India’s heat wave warning system is set for its most significant revision in years. The India Meteorological Department (IMD) is planning to expand its criteria for heat alerts to include a percentile-based threshold — a shift that would, for the first time, formally capture the kind of humid, oppressive heat that coastal regions routinely experience but that existing guidelines have long failed to flag.

Under the new framework, a heat alert will be triggered when maximum temperatures exceed the 95th percentile for a given location — the point above which only 5% of all historically recorded temperatures fall.

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“For example, in coastal parts of South India, we may not see temperatures reaching 44 or 45 degree C but it is very hot because the humidity index is very high. If the models are showing that in the next couple of days, the temperature will be higher than the 95th percentile, we will give a heat wave alert. So, we will not wait for the maximum temperature to exceed 5 degrees above normal,” said M Mohapatra. “We have started giving percentile-based alerts already,” he added.

Under the existing framework, IMD issues heat wave warnings for plains when maximum temperatures hit 45°C or when daytime temperatures exceed the normal by 4.5°C to 6.4°C. For coastal areas, the threshold is 37°C or above and at least 4.5°C above normal — criteria that leave little room for the compounding effect of humidity.

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The revision takes on greater urgency in the context of what is shaping up to be a punishing summer. On Friday, IMD’s alert indicated that the 95th and 98th percentile thresholds would be breached for minimum temperatures near coastal Karnataka and at a few other locations across the country.

WMO has also warned of near-global dominance of above-normal land surface temperatures through May, June and July, and of below-normal rainfall over India until July.

The science behind why humidity changes the calculus of heat stress has been evolving rapidly. Wet-bulb temperature — a measure that combines heat and humidity to capture how effectively the human body can cool itself through sweating — has emerged as a more reliable indicator of heat danger than air temperature alone.

When the wet-bulb temperature rises to the level of human skin temperature, sweat can no longer evaporate, and the body’s primary cooling mechanism fails entirely.

The widely accepted survival threshold for wet-bulb temperature has long been set at 35°C. But as HT reported last April, Harvard researchers presenting at an interdisciplinary conference with the Union environment ministry said physiological studies now suggest the true limit may be closer to 31°C.

The temperature thresholds that should trigger emergency responses, the scientists concluded, are “far more complex than presently understood.”