McArthur Forest Fire Danger Meter Mk5
- Published: Sunday, 28 December 2014 11:24
- Written by Lance Hartley
- Set last rainfall (A) against number of days since rain (B) and read off Drought Factor (C) in window corresponding to the current Drought Index.
- Set arrow D against the derived Drought Factor.
- Set Air Temperature (slide 3) against Relative Humidity (slide 2).
- Read off the Fire Danger Index / Danger Classification on outermost rim opposite Wind Speed.
- Refer to back of meter for additional fire behaviour information.
Fuel quantity is expressed in tonne/ha of combustible material less than 6mm
- R = rate of forward spread (km / hr)
- H = flame height (m)
- S = average spotting distance (km)
FOREST FDI's - Fire Behaviour Relationships
- An index of one (1) means that fires either will not burn, or burn so slowly that control presents little difficulty. An index of one hundred (100) means that fires will burn so fast and hot that control is virtually impossible. The meter is designed for general fire danger forecasting purposes and is based on the expected behaviour of fires burning for an extended period in high eucalypt forest carrying a fuel quantity of 12.5 tonnes per hectare and travelling over level to undulating topography.
- Fuel Quantity is expressed in tonnes per hectare of combustible material less than 6 millimetres in diameter. The behaviour of individual fires can be predicted with reasonable accuracy providing the effect of fuel quantity and slope is taken into account.
- The rate of perimeter spread is generally three times the rate of forward progress but may increase to a factor of four on a large irregular fire.
- Fires travel upslope with the prevailing wind faster than on level ground. A five degree slope increases spread by 33 per cent; a ten degree slope by a factor of two and a twenty degree slope by a factor of four. Corresponding reductions occur on downslopes.
- Fires in low quality eucalypt forest tend to spread at faster rates than those shown, due to greater wind movement near the ground. However, the spotting potential is generally lower. The meter can be used to determine broad control burning conditions although a high degree of precision should not be expected. Burning at an index greater than twelve (12) would be very risky except in very light fuel types. The rate of forward spread refers to a moving flame front which is only affected by relatively short distance spotting. When long distance spotting occurs, the rate of spread may be greater than indicated. The spotting distances given in the table below are for fuel types containing a high proportion of fibrous-barked eucalypts. Gum-type eucalypts will only throw long distance spot fires after a crown fire develops.
- Atmospheric instability is not included as a factor affecting fire behaviour. However the rates of spread indicated are for generally unstable conditions and may be less under conditions of stability.
A.G. McArthur, Forest Fire Danger Meter Mk IV 1973.
Bush Fire Council of N.S.W. Officer Training Module CL/4 - Fire Behaviour Second Edition