Assessing the Deadly Heat Waves Over South Asian Cities: An Insights From UTCI and WBGT Indices

Abstract

This study analysed South Asia's most devastating heat waves over the past four decades using ERA5 hourly reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF). Two heat stress indices, the Universal Thermal Climate Index (UTCI) and Wet Bulb Globe Temperature (WBGT), were employed to evaluate the characteristics, meteorological variable behaviour and diurnal temperature range (DTR) across four selected cities. The findings revealed that while both indices effectively captured heat wave events, WBGT demonstrated superior performance, particularly in assessing nighttime heat stress, where UTCI often underestimated the severity. For instance, during the 1995 heat wave in Bareilly, daytime UTCI exceeded the extreme heat stress threshold of 46.0°C during the second event but dropped close to the lower limit of 26.0°C at night. In contrast, nighttime WBGT consistently remained above the lowest threshold of 25.6°C. In Karachi (2015), WBGT surpassed the extreme heat stress level of 32.2°C during the day and remained above 25.6°C at night for consecutive days. Inland heat waves were marked by air temperature spikes above 40°C and reduced wind speeds (below 2 m/s), while coastal cities like Visakhapatnam experienced prolonged nighttime UTCI above 26.0°C and daytime values fluctuating between 38.0°C and 46.0°C. DTR analysis further demonstrated UTCI fluctuations exceeding 20°C during inland events, while WBGT remained within a 6.6°C range, highlighting its stability and better representation of persistent heat stress. Further analysis of the results revealed that higher WBGT sensitivity to local meteorological variables like relative humidity and wind speed has made it particularly effective in regions with high humidity.