McArthur Grassland Fire Meter Mk5
- Published: Sunday, 28 December 2014 11:25
- Written by Lance Hartley
- Set arrow (A) against degree of curing.
- Set relative humidity (B) against air Temperature.
- Read off the Fire Danger Index against Wind Speed (D).
- Refer to back of meter for additional information.
- ** Rate of forward spread of fire on level to undulating ground on "average" pasture.
- With an index of 1 or 2 fires will either not burn or burn so slowly that control presents little difficulty. At an index of 100 they will burn so hot and fast that control is almost impossible.
- The intensity of a fire and its difficulty of control is also affected by the quantity of grass in the pasture. Heavy pastures burn faster and with a greater intensity than light pastures. In addition the finer the grass the faster a fire will travel. The rates of spread are average values for fires in annual and perennial pastures carrying a continuous body of fuel and occurring on level to undulating ground.
- Spread rates will be less than indicated in sparse, discontinuous pastures and will also vary according to topography.
Degree of Curing:
The amount of greenness in the pasture or degree of curing is estimated visually for large areas. 100% cured is when all pastures are fully cured and there is no green material at all. When using the meter for estimating fire behaviour over broad areas, an aerial inspection is often the best way to determine a general degree of curing for the area. Some grass species such as wild oats or barley grass will be fully cured while other species will only be partially cured.
Always ensure that the degree of curing is appropriate to the area under consideration. For small areas, curing can be estimated by taking a handful of grass and expressing the amount of dead stalks as a percentage of the total.
The Effect of Slope:
The rates of spread given by the meter apply to level or gently undulating ground. Over short distances the effect of slope is very pronounced. The rate of forward spread will double up a 10 degree slope and will be four times greater up a 20 degree slope. The rate of spread will be correspondingly reduced on a downslope, except that massive fire whirlwinds are likely to develop under severe burning conditions.
Is measured at a height of 10 m in the open and refers to the average wind speed (over 10 minutes). When the wind is gusty, fire behaviour will be erratic, particularly in fine fuels which respond rapidly to 'changes in wind speed.
For all practical purposes, the perimeter of a grassfire can be taken as 2.5 times the forward spread, e.g. if the forward spread is 10 km/h, the perimeter spread will be 25 km/h.
The area of a fire increases as the square of the burning time, e.g. the area at 4 hours from start will be 16 times the area at 1 hour. This indicates the need for very fast initial attack and quick control.
Abundant, fully cured grasslands occurring after a rainfall deficient period of 4-6 weeks; increasing temperatures, falling humidities and rising winds immediately preceding a cool change. Always remember wind changes associated with a cool change.
A.G. McArthur, Grassland Fire Danger Meter Mk IV 1973. Bush Fire Council of N.S.W. Officer Training Module CL/4 - Fire Behaviour Second Edition