University of Reading study shows lethal moist heatwaves during southwest monsoon can be predicted weeks in advance

Key facts

• Moist heatwaves occur when high temperature combines with high humidity, preventing sweat evaporation and raising risk of fatal heatstroke within hours.
• University of Reading (UoR) scientists analysed 1940-2023 weather data and found timings and locations of moist heatwaves align with active and break phases of the southwest monsoon (SWM).
• During active SWM phase, northern plains and central India receiving rainfall face moist heatwave risk; during break phases the risk shifts to southern and eastern India.
• The large-scale weather pattern governing these events is the Boreal Summer Intraseasonal Oscillation, a 30-90 day pattern of storm movement between Indian and western Pacific Oceans from June to September.
• Forecast lead time for moist heatwaves is two to four weeks, with heatwave occurrence over northern India rising 125% above normal during a specific phase of this pattern.
• Globally accepted human survivability wet bulb temperature limit is 35°C, but November 2023 Nature Communications paper revised limits to 25.8-34.1°C for younger adults and 21.9-33.7°C for older adults.
• Between 1940 and 2023 there were 188 monsoon break periods and 261 active phases, with breaks influenced by dry desert winds from the west.