Forty Millimetres Means Nothing — What an Impact-Based District Warning Actually Says
Most weather products answer the question "how much?". Forty millimetres of rain, gusts to 60 km/h, a maximum of 43°C. Those are measurements, not warnings. Forty millimetres in Kozhikode during the monsoon is an ordinary Tuesday; the same forty millimetres falling on a dry catchment in Jaisalmer is a flash-flood problem. The number did not change — the context did. An impact-based warning is the step past the number: it states what the weather is likely to do to someone specific, in a specific place. This article explains what VayuMet's district warnings actually contain, the hazard classes and colour ladder behind them, how they are computed, and — importantly — where the method stops.
The core idea in one line. One district forecast produces four different advisories, because the same weather is a different problem depending on who is reading. A thunderstorm is a sightseeing cancellation to a tourist, a lodging risk to a farmer, a convective penetration hazard to a dispatcher, and a steep ramp event to a grid scheduler.
One Thunderstorm, Four Warnings
This is the clearest way to see what "impact-based" means in practice. Take a single flagged thunderstorm day over one district. The meteorology is identical in all four rows. The advisory is not:
| Reader | What the same thunderstorm means |
|---|---|
| PUBLIC & TOURISM |
Move indoors at first thunder. Unsafe for open-area sightseeing, high-profile vehicles, and coastal or boating activity. |
| AGRICULTURE | Lodging risk in standing crops; provide support to orchards and trellises. Squall gusts paired with the thunderstorm are the damaging element, not the rainfall total. |
| AVIATION | Embedded cumulonimbus with cloud tops to a stated flight level — expect convective significant-meteorological-information conditions; avoid penetration. |
| RENEWABLE ENERGY |
If gusts reach the turbine cut-out threshold, expect shutdowns and steep ramp events — a scheduling problem, not a safety one. |
A single rainfall figure cannot produce any of those four sentences. That is the entire argument for impact-based warning, and it is why the advisory rules sit in one shared rule set rather than being written separately for each page — a threshold changed once changes every surface that reads it.
The Six Hazard Classes and the Colour Ladder
VayuMet's district warnings span six hazard classes, only one of which is monsoon-specific. This is a year-round product: heat dominates March to June, fog and cold wave December to February, thunderstorm and gale wind through the pre-monsoon and post-monsoon transitions.
| Hazard class | Principal season | Primary exposure |
|---|---|---|
| Heavy rain | June–September, and the Northeast Monsoon over the southeast peninsula | Waterlogging, flooding, travel disruption |
| Heatwave | March–June | Outdoor exposure, solar panel derating, livestock |
| Cold wave | December–February | Elderly and outdoor workers, overnight exposure |
| Thunderstorm | Pre-monsoon and monsoon; year-round in parts of the peninsula | Sudden gusts, hail, wind shear, lightning |
| Gale / gusty wind | Year-round; peaks with systems and squalls | High-profile vehicles, hoardings, turbine cut-out |
| Fog | December–January, mainly the Indo-Gangetic Plain | Flight and rail delay, road visibility |
The colour ladder
Alert levels follow the India Meteorological Department (IMD) colour convention, which is an action scale rather than an intensity scale — the colour tells you what to do, not merely how much is coming:
| Colour | Meaning | Expected response |
|---|---|---|
| Green | No warning | No action required |
| Yellow | Be updated / watch | Monitor advisories |
| Orange | Be prepared / alert | Implement safety protocols |
| Red | Take action / warning | Comply with disaster directives |
For rainfall the tiers map to IMD's standard 24-hour categories: Heavy Rain (64.5–115.5 mm) carries a yellow badge, Very Heavy Rain (115.6–204.4 mm) orange, and Extremely Heavy Rain (≥ 204.5 mm) red. For temperature, a heatwave requires both an absolute maximum at or above 40°C and a departure from normal of +4.5 to +6.4°C — which is why a 41°C day in May may carry no alert while a 39°C day in March can.
How VayuMet Computes It
Alerts are generated from VayuMet model fields resolved onto district polygons — 754 of them — for ten forecast days. The rule set follows IMD warning criteria, International Civil Aviation Organization (ICAO) meteorological practice, and Indian agromet advisory practice, depending on which sector bundle is being produced.
The step that does the real work
Rainfall amount alone is a weak predictor of damage, because two very different events can produce the same total. The alert pipeline therefore applies an experimental classification that discriminates convective from non-convective (stratiform) precipitation, and derives an associated peak gust forecast for each district polygon at roughly 25–50 km scale:
- Convective events are flagged for rapid-onset hazards — sudden gusts, hail, wind shear. The danger arrives in minutes and the gust matters more than the total.
- Stratiform events are flagged for persistent-rainfall and flooding risk. The danger accumulates over hours and the total matters more than the gust.
Those two demand opposite responses, and a single rainfall-amount alert cannot separate them. This event-type and gust pairing is what feeds the sector-specific wording.
Sector thresholds are explicit, not editorial
Each advisory line is produced by a stated numeric rule rather than a judgement call. A few examples, to show the character of the thing:
Worked thresholds
| Advisory | Rule |
|---|---|
| Sowing window suitable | Seed-zone soil moisture at or above 0.18 volumetric (or 20 mm rain within 72 hours), soil temperature 18–35°C, and no 6-hour downpour above 25 mm |
| Irrigation advised | Period rainfall below half of reference evapotranspiration (ETo) for the same period |
| Excess-moisture period | Period rainfall above 1.5× ETo — ensure field drainage, postpone irrigation |
| Prolonged leaf wetness | Relative humidity above 92%, or fog probability above 40%, on two or more days — elevated fungal-disease pressure |
| Sustained wind generation | 100 m wind inside the full-power band (46.8–90 km/h) for 12 or more hours in a day |
| Storm cut-out risk | 100 m wind at or above 90 km/h — turbine shutdowns and steep ramp events |
| Wind lull | Day maximum 100 m wind below the 12.6 km/h cut-in speed — minimal generation |
| Cell-temperature derating | Maximum temperature at or above 40°C — roughly 8–15% below rated photovoltaic output |
Publishing the thresholds is deliberate. A warning you cannot interrogate is a warning you cannot calibrate against your own operation — a farm with irrigation available reads the irrigation rule differently from one without.
What It Cannot Tell You
The limits are as much a part of the product as the thresholds:
- District scale is not local scale. Polygons resolve at roughly 25–50 km. A district alert does not distinguish one tehsil from the next, and it cannot represent a single catchment, a valley, or an urban drainage basin.
- It is model guidance, not an official warning. IMD issues the authoritative district bulletins for India. VayuMet's alerts are independent model-derived guidance and carry no statutory standing.
- The convective and stratiform separation is experimental. It is the most useful step in the chain and also the least settled — treat it as a strong hint about event character, not a verified classification.
- Aviation advisories are advisory only. For flight operations, use current Meteorological Aerodrome Report (METAR), Terminal Aerodrome Forecast (TAF) and significant-meteorological-information messages from the aerodrome meteorological office, per ICAO Annex 3. The VayuMet Aviation Dashboard carries those live.
- Confidence falls with lead time. Ten days of alerts are produced, but a day-nine thunderstorm flag is a pattern signal, not an appointment. The first three to four days carry the usable detail.
Reading It Yourself
The district warning map sits at Multi-Hazard Early Warning, with the per-district ten-day breakdown — including the four sector advisories — on the District 10-Day Forecast. Select any district and the advisories are generated for that polygon specifically, which is the level at which these rules are meant to be read.
Open the Warning Map →