifying days. Any heatwave series should state its baseline.
This site reads fourteen stations across thirty-five years of ERA5 reanalysis, applies the zone base and both routes above, and publishes the count per station. It does not sum them into a national figure — that would be station-days, not heatwave days.
What this number cannot tell you
- The criteria say nothing about duration. One qualifying day and a nine-day run are both "a heatwave" under this definition.
- They say nothing about night temperature, which IMD handles under a separate warm-night criterion.
- They cannot be applied to a region without stations in it.
- They are not a health threshold. Nothing in the rule refers to who is harmed or at what point.
Where this comes from
Every figure above is read out of one of these, at the address printed beside it.
- Heat wave criteria India Meteorological Department · Every threshold in the table
- ERA5 reanalysis archive ECMWF, served by Open-Meteo · The temperature series the criteria are applied to here
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.
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