Initial Spread Index (ISI)
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The Initial Spread Index (ISI) is a component of the FWI system that estimates the expected rate of fire spread. It combines the two factors that most directly control how quickly a fire moves: wind speed and fine fuel moisture.
Why it matters for fire weather
Section titled “Why it matters for fire weather”The ISI captures the most volatile, hour-to-hour dimension of fire danger. While the BUI changes slowly over days and weeks, the ISI can swing dramatically within a single day as wind picks up and humidity drops.
High ISI values indicate conditions where:
- Fire fronts advance rapidly
- Embers are transported ahead of the fire, creating spot fires
- The window for fire growth narrows quickly
The ISI is particularly relevant for wind-driven fires — the fire behaviour category common during Mediterranean wind events (mistral, tramontana, foehn) where wind dominates fire spread (Castellnou et al., 2009).
How it works
Section titled “How it works”The ISI is calculated as the product of a wind effect and a moisture effect (Van Wagner, 1987):
ISI = 0.208 × f(W) × f(m)
Wind effect
Section titled “Wind effect”The wind function is exponential:
f(W) = exp(0.05039 × W)
Where W is the noon wind speed in km/h. This means the effect of wind on spread rate accelerates disproportionately:
| Wind speed | Wind effect f(W) | Relative to calm |
|---|---|---|
| 0 km/h | 1.0 | Baseline |
| 10 km/h | 1.7 | 1.7× |
| 20 km/h | 2.7 | 2.7× |
| 30 km/h | 4.5 | 4.5× |
| 40 km/h | 7.4 | 7.4× |
Moisture effect
Section titled “Moisture effect”The FFMC (Fine Fuel Moisture Code) is converted to a fine fuel moisture content, which feeds into a function where drier fuels dramatically increase the spread rate. When FFMC is high (dry fuels), the moisture effect amplifies the ISI; when FFMC is low (wet fuels), it suppresses it.
The interaction means that strong wind with wet fuels produces a moderate ISI, while strong wind with dry fuels produces a very high ISI. Neither factor alone tells the full story.
Key thresholds
Section titled “Key thresholds”The ISI is a unitless index. Its absolute values are most meaningful when compared to climatological norms for a given location and time of year.
| ISI | Typical conditions |
|---|---|
| 0–5 | Light wind or moist fuels. Fire spread is slow and primarily influenced by slope and fuel arrangement. |
| 5–10 | Moderate wind with drying fuels. Active fire spread. |
| 10–15 | Strong wind or very dry fuels. Rapid spread, with ember transport becoming significant. |
| 15–25 | Very high spread potential. Conditions associated with fast-moving fires. |
| > 25 | Extreme spread rate. Uncommon values indicating exceptional wind-drought combinations. |
How to read it in Wildflyer
Section titled “How to read it in Wildflyer”ISI is computed as part of the FWI system and displayed alongside other components in the expert view. Because it responds quickly to wind and humidity changes, the ISI is the FWI component that varies most through the day — it typically rises as afternoon winds pick up and humidity drops.
Compare the ISI with the BUI to understand the character of fire danger: high ISI with low BUI suggests fast-moving but shallow fires; high ISI with high BUI suggests fast-moving and deep-burning fires.
Sources
Section titled “Sources”- Van Wagner, C.E. (1987). Development and structure of the Canadian Forest Fire Weather Index System. Forestry Technical Report 35, Canadian Forest Service.
- Castellnou, M., Miralles, M., & Molina, D. (2009). Wildfire management in Mediterranean-type regions: paradigm change in Southern Europe. Wildfire, 18(3): 18–23.