The National Fire Danger Rating System or NFDRS is a calculated set of usable guidelines based on a complex set of equations that uses collected field data. These calculations are used to develop daily indices that can be charted, tracked and immediately made available to all fire managers, support supervisors and on-ground wildland fire fighters.
NFDRS data collected at thousands of "collecting stations" and the calculations made are determined by current, past and predicted weather, the fuel types present and both live and dead fuel moisture. This data is based on sound fire-weather science and easily collected by local support personnel anywhere in North America. It not only is easy to use but is also reasonably inexpensive to operate and portable stations can be used.
The System was developed in the early 1970s and the NFDRS was was released in 1972 for general use by agencies throughout the United States. Modifications to the original system were made in 1978 and 1988. The current system is based on the physics of combustion (or more simply the fire triangle) and laboratory developed constants and coefficients reflecting the relationships between various fuels, weather, topography and risk conditions.
These most relevant and workable outputs are expressed in simple terms which are easily understood and collected by users. The current National Fire Danger Rating System is utilized by all federal and most state agencies to assess fire danger conditions.
What are National Fire Danger Rating System Classes?
Although individual calculations determining drought, current weather conditions, fuel moisture are frequently used independently by fire fighters, the collective results are also reported as fire danger classes and often publicly displayed at local and regional wildfire support units. These color coded classes indicate the rates at which fires can occur and rates of outdoor fire spread.
CLASS 1: Low Danger - COLOR CODE: Green
Fuels do not ignite readily from small firebrands, although a more intense heat source – such as lightning – may start many fires in duff or punky wood. Fires in open or cured grassland may burn freely a few hours after rain, but wood fires spread slowly by creeping or smoldering and burn in irregular fingers. There is little danger of spotting.
CLASS 2: Moderate Danger - COLOR CODE: Blue
Fires can start from most accidental causes, but with the exception of lightning in some areas, the number of starts is generally low. Fires in open cured grassland will burn briskly and spread rapidly on windy days. Woods fires spread slowly to moderately fast. The average fire is of moderate intensity, although heavy concentrations of fuel – especially draped fuel -- may burn hot. Short-distance spotting may occur, but is not persistent. Fires are not likely to become serious and control is relatively easy.
CLASS 3: High Danger - COLOR CODE: Yellow
All fine dead fuels ignite readily and fires start easily from most causes. Unattended brush and campfires are likely to escape. Fires spread rapidly and short-distance spotting is common. High intensity burning may develop on slopes or in concentrations of fine fuel. Fires may become serious and their control difficult, unless they are hit hard and fast while small.
CLASS 4: Very High Danger - COLOR CODE: Orange
Fires start easily from all causes and immediately after ignition, spread rapidly and increase quickly in intensity. Spot fires are a constant danger. Fires burning in light fuels may quickly develop high-intensity characteristics - such as long-distance spotting - and fire whirlwinds, when they burn into heavier fuels. Direct attack at the head of such fires is rarely possible after they have been burning more than a few minutes.
CLASS 5: Extreme - COLOR CODE: Red
Fires under extreme conditions start quickly, spread furiously and burn intensely. All fires are potentially serious. Development into high-intensity burning will usually be faster and occur from smaller fires than in the Very High Danger class (4). Direct attack is rarely possible and may be dangerous, except immediately after ignition. Fires that develop headway in heavy slash or in conifer stands may be unmanageable while the extreme burning condition lasts. Under these conditions, the only effective and safe control action is on the flanks, until the weather changes or the fuel supply lessens.