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The IMD declares a heatwave using both absolute thresholds and departure from normal. Understanding the criteria explains why coastal warnings arrive at lower temperatures.
Monalisa DuttaHeat is among the most lethal weather hazards in India, and among the most underestimated, in part because its effects are cumulative and frequently attributed to other causes.
The India Meteorological Department applies defined criteria to heatwave declaration, and those criteria are more considered than a single national temperature threshold would be.
Before any heatwave may be declared, the maximum temperature must reach a minimum level appropriate to the region:
| Region | Minimum maximum temperature |
|---|---|
| Plains | 40 °C or above |
| Coastal areas | 37 °C or above |
| Hilly regions | 30 °C or above |
The regional variation reflects both physiological acclimatisation and, in coastal areas, the effect of humidity, which is discussed below.
Once the minimum threshold is met, the declaration depends upon how far the temperature has departed from the normal value for that station and date:
Alternatively, and irrespective of departure, a heatwave is declared when the actual maximum reaches 45 °C, and a severe heatwave when it reaches 47 °C.
A heatwave is generally declared when the criteria are met at two or more stations within a meteorological subdivision, or when they persist at a single station for two consecutive days.
This is, in my view, the aspect of heat risk least well understood by the public.
The human body regulates temperature principally through the evaporation of perspiration. Evaporation requires that the surrounding air be capable of accepting additional moisture. As relative humidity rises, evaporation slows, and above roughly 75 per cent it becomes substantially impaired.
The consequence is that 38 °C at 80 per cent humidity places considerably greater physiological stress upon the body than 44 °C at 20 per cent humidity, notwithstanding that the latter figure is six degrees higher and considerably more alarming in a headline.
This is why the coastal threshold is set at 37 °C. Coastal humidity is persistently high, and the same air temperature is far more dangerous there than in the dry interior.
The combined effect is expressed through indices such as the heat index or wet bulb globe temperature, which account for both temperature and humidity. Wet bulb temperature in particular is significant: sustained wet bulb temperatures approaching 35 °C represent a physiological limit beyond which the body cannot shed heat by evaporation regardless of behaviour, and parts of coastal South Asia are among the regions where such conditions have been approached.
A further criterion which receives limited attention concerns the minimum temperature.
Recovery from heat stress occurs largely at night. When the minimum temperature remains elevated, the body is denied that recovery period, and the cumulative physiological burden increases substantially across successive days.
The IMD accordingly issues warm night criteria alongside heatwave declarations. A sequence of hot days combined with warm nights is considerably more dangerous than the daytime figures alone would suggest, and it is this combination which produces the greatest excess mortality.
The heat season runs broadly from March to June, with peak intensity ordinarily during May and early June immediately preceding monsoon onset.
The regions most affected are north-western and central India — Rajasthan, Gujarat, Madhya Pradesh, Vidarbha, Punjab, Haryana, Delhi, Uttar Pradesh and parts of Bihar and Jharkhand. Coastal Andhra Pradesh and Odisha experience the particularly hazardous combination of high temperature with high humidity.
Heat conditions are ordinarily terminated by monsoon onset, which is one reason a delayed onset carries public health implications extending well beyond agriculture.
Current temperatures across Indian cities are available on our Live Weather Dashboard, and the IMD issues heat warnings using the colour-coded system during the season.
Meteorology consultant and former Squadron Leader in the Indian Air Force, with over eleven years of experience in operational aviation meteorology, weather forecasting and flight safety support. She now advises technology organisations on AI-driven weather forecasting and decision-support systems. Consultancy services →