Monsoon Patterns Drive Moist Heatwave Risk Across India With Kerala Most Exposed

A study published in Climate Dynamics drawing on more than 80 years of weather data shows that the Boreal Summer Intraseasonal Oscillation, the same atmospheric system that drives the monsoon's active and break phases, governs where and when dangerous combinations of high temperature and high humidity develop across India. During specific oscillation phases, moist heatwave occurrence over northern India rises by 125 per cent above normal levels, while peninsular India including Kerala becomes most vulnerable during monsoon break phases.

The study, led by Akshay Deoras at the University of Reading, demonstrates that wet-bulb temperature combining heat and humidity reflects human survivability limits more accurately than air temperature alone because the body relies on evaporative sweating to cool itself. When humidity is high, evaporation becomes inefficient regardless of air temperature, raising core body temperature to levels where nervous system function degrades and organ damage begins. In Kerala, recent years have seen rising cases of heat-related illness even when temperatures stayed below official heatwave thresholds, reflecting how humidity creates risk that standard metrics miss.

The practical significance is that the same oscillation driving these moist heatwave patterns can be forecast up to four weeks in advance, creating an actionable warning window. Advance warnings could allow hospitals to increase staffing before admissions rise, city authorities to open cooling centres and adjust school hours and power systems to manage demand spikes. Kerala's disaster preparedness frameworks have been strengthened by repeated flood and landslide experiences, but heat has not received equivalent institutional attention. A Council on Energy, Environment and Water assessment found that a majority of Indian districts now face high to very high heat risk.

Deoras notes that dry heatwaves receive more public attention in India because of the dramatic scorching summer season, while moist heat remains less recognised and therefore more dangerous. Outdoor public gatherings are particularly vulnerable since advance warnings of a moist heatwave could allow organisers to reschedule activities and strengthen medical preparedness. Experts call for heat index and wet-bulb temperature to be integrated into early warning systems and public health alerts, especially in humid coastal states. Rising baseline temperatures and unchanged high humidity levels mean that the trend of moist heat risk is expected to intensify as climate change progresses.

Key figure — 125% above-normal moist heatwave occurrence during specific monsoon oscillation phases