Moist Heatwaves During Monsoon Now Predictable Weeks Ahead

Many parts of India experience moist heatwaves during the southwest monsoon (SWM), yet the phenomenon has remained poorly understood. A new study published in the journal “Climate Dynamics” finds that the timing and location of these events are closely linked to the active and break phases of the monsoon. Crucially, researchers suggest that moist heatwaves can now be predicted two to four weeks in advance, enabling better public health preparedness.

What Are Moist Heatwaves?

Unlike dry summer heatwaves driven mainly by high temperatures, moist heatwaves result from a combination of elevated temperatures and high atmospheric humidity. During such events, the body’s ability to cool itself through sweating is severely impaired.

High humidity limits evaporation from the skin, increasing physiological stress. This can lead to hyperthermia, heat exhaustion and, in extreme cases, fatal heatstroke. The risks are particularly acute during outdoor gatherings and prolonged exposure.

Wet Bulb Temperature and Human Survivability

The combined impact of heat and humidity is best measured through the wet bulb temperature, which represents the lowest temperature air can reach through evaporation at constant pressure. It reflects how effectively sweat can cool the human body.

Traditionally, 35°C wet bulb temperature has been considered the upper survivability limit. However, a 2023 study in “Nature Communications” found lower tolerance thresholds. Survivability limits ranged from 25.8°C to 34.1°C for younger adults and 21.9°C to 33.7°C for older adults. Older women showed the lowest tolerance levels, well below the conventional benchmark.

Link with Monsoon Active and Break Phases

The research analysed data from 1940 to 2023 and found that moist heatwave risk shifts geographically depending on the SWM phase. During active monsoon phases, northern plains and central India face higher risk due to elevated humidity combined with heat.

During break phases, when rainfall shifts towards the Himalayan foothills and the Northeast, the risk moves to southern and eastern India. This shift is influenced by the movement of the monsoon trough and the interaction of dry desert winds.

Important Facts for Exams

• Wet bulb temperature measures combined heat and humidity stress.
• Southwest Monsoon has active and break phases linked to rainfall distribution.
• Boreal Summer Intraseasonal Oscillation (30–90 days) influences monsoon variability.
• Moist heatwaves differ from dry heatwaves due to high humidity impact.

Forecasting and Public Health Preparedness

The study links moist heatwaves to the Boreal Summer Intraseasonal Oscillation, a 30–90 day large-scale weather pattern moving from the Indian Ocean to the western Pacific. Certain phases increase northern India’s heatwave risk by over 125% above normal.

Advanced forecasting can support timely adaptation measures. Authorities can reschedule mass events, expand hospital capacity, open cooling centres and manage power demand. As climate variability intensifies, early warning systems for moist heatwaves could play a critical role in reducing health impacts across India’s monsoon-affected regions.