The Fire Weather Index (FWI) Calculator estimates the potential risk of wildfire spread based on key weather and fuel conditions. It is widely used by forest fire agencies, meteorologists, and land managers to make informed decisions about fire prevention, suppression readiness, and public safety warnings. This tool belongs to the Wildfire Risk and Weather Assessment Calculator category.
Formula of Fire Weather Index Calculator
1. Initial Spread Index (ISI)
This measures the expected rate of fire spread immediately after ignition.
ISI = 0.208 × exp[0.05039 × W] × f(W) × f(FMC)
Where:
- W = wind speed (km/h)
- f(W) = wind adjustment function
- f(FMC) = function of fine fuel moisture content
The ISI increases with higher wind speed and lower fuel moisture.
2. Buildup Index (BUI)
The BUI represents the amount of fuel available to sustain a fire.
BUI = [0.8 × DMC × DC] / [DMC + 0.4 × DC]
Where:
- DMC = Duff Moisture Code (medium-term moisture)
- DC = Drought Code (long-term moisture)
Higher BUI means more fuel is ready to burn.
3. Fire Weather Index (FWI)
Finally, the FWI combines spread and fuel build-up to assess overall fire risk.
FWI = 0.1 × ISI × f(BUI)
Where:
- If BUI ≤ 80:
f(BUI) = 0.626 × BUI^0.809 + 2.0 - If BUI > 80:
f(BUI) = 1000 / [25 + 108.64 × exp(−0.023 × BUI)]
This calculation results in an index value used to classify fire danger from low to extreme.
Reference Table
| Term | Description | Typical Range |
|---|---|---|
| Wind Speed (W) | Speed of wind in km/h | 0–40+ |
| Fine Fuel Moisture (FMC) | Moisture content in light fuels (%) | 5–25% |
| Duff Moisture Code (DMC) | Soil and organic layer moisture | 0–100 |
| Drought Code (DC) | Deep soil moisture | 0–800 |
These help users input correct values when using the calculator.
Example of Fire Weather Index Calculator
Scenario:
- Wind Speed: 20 km/h
- Fine Fuel Moisture: adjusted through f(FMC) based on tables (assume 0.8 for example)
- DMC: 35
- DC: 100
Step 1: Calculate ISI
ISI = 0.208 × exp(0.05039 × 20) × f(W) × f(FMC)
Assume f(W) = 1.2 (typical)
ISI ≈ 0.208 × exp(1.0078) × 1.2 × 0.8 ≈ 0.208 × 2.74 × 0.96 ≈ 0.55
Step 2: Calculate BUI
BUI = (0.8 × 35 × 100) / (35 + 0.4 × 100)
= (2800) / (35 + 40)
= 2800 / 75
= 37.33
Step 3: Compute f(BUI)
Since BUI ≤ 80:
f(BUI) = 0.626 × 37.33^0.809 + 2.0 ≈ 0.626 × 14.8 + 2.0 ≈ 9.27 + 2.0 = 11.27
Step 4: FWI = 0.1 × ISI × f(BUI)
FWI = 0.1 × 0.55 × 11.27 ≈ 0.62
So, the Fire Weather Index is approximately 0.6, indicating low risk.
Most Common FAQs
The FWI is an industry-standard method used globally for daily fire risk forecasting. However, local variations and sudden weather changes can affect accuracy.
Use it daily during fire seasons or whenever planning controlled burns or outdoor activities in fire-prone areas.
No. It is a valuable tool but should complement local observations, weather forecasts, and professional fire monitoring services.