More frequent and intense heatwaves in the last decade have challenged humanitarian, health and meteorological authorities to mitigate impact. Meteorological heatwave monitoring and prediction services vary between heatwave definitions which either include humidity or are based only on temperature. Incorporation of humidity into human health heatwave studies and warning services has been variable. Whilst higher humidity is a known stressor during heatwaves, humidity is known to confound interpretation of heatwave data and can be difficult to monitor and forecast.

This study examines the effect of humidity on diagnosed heatwave severity across Australia’s diverse climate zones. Dry bulb temperature is used as the only input into the Bureau of Meteorology’s current operational Excess Heat Factor (EHF) index. Alternative humidity-affected temperature indices (Apparent Temperature, Wet Bulb Globe Temperature and Heat Index) are examined for suitability as input to EHF to compare the incidence of dry and humidity-affected heatwave severity within Australia. This paper uses maximum and minimum dry and humidity affected temperature indices extracted from Australia’s Bureau of Meteorology Atmospheric high-resolution Regional Reanalysis for Australia (BARRA).

Our investigation demonstrates Australia’s operational temperature-only percentile-based heatwave severity service provides effective heatwave warning guidance for five of Australia’s six diverse climate zones. However, rare very dry or very humid heatwaves in the tropics require both dry bulb temperature-only and Heat Index versions of Excess Heat Factor (EHF) severity index to provide competent operational heatwave early warning guidance.