NWS Fire Spread Index
The Fire Spread Index is an estimate of how fast a fire might spread given the wind, temperature, humidity, and vegetation state (green vs cured). The images shown are using a cured state. Measured in feet per minute.
The fire danger for cured grasses (not forest fires) are as follows: 0 to 29: Low Fire Danger | 30 to 43: High Fire Danger | 44 to 63: Very High Fire Danger | 64 to 100: Extreme High Fire Danger