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.
Applied to fourteen stations over thirty-five years of reanalysis, the count is published per station. Summed, it would be station-days, not heatwave days.
What it does not measure
- 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 comes from one of these.
- 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. Each source above keeps its own terms.