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Initial Spread Index (ISI)
The ISI combines wind speed and fine fuel moisture to estimate how fast a fire will spread. Higher values indicate faster-spreading fire.
| Color |
Value |
Category |
Description |
| | 0 – 5 | Very Low | Slow fire spread. Easy containment. |
| | 6 – 10 | Low | Moderate spread. Manageable under most conditions. |
| | 11 – 20 | Moderate | Rapid spread. Increased suppression effort needed. |
| | 21 – 30 | High | Very rapid spread. Aerial support likely needed. |
| | 30+ | Extreme | Explosive spread. Extreme fire behavior expected. |
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