BharatNotes What is Heat Wave (IMD Criteria)?
A heat wave is an extended spell of unusually high day temperatures that can cause physiological stress, dehydration, heat stroke and excess mortality. The India Meteorological Department (IMD) does not declare a heat wave on temperature alone — it applies region-specific thresholds combined with either a departure-from-normal rule or an absolute-temperature rule.
A heat wave is considered only when the maximum temperature of a station reaches at least 40°C in the plains, 37°C in coastal areas, and 30°C in hilly regions (IMD criteria, current as of June 2026).
Departure and Absolute Criteria
Once the baseline threshold is met, IMD classifies the event using two parallel rules:
| Criterion | Heat Wave | Severe Heat Wave |
|---|---|---|
| Departure from normal maximum | 4.5°C to 6.4°C above normal | More than 6.4°C above normal |
| Absolute maximum temperature | Actual maximum ≥ 45°C | Actual maximum ≥ 47°C |
Either rule can trigger a declaration. The "normal" is computed from the 1991-2020 climatological average of maximum temperatures for that station and date.
For coastal stations, a heat wave may be declared when the departure from normal is 4.5°C or more, provided the actual maximum reaches 37°C or above.
Persistence and Warm Nights
IMD adds a spatial-temporal persistence rule to avoid flagging isolated spikes: the above criteria must be met at least at two stations in a meteorological sub-division for at least two consecutive days, and the heat wave is declared on the second day.
IMD has also operationalised a "warm night" concept (relevant for night-time heat stress in cities). A warm night is declared only when the maximum temperature is 40°C or more and the minimum temperature departure is 4.5°C-6.4°C (very warm night when the departure exceeds 6.4°C). Heat-wave warnings take priority over warm-night warnings.
Significance and Current Status
Heat waves are a major public-health hazard in India and a recognised disaster in disaster-management planning. IMD issues impact-based, colour-coded warnings (Green/Yellow/Orange/Red), feeding into Heat Action Plans coordinated by NDMA and state authorities. The pre-monsoon season of 2026 again saw extended-range heat-wave guidance issued by IMD (early-June 2026 bulletins), underscoring the recurring nature of the hazard.
These criteria matter because they translate a vague notion of "extreme heat" into an objective, station-level trigger that drives early-warning dissemination, hospital preparedness, work-hour adjustments and water provisioning.
UPSC Angle
This is a foundational concept that supports GS3 questions on disaster management and extreme-weather events, and GS1 questions on the seasonal temperature regime of India. Aspirants should remember the three regional thresholds, the 4.5-6.4°C / >6.4°C departure split, and the 45°C/47°C absolute markers. Link the topic to NDMA's Heat Action Plans, urban heat-island mitigation, and climate-change adaptation for a strong Mains answer. For current-affairs integration, cross-reference seasonal IMD bulletins on Ujiyari.com.
