The diurnal variation of wet bulb temperatures and exceedance of physiological thresholds relevant to human health in South Asia

Abstract

Extreme heat is increasingly being acknowledged as a serious hazard to human health, through a combination of physiological responses to heat, expressed as dry and wet bulb temperatures, and personal factors. Here we present an analysis of the diurnal variability of dry and wet bulb temperatures using station data in South Asia during both regular and heatwave days. We find that diurnal cycles differ, with the daily maximum wet bulb temperature occurring several hours after the daily maximum dry bulb temperature. Using radiosonde profiles, we show that the timing and amplitude of the diurnal variability of wet bulb temperature can be explained by changes in boundary layer depths and water content. Physiological thresholds for uncompensable heat stress were exceeded even in the evenings, many hours after dry bulb temperature peaks. Cumulative exceedances occurred in 105 instances, corresponding to at least 300 hours of exposure to uncompensable heat stress in South Asia between 1995 and 2020. We conclude that physiologically relevant thresholds provide a more robust way to estimate health impacts, and that wet bulb temperature alone is insufficient as an indicator of hazardous heat.