Evolution of Heat Index (HI) and Physiological Equivalent Temperature (PET) Index at Mumbai and Pune Cities, India

Abstract

In the present study, trends in heat stress during summer and monsoon season months were assessed for two cities, Pune and Mumbai, for the period of 47 years from 1969 to2015 with the application of empirically derived Heat Index (HI) and rational heat balance based Physiological Equivalent Temperature (PET) index. A stepwise multiple regression analysis was applied to determine contributing meteorological parameters responsible for changes in heat stress incidences. The study reveals a considerable increase in heat stress during the summer months over Mumbai compared to Pune city. Similarly, during the end months of monsoon season, thermal discomfort conditions aggravate over both the cities, with statistically significant rising trends. The actual identification and categorization of thermally discomfortable days during the study period in accordance with the Heat Index were moderate. They remained consistent in Pune during summer, however, in monsoon, heat stress incidences were meager. While at Mumbai days with 'High' and 'Very High,' heat stress have increased towards recent years. Categorization according to PET index depicted conspicuous presence of 'Strong' and 'Extreme heat stress' at Pune, while at Mumbai, 'Warm' and 'Hot' days portrayeda slight increase.  The assessment of meteorological parameters depicted that increased humidity and temperature were the main concern for the increase in heat stress over Mumbai. In contrast, mean radiant temperature, ambient air temperature with restricted wind speed leading to high sensible heat may be responsible for the significant increasing trend in PET. The study infers that both the cities are vulnerable to escalating heat stress and may have adverse implications on the health of city dwellers.