CHENNAI: Monsoon is associated with relief from extreme summer temperatures. But a recent study has shed new light on a lesser known but highly dangerous climate threat: moist heatwaves during the monsoon season, the combination of heat and humidity that can be more lethal than dry heatwaves, particularly for regions like Tamil Nadu.
The study by scientists at the University of Reading in collaboration with Indian and UK researchers, analysed weather and rainfall data over 80 years to understand how monsoon dynamics influence the occurrence of moist heatwaves across India.
Their findings reveal a pattern that has direct implications for southern states. Unlike northern India, where the risk peaks during active monsoon phases, Tamil Nadu and other parts of coastal India face greater danger during "break" periods, the intervals when rainfall activity weakens or pauses. During such breaks, reduced cloud cover allows solar radiation to heat the land surface, while the atmosphere remains loaded with moisture, creating ideal conditions for moist heatwaves.
In Tamil Nadu, where coastal humidity is already high for most part of the year, even a modest rise in temperature during these break periods can push wet-bulb temperatures into dangerous territory. Wet-bulb temperature is considered a more accurate indicator of heat stress on the human body because it reflects the body's ability to cool itself through sweating. When humidity is high, sweat evaporation slows, causing rapid increases in core body temperature, significantly raising the risk of heat exhaustion and heatstroke.
The study identifies the Boreal Summer Intraseasonal Oscillation (BSISO), a large-scale atmospheric pattern that drives the monsoon's internal variability, as the key mechanism behind these shifts. While this oscillation increases moist heat wave risk in northern India during active phases, it shifts the threat southward during breaks directly affecting states like Tamil Nadu, Andhra Pradesh, and parts of coastal Karnataka.
One of the most important outcomes of the research is predictability. Scientists have shown that the BSISO can be forecast two to four weeks in advance, allowing authorities to implement targeted interventions.
Urban centres such as Chennai, Coimbatore, Madurai, and Tiruchy are particularly vulnerable due to dense construction and limited green cover that amplify heat retention, while coastal districts may experience compounded effects due to persistent humidity from the Bay of Bengal.
The study also highlights the disproportionate impact on vulnerable populations. In Tamil Nadu, this includes daily wage workers, fishermen operating in humid coastal environments, agricultural labourers in Delta regions, and residents of informal houses like metal roofing houses where ventilation and cooling options are limited. The elderly and those with pre-existing health conditions face additional risk.
Speaking to DT Next, G Sundarrajan of Poovulagin Nanbargal pointed out that there is a shortfall in the heat-related warnings usually issued by the IMD. The government should take note of the warning in the study that monsoon can intensify humid heatwaves, he added.
The wet-bulb effect is the inability of the human body to cool itself through sweat evaporation when both air temperature and humidity are high. When wet-bulb temperatures exceed 31 to 35°C, sweating becomes ineffective, leading to rapid, potentially fatal overheating, even for healthy individuals staying in the shade.