India’s heatwave warning system is set to undergo its most significant review in years. The India Meteorological Department (IMD) plans to expand its criteria for heat alerts to include a percentage-based threshold – a shift that would, for the first time, officially capture the kind of oppressive humid heat that coastal areas routinely experience, but which 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.
Read also: IMD predicts a heatwave week in Delhi with temperatures likely to reach 44 degrees Celsius
Why change?
This change is designed to address a specific gap: areas, especially along the coast, where temperatures may not reach the absolute thresholds required under current rules but where high humidity makes the heat truly dangerous.
Mohapatra: “For example, in coastal parts of southern India, we may not see temperatures reaching 44 or 45 degrees Celsius, but it is very hot because the humidity index is very high. If the models show that in the next two days, the temperature will be above the 95th percentile, we will issue a heatwave alert. So, we will not wait until the maximum temperature exceeds 5 degrees above normal.” “We have already started issuing alerts on a percentage basis,” he added.
Under the current framework, the IMD issues heatwave warnings for plains when maximum temperatures reach 45°C or when daytime temperatures exceed the normal range 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 multiplier effect of humidity.
Read also: IMD to change parameters for announcing heat waves
“Good decision,” say scientists.
Scientists have welcomed the percentage-based approach. “It is a good decision. Using percentages is much more important than absolute values. This is what we have suggested in our research papers and study. Percentages help in identifying extremes at a particular location irrespective of the background average. There is no universal rule for defining a heatwave. It should reflect the meaning and purpose of a heatwave warning,” said former secretary of the Ministry of Earth Sciences and a climate scientist.
The review takes on greater urgency in the context of what appears to be a harsh summer. On Friday, an IMD alert indicated that the 95th and 98th percentile thresholds for minimum temperatures will be breached near coastal Karnataka and in a few other locations across the country.
El Nino in India
The El Niño phenomenon is expected to develop by mid-2026, with the World Meteorological Organization and the US National Oceanic and Atmospheric Administration estimating the probability of its appearance between June and August at about 80%.
The World Meteorological Organization (WMO) also warned of a near-global dominance of above-normal surface temperatures during May, June and July, and of below-normal rainfall over India until July.
The science behind why humidity changes the heat stress calculation has evolved rapidly. Wet bulb temperature — a measure that combines heat and humidity to see how effectively the human body cools itself through sweating — has emerged as a more reliable indicator of heat risk than air temperature alone.
When the wet bulb temperature rises to the level of human skin temperature, sweat is no longer able to evaporate, and the body’s basic cooling mechanism fails completely.
The widely accepted survival threshold for wet bulb temperature has long been set at 35°C. But as HT reported last April, researchers from Harvard University, who attended an interdisciplinary conference with the Union environment ministry, said physiological studies now suggest the true limit may be closer to 31 degrees Celsius.
The scientists concluded that the temperature thresholds that should trigger emergency responses are “far more complex than currently understood.”
