A climatology of the Hot-Dry-Windy Index for the major wildfires of 2017 in Portugal

The Hot-Dry-Windy Index (HDWI) is a fire weather index used to predict the likelihood of intense wildfire activity. This index was developed to consider the weather conditions that are most likely to interact with the surface throughout the burning period on a given day since the depth of the atmosphere that would affect a fire might vary significantly from one fire to another. Studies have shown that HDWI can identify days when physical processes on a synoptic and mesoscale contribute to especially dangerous fire behavior. It particularly analyses the combined effects of three important elements that affect the intensity and spread of wildfires: high temperatures, low relative humidity, and strong winds. Even if other conventional indices (such as the Continuous Haines index (CHI) or the Fire Weather Index (FWI)) fail to indicate excessive dangers, like aforementioned, this indicator might be very helpful in identifying days when fire conditions are exceptionally hazardous. HDWI is more susceptible to short-term, extreme weather events, such as dry thunderstorms or heatwaves with high winds, than standard indices, therefore, the HDWI for the 2017 major wildfires in Portugal was computed, and a brief climatology was analysed. The study comprises the major wildfires of June and October 2017, and the results are focused on the annual, daily, and hourly (12h, 15h, and 18h) HDWI values and related percentiles. Since HDWI is built on the understanding that fire behaviour is not just a function of fuel conditions, but is often driven by immediate weather extremes, this case study underlines the need to examine more than one index due to the increasing severity and complexity of conditions prone to the occurrence of extreme wildfires.

Acknowledgments: This work is supported by National Funds by FCT –Portuguese Foundation for Science and Technology, under the projects UID/04033 and LA/P/0126/2020 (https://doi.org/10.54499/LA/P/0126/2020). This research was supported by the European Union under the Breath IN Erasmus+ project 2023-1-PT01-KA220_HED-00153118.