Fire Danger Index Components

Sierra Front Interagency Dispatch Center

FIRE DANGER RATING SYSTEM

The National Fire Danger Rating System (NFDRS) is a system that integrates the effects of existing and potential fire danger factors into a series of numeric “components” and “index’s” that indicate fire spread and intensity. Fire Danger is a broad scale assessment that assists the fire manager in being able to determine the potential of overall fire behavior within a geographic area. This geographical area is referred to as a Fire Danger Zone. Due to the size, topographical variance, slopes, aspects, and multiple vegetation types in a typical fire danger zone, fire danger predictions are taken as relative indicators of fire behavior. This means actual fires will have behavior characteristics that are influenced by local conditions that the model does not predict exactly. Fire Danger Ratings describe conditions that reflect the potential, over a large geographical area, for fires to ignite, spread, and require suppression action. The Sierra Front wildland fire cooperators utilizes the National Fire Danger Rating System to assess wildland fire potential and behavior. The Sierra Front Interagency Dispatch Center processes weather observations for ten local fire weather stations to obtain the Fire Danger assessment within its jurisdiction.

FIRE DANGER INDEX COMPONENTS AND INDEX’S

Ignition Component: Chances of a firebrand, if present, would start a fire. The easiest way to look at this is to visualize if you had a hundred matches, and you individually lit each one and flipped it into a stack of pine needles on the ground, how many of the matches would actually start a fire. This value is expressed as a percentage (Ie. Value 75 means that there is a 75% chance of a fire starting if a firebrand is present). Range is from 0-100.

Spread Component: The rating of the forward rate of spread of a head fire. The value is read in feet per minute Ie. If the SC value is 25, it means that the forward rate of spread is 25 feet per minute.

Energy Release Component: A number related to the available energy (Btu/square foot) within the flaming front at the head of a fire. The significance of ERC values is not obvious to the field user, unless he/she is able to compare the current ERC value against the history of fire weather, as the numbers vary from weather station to weather station. Pocket Cards and Seasonal Fire Danger graphs demonstrate best the significance of the numbers. ERC’s work best in the Timber fuel models (including Pinion-Juniper) and in indicating the fuel moisture content throughout the fire season.

Burning Index: A number related to how hard the fire is to contain. The Burning Index value is flame length X 10. To get the flame length, just divide the BI by 10.

Adjective Rating: Fire danger expressed as a rating value that can be used for public information and fire prevention signing (Smokey Bear Signs that display the Fire Danger Ratings). Ranges are Low, Moderate, High, Very High, and Extreme.

NOTICE: The National Fire Danger Rating System is a model that, based on particular parameters, gives a GENERAL picture of the Fire Danger. It is not specific to any one fire. The actual fire may demonstrate behavior that is greater than the model prediction. Always expect the actual fire to exemplify fire behavior not typical of the model.

(Source: Gaining an Understanding of the NFDRS / NFDRS 1978 Deeming)