HDWI (EN)

What is the HDWI?

The Hot-Dry-Windy Index is a meteorological index designed to quantify the exclusive influence of the atmosphere on the behavior of wildfires, regardless of the type of fuel or topography. The HDWI isolates the contribution of the weather to measure the atmosphere's ability to worsen a fire.


The HDWI aims to identify synoptic and mesoscale situations (greater than 200 km) favorable to the development of extreme fire behavior. It is designed to detect days when the combination of heat–dry–wind creates an atmospheric environment conducive to difficult-to-control fires, particularly in the medium term, within a meteorological forecasting framework.
This index seeks to answer a simple question:
To what extent does the atmosphere alone contribute to making a fire potentially uncontrollable?

What parameters does it take into account?

The HDWI relies solely on three fundamental weather variables:
  • the air temperature,
  • the atmospheric humidity,
  • the wind speed.

These variables are used to calculate the vapor pressure deficit (VPD), which represents the difference between the maximum amount of water vapor that air can hold at a given temperature and the amount actually present.
The Vapor Pressure Deficit (VPD) represents the difference between:
  • the saturated vapor pressure (es) — the maximum capacity of air to hold moisture;
  • the actual vapor pressure (e) — the amount of moisture actually present;
that is: VPD = es − e

The higher the VPD, the drier and hotter the air, and the more capable it is of rapidly drying out vegetation fuels.
Unlike relative humidity, VPD accounts for the air's ability to dry out vegetation fuels. At equal relative humidity, a warm atmosphere will have a much higher VPD than cold air. This explains why fires become severe during heat waves even at constant relative humidity.

Calculation Formula

HDWI is defined by the relationship:
HDW = VPD × U
where:
  • VPD is the vapor pressure deficit;
  • U is the maximum wind speed.
The calculation is performed in the lower atmospheric layer of 500 meters above the ground, corresponding to the daytime boundary layer in which thermal and turbulent exchanges directly influence fires. For each day, the maximum observed value is retained as representative of the potential danger.
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Advantages
Limits
  • simple to calculate,
  • effective for detecting extreme situations (heat, dry air, strong wind),
  • comparable between regions and applicable to multi-day forecasting.
  • complements classic indices by focusing solely on the role of the atmosphere.
  • limited spatial resolution (~50 km): local phenomena are not represented;
  • total absence of consideration:
  • of vegetation;
  • of fuel moisture;
  • of topography;
  • of ignition sources;
  • strong dependence on the quality of atmospheric models.
The HDWI of Wildflyer differs from the original academic formulation by choices aimed at improving operational readability and the temporal resolution of the index.
The original formulation of the HDWI uses wind speed at different levels of atmospheric pressure, particularly around the 900 hPa level, in addition to surface wind. In the Wildflyer tool, a deliberately simplified version is retained, based exclusively on: HDWI = VPD × UThe creators of the HDWI prioritize a daily calculation based on maximum values, in order to facilitate statistical comparisons with burned areas, Wildflyer calculates the HDWI on an hourly basis.This approach allows: visualizing the most dangerous periods of the day, tracking the evolution of weather conditions at a fine scale, and consequently adapting operational strategies more precisely (patrols, restrictions, pre-positioning of resources).Wildflyer allows the calculation of the HDWI from different numerical weather models, which is not the case in the initial model.

Scientific References:

Srock, A. F., Charney, J. J., Potter, B. E., & Goodrick, S. L. (2018).The Hot-Dry-Windy Index: A New Fire Weather Index. Atmosphere, 9(7), 279. https://doi.org/10.3390/atmos9070279