IMD uses heavy rainfall for a station's accumulated rainfall of 64.5 to 115.5 millimetres over 24 hours. Very heavy rainfall is 115.6 to 204.4 mm, and extremely heavy rainfall begins at 204.5 mm. These are technical categories, not interchangeable descriptions of a wet day. This guide checks the terminology on 6 November 2026 and does not issue a weather forecast for that date.
| IMD category | 24-hour rainfall |
|---|---|
| Very light | Trace to 2.4 mm |
| Light | 2.5 to 15.5 mm |
| Moderate | 15.6 to 64.4 mm |
| Heavy | 64.5 to 115.5 mm |
| Very heavy | 115.6 to 204.4 mm |
| Extremely heavy | 204.5 mm or more |
The useful distinction for a reader is between amount, location, probability and impact. A sentence such as isolated heavy rain very likely contains several pieces of information. Heavy describes the amount, isolated describes its geographical distribution, and very likely expresses a probability. The warning colour adds another layer about expected consequences and the response required.
How much rain counts as heavy rainfall?
The rainfall table in IMD's Standard Operating Procedure for Weather Forecasting and Warning sets out the 24-hour bands. The accumulation period is essential. A figure from a short spell cannot be compared directly with a daily total without identifying the interval it covers. Equally, the amount at one observing station does not describe every neighbourhood in a district.
Consider a hypothetical daily reading of 70 mm. It falls in the heavy category. A hypothetical 120 mm daily total falls in very heavy, while 210 mm falls in extremely heavy. These examples only demonstrate the published thresholds; they are not observations from any Indian city on 6 November. Keeping examples separate from measurements prevents an explainer from being mistaken for a current bulletin.
A second useful example is a week with a combined total of 70 mm. That weekly sum does not, on its own, establish a heavy-rainfall day. The daily distribution matters. Likewise, two neighbouring stations may report different daily amounts and therefore different categories. The correct comparison uses the same accumulation period and the relevant station or area named in the official product.
What does isolated heavy rainfall mean?
The terminology annex in an IMD Ahmedabad bulletin distinguishes isolated, scattered, fairly widespread and widespread rainfall. Isolated refers to up to 25 per cent of stations; scattered to 26 to 50 per cent; fairly widespread to 51 to 75 per cent; and widespread to 76 to 100 per cent. Read the applicable table heading to see which rainfall event the distribution describes.
Isolated therefore does not mean weak. A severe local spell can occupy a limited part of a region. Conversely, widespread light rain describes broad coverage with a smaller amount at the affected locations. For a travel decision, find both the area and the intensity instead of treating either adjective as a complete description of the weather.
Distribution is also different from probability. If a forecast calls rain likely, that language is about its chance of occurring, not the percentage of a district guaranteed to get wet. The Ahmedabad terminology table lists likely as 25 to 50 per cent, very likely as 50 to 75 per cent and most likely as more than 75 per cent. Those probability bands answer a different question from the coverage bands.
Does heavy rain always mean an orange or red warning?
No single rainfall number should be converted automatically into a warning colour. IMD's operating procedure says the forecaster considers the expected event and its impacts. Existing flood conditions, the location and the duration of rain can change the consequences. A subdivision's warning colour can also differ from the warning for an individual district within it.
For example, imagine two places expecting similar rainfall amounts but with different existing flood conditions. It would be misleading to decide that their warnings must match solely because the forecast amount matches. Use the actual district warning and accompanying advice. The purpose of a colour-coded warning is to communicate the assessed situation, rather than provide a substitute rainfall gauge.
IMD's public Mausam website provides links to forecasts and warnings. Open the product that names your district or region, then check its issue time and validity. A document can remain available at a familiar URL after its forecast period ends. The Ahmedabad PDF opened for this explainer was dated 15 September 2026, issued at 1:30 pm IST; its forecast is historical, and only its terminology annex is used here.
For related reading, our cold-wave criteria guide covers temperature thresholds, while our fog-warning explainer discusses another hazard. Our dated Delhi AQI report concerns air pollution. Rainfall intensity, visibility and air quality answer different questions, so check each relevant official product before making a plan.
