Wildland Fire Danger Index (FDI) FAQ

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 1 p.m.

Although the FDI does tell us something about wildfire activity on past days with similar conditions, it does not tell us anything about how difficult any new fires may be to control should a wildfire begin.

The observed fire danger is derived from a matrix of Energy Release Component (ERC) and Relative Humidity (RH). The value for the ERC is based on the actual weather observations at 1 p.m. local time. The relative humidity reading is also for 1 p.m. We take the reading at 1 p.m. to be consistent with the rest of the country and it allows us to issue the product before 5 p.m. Eastern Time. This Observed Fire Danger is valid for 1 p.m. on the date shown on the observed fire danger map. The Forecast Fire Danger is valid through 1 p.m. local time for the date shown on its map. In most cases, this rating can be extended through the rest of the afternoon. However, there are instances where this may not be the case. One example of this is when a dry air mass behind a cold front does not enter the state until later in the afternoon, causing relative humidities to fall dramatically after 1 p.m.

When using relative humidity measurements, our observation system requires us to use the actual observation or the minimum value from the past 24 hours. Unfortunately, the minimum relative humidity over 24 hours at 1 p.m. is generally from the previous afternoon. A relative humidity value from the previous day does not always hold much significance for the current day. On most days, the 1 p.m. relative humidity should be near the minimum value. While not ideal, it is more valuable than a relative humidity from the previous day. The forecast FDI instead uses the forecast minimum relative humidity through 1 p.m. on the next day, in case the minimum relative humidity might end up occurring before 1 p.m.

As moisture levels in both live and dead fuels go down, more fuel becomes available to a fire in that area. More fuel burning results in more heat being released. This amount of energy release is what the ERC measures (in BTU/ft2). If the environment were to be compared to a charcoal grill, the ERC could be seen as a measure of how many briquettes are in the grill. The ERC is valuable to determining fire danger because it acts as a composite fuel moisture index, as it has contributions from all types of live and dead fuels. Changes in the "heavy" fuels, which are 6 inches or larger in diameter, have a greater impact on the ERC than a change in the "fine" fuels, which are 1/4 inch or less in diameter.

A significant drop in the ERC is most likely due to a rain event that lasted for several hours. The duration of rainfall is more important to the ERC than the amount of rainfall. Moisture can more effectively saturate fuels if it comes from rain that falls over a long period of time. Heavy rainfall that falls over a very short period of time is not as effective in saturating available fuels. A rise in the ERC value can happen after a rain event if the RH values drop suddenly. For example, if rain on a particular day was enough to significantly impact the ERC, and the following day the RH values fall to 20% or below, you could see a significant rise in the ERC. Generally, the day to day change in ERC will not be large in the absence of rainfall because only the fine fuels are very responsive to changes in humidity, while the more important heavy fuels respond to precipitation (or the lack thereof). However, when the change in humidity is great, it may have a more noticeable impact on the ERC calculation than usual.

Although rainfall is the most effective way to bring down the ERC, a significant rise in the forecast minimum RH value or the observed RH value at 1 p.m. can add enough moisture to the fine fuels to lower the ERC. The drop should not be more than a few points at most since fine fuels are not as important to the ERC calculation as heavy fuels, but it can drop without rainfall.

The Fire Danger Index as well as its components, the ERC and relative humidity, are based on the information that is measured at individual weather stations assigned to each county, and in forecasts by the NWS for the county center. At certain times of the year, especially during the spring and summer thunderstorm season, rainfall does not fall uniformly over large areas. While this rainfall should even out over time, it is possible that some points may see considerably more or less rain than other places, resulting in differing ERC values. Relative humidity may also play a role, since the FDI is a matrix of the ERC and the RH. Perhaps the most common instance of this will come with the sea breeze. Air brought onshore by the sea breeze is cooler and generally more moist, which will lower the fire danger. If one county's observation station is reached by the sea breeze while a neighboring county's station is not, there could be a significant difference in fire danger. The stations assigned to each county can be obtained by contacting the Forest Protection Bureau of the Florida Forest Service.

If the county color is gray, that means the observation or forecast for your county is missing. Though weather observation stations fail infrequently, they do occasionally have problems, and this is the most likely reason for a missing observed fire danger. If a forecast fire danger is missing, the forecast product from the National Weather Service was not submitted in time to be included in the product. This is also an infrequent occurrence, and is most likely to happen when severe weather occurs. In these cases, the necessity of providing life-saving warnings prevents offices from being able to complete all of their routine tasks. In the case of missing observations or forecasts, the tables will display a relative humidity of -999, while the ERC and the FDI will be -1. These numbers are used to make it obvious that the value is missing.

The FDI in this example tells us that the danger of seeing new wildfires is moderate, but other weather conditions may support the explosive growth of any wildfire. A moderate fire danger with unstable fire behavior parameters can create a dangerous situation for any fire management program. From the manager's point of view, he/she knows their capabilities and understands that on certain days, a wildfire could require all of their available resources to protect life and property. The best idea under those conditions is to keep any fires from starting. It may be impossible to completely remove every potential cause of wildfire, but Florida has been very successful in preventing many wildfire disasters by properly managing the open burning authorization program.

This situation is the reverse of the question just above. In this case, the environment may be conducive to the start of new wildfires, indicated by a high FDI. Other weather conditions, however, may not support the rapid growth of wildfires, and the manager knows that their resources are capable of effectively combating any new fires quickly. In such cases, the manager may determine that only select, well trained people (certified burn managers, for example) may be allowed to burn. The manager knows these people have the training, experience, and resources to help them complete a prescribed burn in these conditions successfully. These managers, in order to effectively supervise the largest open burning program in the country, must weigh potential danger against the training and experience of the person requesting an authorization to burn. Based on this, they make the best decision to protect life and property.