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Fire Behavior Index (ISI)
The ISI measures the expected rate of fire spread based on wind speed and fine fuel moisture. Higher values indicate faster spreading fire.
| Color |
Value |
Category |
Description |
| | 0 – 5 | Very Low | Slow spread. Fire easily controlled. |
| | 6 – 10 | Low | Moderate spread under dry conditions. |
| | 11 – 20 | Moderate | Rapid spread. Increasing suppression difficulty. |
| | 21 – 30 | High | Very rapid spread. Difficult to control. |
| | 30+ | Extreme | Explosive spread. Extreme fire behavior. |
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