The Chandler Burning Index (CBI) uses the air temperature and relative humidity to calculate a numerical index of fire danger. That number is then equated to the Fire Danger severity of either extreme, very high, high, moderate, or low. It's based solely on weather conditions, with no adjustment for fuel moisture.
In FWICalc daily CBI ratings are calculated, and an averaged 30 day historical rating is also calculated.
Daily CBI = (0.0167 * (104.5 - (1.373 * H) + (0.54 * T)) * (124 * Power(10,(-0.0142 * H))))
In the United States the National Weather Service calculates a 30 day Chandler Burning Index based on forecasted weather conditions for the month.
Monthly = (((110 - 1.373*H) - 0.54 * (10.20 - T)) * (124 * power(10,(-0.0142*H))))/60
LOW (Green) <50
Fuels do not ignite readily from small firebrands although a more intense heat source, such as lightning, may start fires in duff or punky wood. Fires in open cured grasslands may burn freely a few hours after rain, but woods fires spread slowly by creeping or smoldering, and burn in irregular fingers. There is little danger of spotting.
MODERATE (Blue) 50 - 75
Fires can start from most accidental causes but, with the exception of lightning fires in some areas, the number of starts is generally low. Fires in open cured grasslands will burn briskly and spread rapidly on windy days. Timber 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.
HIGH (Yellow) >75 - 90
- 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 fuels. Fires may become serious and their control difficult unless they are attacked successfully while small.
VERY HIGH (Orange) >90 - <97.5
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.
EXTREME (Red) 97.5+
Fires 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 fire danger class. 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.
The Angtstrom Index (Angstrom) is used primarily in Sweden, it uses the air temperature and relative humidity to calculate a numerical index of fire danger. That number is then equated to the Fire Danger severity of either Extreme, High, Moderate, or Low. It's based solely on weather conditions, with no adjustment for fuel moisture. The lower the number, the higher the fire risk.
Angstrom Index = (H/ 20)+ ((29 ?T) /10)
where H = Relative Humidity as % (0-100)
and T = Temperature in Degrees Celcius.
> 4.0 Fire occurrence unlikely
4.0 – 2.5 Fire conditions unfavourable
2.5 – 2.0 Fire conditions favourable
< 2.0 Fire occurrence very likely
The Fuel Moisture Index (FMI) is very basic but showed in a study presented by its developer Sharpes a very good
performance in comparison with other fuel moisture indices. It uses the air temperature and relative humidity to calculate a numerical index of fire danger. No fire danger classification exists for this index. The lower the number, the higher the fire risk.
FMI = 10 - 0.25 (T - H)
where H = Relative Humidity as % (0-100)
and T = Temperature in Degrees Celcius.
FOREST FIRE DANGER INDEX (FFDI)
The Forest Fire Danger Index (FFDI) was developed in the 1960s by CSIRO scientist A.G. McArthur to measure the degree of danger of fire in Australian forests. The index combines a record of dryness, based on rainfall and evaporation, with meteorological variables for windspeed, temperature and humidity.
A FFDI of between 12 and 25 on the index is considered a "high" degree of danger, while a day having a danger rating of over 50 is considered an "Severe" fire danger rating. Above this level in 2010 a distinction was made between Forest and Grassland fuels. For Forest fuels, an FDI over 75 is categorised as "Extreme" and over 100 as "Catastrophic" (In Victoria the alternate rating name of "Code Red" has been adopted). For Grassland Fuels the threshold FDI values for the Extreme and Catastrophic Ratings was increased to 100 and 150 respectively.
GRASSLAND FIRE DANGER INDEX (GFDI)
The Grassland Fire Danger Index (GFDI) is calculated from air temperature, relative humidity and wind speed. It also varies according to the greeness or curing of the pasture. It provides a figure directly related to the chances of a fire starting, its rate of spread, difficulty of control and the amount of damage it will do. The formula used in FWICalc is generally for Australian conditions, but can be adapted by varying the environment variables for any part of the world.
THE BYRAM-KEETCH DROUGHT INDEX (BKDI) & DROUGHT FACTOR (DF)
The Byram-Keetch Drought Index (BKDI) attempts to measure the amount of precipitation necessary to return the soil to full field capacity. It is a closed system ranging from 0 to 203 units (0 - 800 imperial) and represents a moisture regime from 0 to 203 millimetres (0 - 8 inches) of water through the soil layer. At 203 millimetres (8in) of water, the KBDI assumes saturation. Zero is the point of no moisture deficiency and 203 (800) is the maximum drought that is possible. At any point along the scale, the index number indicates the amount of net rainfall that is required to reduce the index to zero, or saturation.
The inputs for KBDI are weather station latitude, mean annual precipitation, maximum dry bulb temperature, and the last 24 hours of rainfall. Reduction in drought occurs only when rainfall exceeds 5mm (0.20 inch) (called net rainfall).
Drought Factor (DF): A key component of the FFDI is the modelling of the dryness of the fuel. This is expressed by the Drought Factor (DF), which ranges from 0 to 10. If this is multiplied by 10 and called a percent, it gives the percentage of fine fuel that would be removed by a fire under the current conditions.
The DF is based on recent rainfall and on the Byram-Keetch Drought Index (BKDI).