Analysis of the Impact of Heatwaves on Thermal Stress in the Cities of Tehran and Mashhad, Incorporating the Excess Heat Factor (EHF) and the Heat Index (HI)

Heatwaves amplify many risks, including increased mortality, drought, wildfire, power shortages, and agricultural losses. This study analyzes the influence of heatwaves on thermal stress in the cities of Tehran and Mashhad using long-term station-based meteorological records provided by the Iran Meteorological Organization for the period ۱۹۹۱–۲۰۲۰, filtered to include March through September. Heatwaves were detected using the Excess Heat Factor (EHF), which identifies events when temperature exceeds the local ۹۰th percentile together with a positive three-day temperature anomaly relative to the preceding ۳۰-day baseline. Thermal stress was quantified using the Heat Index (HI), which integrates temperature and humidity to estimate perceived heat. The comparison of the two indicators reveals a hidden but non-synchronous relationship: EHF responds primarily to short-term temperature anomalies, whereas HI reflects absolute thermal load strongly modulated by humidity. Consequently, years with high HI may not necessarily exhibit long or intense heatwaves, and vice versa. Furthermore, humidity (which plays no role in EHF) significantly elevates HI, and the indicators operate on different temporal scales (event-based for EHF vs. seasonal mean for HI). Results indicate that in Tehran, heatwave intensity based on EHF has declined since the mid-۲۰۰۰s, whereas HI exhibits a positive trend of +۰.۰۰۹ °C yr⁻¹. In Mashhad, EHF intensity has similarly decreased but heatwave frequency has increased, and HI shows a stronger upward trend of +۰.۰۴۹ °C yr⁻¹. These findings highlight the differentiated yet interconnected responses of thermal stress indicators under ongoing climate warming.

کلیدواژه ها:

heatwaves ، Excess Heat Factor ، Heat Index ، Risk ، Health