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Ignition Component (IC)
The Ignition Component is a number which relates the probability that a fire will result if a firebrand is introduced into a fine fuel complex. The ignition component can range from 0 when conditions are cool and damp, to 100 on days when the weather is dry and windy. Theoretically, on a day when the ignition component registers a 60 approximately 60% of all fire brands that come into contact with wildland fuels will require suppression action.

Ignition normally takes place in the dead component of the fine fuels. Three distinct steps must be considered in this ignition process. They are 1) a firebrand must come into contact with the dead fuel, 2) the fuel particle must be dry, and 3) the temperature of the fuel particle must be raised to the kindling point which is about 380 degrees centigrade.

Living material in the fine fuel complex reduces the efficiency of ignition. Therefore, an adjustment to the ignition component is made based on the percentage of live fuel (herbaceous vegetation) in the fine fuel complex.

The moisture content of the dead component of the fine fuel (1-hr. timelag fuel moisture) is determined by the state of the weather (sunny or cloudy), air temperature, and relative humidity at the time of the 2 p.m. fire weather observation.

The condition of the herbaceous (live) vegetation and the 1-hr. timelag fuel moisture are then integrated in the calculation the fine fuel moisture (FFM) which expresses the effective moisture content of the fine fuels.

The closer the initial temperature of the fuel is to the ignition temperature, the more likely a fire will result when a firebrand in introduced into the fine fuel complex, since not a much energy is required to raise the fuel particle to its ignition temperature.


Spread Component (SC)
The Spread Component is a numerical value derived from a mathematical model that integrates the effects of wind and slope with fuel bed and fuel particle properties to compute the forward rate of spread at the head of the fire. Output is in units of feet per minute. A Spread Component of 31 indicates a worst-case, forward rate of spread of approximately 31 feet per minute.

The inputs required in to calculate the SC are wind speed, slope, fine fuel moisture (including the effects of green herbaceous plants), and the moisture content of the foliage and twigs of living, woody plants.

Since the characteristics through which the fire is burning are so basic in determining the forward rate of spread of the fire front, a unique SC table is required for each fuel type.


Energy Release Component (ERC)
The Energy Release Component is defined as the potential available energy per square foot of flaming fire at the head of the fire and is expressed in units of BTUs per square foot. Like the Spread Component, the Energy Release Component is calculated using tables unique to each fuel model. The rate of combustion is almost totally dependent on the same fuel properties as are considered in the SC calculation. However, the principal difference in the calculation of the two components is that SC is determined primarily by the finer fuels, whereas ERC calculations require moisture inputs for the entire fuel complex, i.e., 1-hr. , 10-hr. , 100-hr. , 1000-hr. , and the live fuel moisture.


CHARACTERISTICS OF NFDRS COMPONENTS
IC SC ERC
FUEL MODEL SENSITIVITY Moderate High High
MEMORY Low Low Moderate/High
VARIABILITY High High Low
PREDICTABILITY Low Low High
FIRE SEASON INDICATOR Low Low High
DRIVER SC, 1 Hr FM Wind, 1 Hr FM, Live FM 1, 10, 100, 1000 Hr FM, Live FM

The importance of the basic aspects of fire behavior -ease of ignition, rate of spread, and the rate of combustion - cannot be overemphasized since they dictate what is needed to control a wildfire. Any system or scheme which attempts to rate fire danger should provide accurate and reliable predictions of these three variables. This is the reason why the fire behavior components are the principle keys to fire-danger rating, since they integrate the effects of fuel, topography, and weather into a set of numbers that fire managers can use to meet his or her needs.

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Please send comments or questions about this fire weather training module to John Werth, WSO(FW) Olympia, Wa.

email: john.werth@noaa.gov

phone: (206) 526-6095 ext 251