Hot-Dry-Windy Index (HDWI)
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The Hot-Dry-Windy Index (HDWI) quantifies the atmosphere’s contribution to fire behaviour, independent of fuel type or topography. It answers a straightforward question: how much is the weather alone making conditions conducive to fire?
Proposed in 2018 by Srock et al. in the United States, the HDWI is designed to detect days when the combination of heat, dry air, and wind creates conditions for significant fire behaviour.
Why it matters for fire weather
Section titled “Why it matters for fire weather”The FWI system integrates weather over time through fuel moisture codes, which makes it excellent at tracking cumulative drying but slower to respond to sudden atmospheric changes. The HDWI, by contrast, captures the immediate atmospheric environment — it responds instantly to today’s heat, humidity, and wind without requiring historical moisture data.
This makes the HDWI particularly valuable for:
- Heat wave detection — identifying days when the combination of temperature and dryness (captured by VPD) is exceptional
- Wind events — capturing the mechanical force that drives fire spread
- Comparing across regions — the HDWI uses the same physical formula everywhere, making it straightforward to compare atmospheric conditions between locations
How it works
Section titled “How it works”The HDWI combines two factors:
HDWI = VPD × Wind Speed
- VPD (Vapour Pressure Deficit) — measures how aggressively the atmosphere dries vegetation. Higher VPD means hotter, drier air that pulls moisture from fuels faster.
- Wind speed — the mechanical force that drives fire spread and ember transport.
The original formulation by Srock et al. (2018) uses data from the lowest 500 m of the atmosphere, corresponding to the daytime boundary layer where thermal exchanges directly influence fires.
Why VPD rather than relative humidity
Section titled “Why VPD rather than relative humidity”At equal relative humidity, warm air has a much higher VPD than cool air. This explains why fires become severe during heat waves even when relative humidity appears similar to cooler days. VPD captures the actual drying power of the atmosphere, not just the percentage saturation.
Key thresholds
Section titled “Key thresholds”The HDWI is a continuous index — higher values mean greater atmospheric contribution to fire conditions. There are no fixed universal thresholds. Values are most useful when compared against local climatology: how does today compare to the typical range for this time of year and location?
Because the HDWI is a product of VPD (kPa) and wind speed (m/s in the original formulation), extreme values occur when both factors are simultaneously elevated — hot, dry, and windy conditions.
Strengths and limitations
Section titled “Strengths and limitations”| Strengths | Limitations |
|---|---|
| Simple, physically-based formula | Does not account for fuel type or moisture |
| Effective at detecting extreme heat-dry-wind events | Does not consider topography |
| Comparable across regions | Depends on quality of weather model data |
| Complements FWI by isolating atmospheric effects | No cumulative drought information (unlike DC/BUI) |
How to read it in Wildflyer
Section titled “How to read it in Wildflyer”Wildflyer’s implementation differs from the original academic formulation in two ways:
- Hourly calculation — the original HDWI uses daily maximum values; Wildflyer computes it hourly, letting you see when during the day conditions peak
- Multiple weather models — the original uses a single atmospheric model; Wildflyer lets you compare outputs from several models
These changes prioritise operational utility: seeing the most dangerous hours of the day and comparing model outputs.
Sources
Section titled “Sources”- 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.