CHENNAI: When Chennai hosted an air show on October 6, 2024, the temperature at Nungambakkam was only 34.3°C, far below the blistering temperatures usually associated with heat emergencies.
Yet, humidity stood at 73%, pushing the wet-bulb temperature to 30.1°C – a level considered highly stressful for human body. Five people died and more than 200 reportedly fainted during the event.
That tragedy is now being cited as a real-world reminder of a growing danger highlighted in a new scientific study: Humid heat or ‘moist heatwaves’ may be more dangerous than the scorching dry summers Indians fear most.
Researchers from the University of Reading, working with scientists from India and the UK, have for the first time decoded how India’s monsoon controls where and when these deadly moist heatwaves strike. The findings were published in the peer-reviewed journal Climate Dynamics earlier this year.
Using 84 years of weather records from 1940 to 2023, the researchers found that a large atmospheric pattern – Boreal Summer Intraseasonal Oscillation – can increase moist heatwave risk in India by 125% above normal levels. More importantly, the same pattern can be forecasted two to four weeks in advance, opening the door for life-saving warnings.
Unlike dry heat, moist heat becomes dangerous because sweat cannot evaporate efficiently when humidity is high, which means the body loses its main cooling mechanism. Core temperatures rise, heart works harder, and heatstroke can develop rapidly.
The Chennai air show episode showed exactly why temperature alone can be misleading. A day in the mid-30s may not seem extreme by TN summer standards, but when humidity is intense, the body experiences far greater stress. Scientists say wet-bulb temperature, which combines heat and humidity, is a more accurate indicator of human risk than air temperature alone.
The study also found a striking geographical pattern. During active monsoon phases, central and northern India face heightened moist heat risk as sudden surges of moisture create oppressive conditions. But during monsoon ‘breaks’, when rainfall weakens, the threat shifts south and east, including coastal regions such as TN. Here, already humid air combines with sunshine and reduced cloud cover to create dangerous heat stress.
The finding is especially relevant for Chennai, where humidity remains high for much of the year. Public gatherings, outdoor work, sporting events, festivals and transport hubs could all become risk zones when humid heat spikes.
Former chief scientist of WHO Soumya Swaminathan described moist heat as a “silent killer” and said, “We should evolve a heat index, which takes into account temperature, humidity and wind, to tell the safe limit for humans.”
Dr Akshay Deoras, the lead researcher of the study, said better awareness is needed because many Indians recognise dry heatwaves but underestimate moist heat.