Rain
What counts as extreme rainfall?
India Meteorological Department classifies rainfall by what falls at a station in 24 hours. Heavy rain begins at 64.5 mm, very heavy at 115.6 mm, and extremely heavy at 204.5 mm. The categories describe a day, not a storm — which is the single most important thing to know about them, because 200 mm in three hours and 200 mm spread over a full day are the same category and are not the same event.
The categories, and what they are a category of
The classification is old, simple and useful. Its unit is the problem.
IMD's 24-hour rainfall categories run from a "rainy day" at 2.5 mm up through heavy, very heavy and extremely heavy. They are applied at a station, over a fixed 24-hour period, and they are the basis of most public reporting of Indian rainfall.
The categories say nothing about intensity within the day. Urban flooding is driven by rate — how much water arrives faster than the drainage can take it — and a 24-hour total cannot distinguish a steady soak from a cloudburst. Two days with identical totals can produce no flooding and a disaster.
They also say nothing about antecedent conditions. The same 90 mm falling on dry ground and on ground already saturated by a week of rain produce very different amounts of runoff. Neither the category nor the total contains that information.
- Rainy day
- 2.5 mm or more in 24 hours. The floor of the classification.
- Cloudburst
- IMD's separate criterion for very high short-duration intensity. Not part of the day categories above.
- Normal
- The long-period seasonal average for a place. A comparison, not a rule — nothing can be over a rainfall limit, because there is no rainfall limit.
IMD's 24-hour thresholds
Four figures, all applied to what a station records in one day.
Rainy day
2.5 mm in 24 hours
India Meteorological Department
Heavy rain
64.5 mm in 24 hours
India Meteorological Department
Very heavy rain
115.6 mm in 24 hours
India Meteorological Department
Extremely heavy rain
204.5 mm in 24 hours
India Meteorological Department
Legal limit on rainfall
None
No statute sets a permitted rainfall
This is one of two subjects on this site with no published limit at all — the other is fire detections. Everywhere else a reading is judged against something notified. Here the only available comparison is the climatological normal, which is an average and not a rule, and nothing can be "over" it in the sense a pollutant can be over a standard.
How to read a rainfall figure
Three questions, and "how much rain fell" is not one of them.
Over what period? A 24-hour category, a monsoon-season total and a single-hour intensity are three different quantities that get reported in the same sentence.
At which station? Rainfall is intensely local. A gauge four kilometres away can record half as much on the same afternoon, and a city figure is usually one gauge.
Against which normal? "Above normal" depends entirely on the baseline period chosen. Different windows give different verdicts on the same season, and a figure without its baseline cannot be checked.
What this number cannot tell you
- It cannot describe intensity. A day category is blind to how fast the water arrived.
- It cannot predict flooding. Drainage capacity, ground saturation and land use decide that, and none of them are in the number.
- It cannot be compared across stations with different exposures, or across gauges of different types.
- It cannot be read as climate change on its own. A single extreme day is weather; a trend needs a long homogeneous series.
Where this comes from
Every figure above is read out of one of these, at the address printed beside it.
- Rainfall statistics and criteria India Meteorological Department · The 24-hour day categories above
- ERA5 reanalysis archive ECMWF, served by Open-Meteo · The multi-decade station series this site applies the categories to
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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