A comparative analysis of accelerating humid and dry heat stress in India

Extreme humid heat stress poses distinct challenges to human health and productivity that cannot be mitigated solely by heat action plans designed for dry heat stress. This study investigates the trends in extreme heat stress, differentiating between dry and moist heat, in India from 1943 to 2022 using the high-resolution hourly ERA5 reanalysis data. The analysis utilizes the Heat Index (HI) as a key parameter to identify instances of extreme humid heat stress, characterized by HI values exceeding 41 degrees Celsius and relative humidity more than 50%. The findings indicate that certain eastern coastal regions in peninsular India experience extreme humid heat stress from May to June due to persistent high humidity levels. In addition, northwest, northcentral and inland eastern coastal regions encounter extreme dry heat stress preceding the monsoon season, followed by a transition to humid heat stress immediately after the onset of the monsoon. The results also show that there has been a significant increase ofsix times (on average) in the extreme humid heat stress hours per grid over the past 80 years compared to a threefold increase in dry heat stress. Our research underscores the need to shift from exclusively addressing dry heat stress to adopting a comprehensive approach that considers the impacts of humid heat stress.