The Grassland Fire Danger Meter uses the same relationships as the McArthur Mk 4 Grassland Fire Danger Rating System, with two important differences.
Firstly, rate of spread has been removed; it should be calculated separately using the CSIRO Fire Spread Prediction System (CSIRO Grassland Fire Spread Meter).
Secondly, the index value can exceed 100; the index is open-ended.
In McArthur’s original system, an index value of 100 was intended to represent the 'worst possible' fire weather conditions likely to be expected in Australia. However, this value has been exceeded on several occasions since 1966.
Calculating fire danger
The Grassland Fire Danger Meter employs only one fuel variable, degree of curing. Combined with temperature, relative humidity and wind speed, this gives an index of the degree of difficulty of suppressing fire in a standard, average pasture carrying four tonnes per hectare.
The five fire danger rating classes - Low, Moderate, High, Very High and Extreme - represent the degree of suppression difficulty in such a pasture. These classes were defined by AG McArthur in 1966 and remain widely accepted.
Fuel characteristics such as fuel load and grass height do influence difficulty of suppression but the differences among grass species vary widely across the landscape.
The Grassland Fire Danger Meter is designed to provide regional warnings of fire danger and so uses the standard average pasture, which uses the common pasture condition during early summer in southern Australia.
In late summer or during drought conditions fuel loads may be greatly reduced and fire suppression will be easier under any combination of weather conditions given by the fire danger index. We feel it would be better to make local adjustments to preparedness levels rather than reduce the fire danger index.
McArthur's system has been used by rural fire authorities across Australia for more than 30 years, and his fire danger classes have been found to be satisfactory for providing public warnings, setting preparedness levels, and generally providing a good indication of the difficulty of fire suppression over a wide range of conditions.
Retaining the same fire danger rating system will make it possible to compare contemporary fire weather with historical records. Separating fire danger from fire spread will allow adjustments to the fire spread prediction system to be made in the future without disrupting organisational arrangements based on fire danger levels.
Obtain a fire danger meter
All fire danger meters produced by CSIRO are available from:
Sandleford Holdings Pty Ltd
T + 61 3 9786 0055
F + 61 3 9786 0077