Mumbai: Scientists have for the first time identified what drives India's most dangerous form of heat during the monsoon season and shown that the risk can be forecast up to four weeks in advance, according to a study published in the journal Climate Dynamics earlier this year.
The research, by scientists at the University of Reading working with institutions in the UK and India, found that a single large-scale monsoon pattern can raise the likelihood of a moist heatwave in northern India by 125 per cent above normal.
Moist heatwaves, measured by wet-bulb temperature, which captures both heat and humidity, are deadlier than dry heat because when the air is already saturated with moisture, the body's primary cooling mechanism, the evaporation of sweat, slows or fails entirely. Core temperature rises, the cardiovascular system is strained, and heatstroke can follow within hours. The temperature on a thermometer may look unremarkable; the physiological effect is not.
East, central and northwest India to witness more heatwave days than usual: IMDThe study maps out a clear geography of risk. During active monsoon phases, the densely populated Indo-Gangetic Plains of northern India , home to hundreds of millions of people, face sharply elevated danger, as sudden surges of monsoon moisture push humidity to physiologically harmful levels even as rainfall continues nearby. When the monsoon weakens or breaks, the risk moves south and east, into peninsular India and along the eastern coast, where absent cloud cover allows heat to build over an already moisture-laden atmosphere.
The populations most at risk are those with least ability to take shelter: farm workers in Uttar Pradesh and Bihar, construction labourers in Delhi and Mumbai, the elderly without air conditioning, and the urban poor in areas where concrete retains heat through the night. A four-week warning window, researchers say, could allow hospitals to adjust staffing, city authorities to open cooling centres, schools to alter hours, and power utilities to prepare for increased grid demand.
The study was led by Dr Akshay Deoras at the University of Reading's National Centre for Atmospheric Science and Department of Meteorology, alongside colleagues from the University of Leeds, the UK Met Office, and the Indian Institute of Tropical Meteorology. Researchers analysed 84 years of atmospheric data (from 1940 to 2023) drawing on ERA5, the European Centre for Medium-Range Weather Forecasts' global reanalysis dataset, and rainfall records from the India Meteorological Department. Across that period, the team identified 261 active monsoon events and 188 break episodes, tracking how moist heatwave risk shifted before, during, and after each. The study used wet-bulb temperature as its core metric, a measure that reflects both heat and humidity and directly captures the body's ability to cool itself, rather than air temperature alone.
"We often find people being more aware of dry heatwaves in India, given the scorching summer season, but moist heat remains less known and is therefore more dangerous," said Dr Deoras. "Because we can forecast these monsoon patterns weeks ahead, this creates real opportunities to prepare and protect people."