Florida Department of Agriculture and Consumer Services

Wildland Fire Danger Index (FDI)

The Wildland Fire Danger Index (FDI) is a continuous reference scale for estimating the potential for a fire to start and require suppression action on any given day. The FDI in Florida uses two separate components: the forecast/actual Energy Release Component and the forecast/actual minimum relative humidity. The forecast FDI is based on the National Weather Service forecast for that particular area, the actual is based on the observations taken at 1300 hours (1 p.m.). 

Energy Release Component (ERC)

The Energy Release Component (ERC) is a calculated output of the 1978/88 National Fire Danger Rating System (NFDRS). The ERC is a number related to the available energy (BTU) per unit area (square foot) within the flaming front at the head of a fire. The ERC is considered a composite fuel moisture index as it reflects the contribution of all available live and dead fuels to potential fire intensity. As live fuels cure and dead fuels dry, the ERC will increase and can be described as a build-up index.

The ERC has memory. Each daily calculation considers the past 7 days in calculating the new number. Daily variations of the ERC are relatively small as wind is not part of the calculation. The Keetch-Byram Drought Index (KBDI) on the other hand, has been used by many private and public fire managers in Florida as a key indicator of daily wildfire potential since June of 1998. Although the KBDI index has value, we have determined that the ERC is a better indicator of fuel availability and correlates better with Florida’s fire business. The Florida Forest Service has decided to use the ERC in place of the KBDI.

Relative Humidity (RH)

The ratio of the actual vapor density (which indicates the amount of water vapor present in the air) to the theoretical maximum (saturation) vapor density at the same temperature, expressed as a percentage.

The FDI uses the Energy Release Component (ERC) and the Relative Humidity (RH) component because they best reflect wildland fire business in Florida, and because they work well together. In some situations a low RH does not necessarily mean high fire danger and of course the reverse is true. By the same token a high ERC does not indicate a potentially active day for wildfire. A good example of this would be a frontal passage in Florida that drops three or four inches of rain. Behind the front the RH will drop, and in some cases to very low values. However the rainfall will lower the ERC so that the low RH values will not cause the FDI to sound the alarm unnecessarily.
View the values for the FDI matrix by FFS Region.