Monsoon Moist Heatwaves Predictable Two to Four Weeks in Advance

Moist heatwaves during India's southwest monsoon season are controlled by the active and break phases of the monsoon and can be predicted two to four weeks in advance, offering critical lead time for public health preparedness, according to a study published in Climate Dynamics by researchers at the University of Reading. The findings address a less-understood but more dangerous heat risk than the dry summer heatwaves that receive greater public attention.

The study found that during active monsoon phases, when northern plains and central India receive rainfall, those same regions face elevated risk of moist heatwaves as monsoon moisture pushes humidity to dangerous levels. During break phases, risk shifts to southern and eastern parts of the country where existing high humidity combines with clear sky conditions that trap heat. Using weather data from 1940 to 2023, lead author Akshay Deoras and colleagues identified 188 monsoon break periods and 261 active phases, and found that moist heatwave occurrence over northern India rises by 125 per cent above normal during one specific phase of the Boreal Summer Intraseasonal Oscillation.

Moist heatwaves are measured using wet bulb temperature, which captures the combined physiological effect of heat and humidity. The globally accepted human survivability limit is 35 degrees Celsius wet bulb, but a November 2023 Nature Communications study revised these limits, finding survivability thresholds between 25.8-34.1 degrees Celsius for younger adults and 21.9-33.7 degrees Celsius for older adults, with older women showing the lowest tolerance at 7-13 degrees below the conventional benchmark. When humidity is high, sweat cannot evaporate from skin, preventing the body from cooling and increasing risks of heat exhaustion and fatal heatstroke within hours.

The study's predictability finding has direct applications for public health planning. Advance warnings could allow event organisers to reschedule large outdoor gatherings such as cricket matches, enable hospitals to increase staffing, allow city authorities to open cooling centres, and help power grid operators manage infrastructure strain. The Boreal Summer Intraseasonal Oscillation, a 30-90 day pattern governing active convection from the Indian Ocean to the western Pacific between June and September, is the large-scale driver of moist heatwave timing and can be modelled weeks in advance using existing forecasting systems.

Key figure — 125% above-normal increase in moist heatwave occurrence during specific monsoon oscillation phase