Understanding the fire danger rating system

As we travel Victoria’s roads many of us will have seen an adjustable fire danger rating sign (click here) used by the CFA to warn us of the bushfire danger in the area.

The Australasian Fire and Emergency Services Authorities Council (AFAC) defines “fire danger” as:

Sum of constant danger and variable danger factors affecting the inception, spread, and resistance to control, and subsequent fire damage; often expressed as an index.

and “fire danger rating” as:

A relative class denoting the potential rates of spread, or suppression difficulty for specific combinations of temperature, relative humidity, drought effects and wind speed, indicating the relative evaluation of fire danger.

Extracts from Bushfires in Australia, by R H Luke and A G McArthur considered relevant to this posting:

Fire danger rating systems serve two major purposes. The first is to provide the basis for the Bureau of Meteorology fire weather forecasts, which are issued through the media as a public information service. When warnings of increasing fire danger are included, such messages may be associated with or contain announcements of fire restrictions [e.g. periods of total fire ban]. General terms must be used as climatic regions cover large areas and many fuel types.

The second major purpose of a fire danger forecast is to provide fire control managers and other responsible persons with reliable daily or even hourly information on which to base their assessment of fire risk, likely fire behaviour and many other important matters such as those relating to the issue of burning-off permits, detection services [e.g. activating fire lookout towers] and the location [pre-positioning] of initial [firefighting] attack crews.   

If a precise prognosis of likely fire behaviour in a specific fuel type is required for a running fire the fire danger rating system should be capable of enabling the fire boss [incident controller] to answer such questions as:

• the rate of spread of the fire;

• difficulty of control; will mechanical equipment [including water bombing aircraft] be required or can it be handled by initial attack forces?

• will it be a high intensity or low intensity fire?

• will it produce a towering convection column or have a wind-driven smoke plume?

• will it be a crown [sustained tree canopy] fire or a ground [or surface] fire?

• what is the probability of it ‘blowing up’ [developing into an especially violent fire] in forest fuels?

• what will be the spotting potential of the fire and the likely distance that spot fires will be thrown?

Referring back to my posting of 16 February (click here) where I mentioned FDI (fire danger index) read this to mean “fire danger rating” in this posting.

The Fire Danger Rating sign (click here) mentioned earlier is graduated according to the FDI:

		Rating                                       FDI
	Code Red                                 100 + Extreme                                   75 – 100 Severe                                     50 – 75 Very High                                 25 – 50 High                                         12 – 25 Low – Moderate                         0 – 12

AFAC defines FDI (Fire Danger Index) as:

A relative number denoting an evaluation of rate of spread, or suppression difficulty for specific combinations of temperature, relative humidity, drought effects and wind speed.

So, what does this “fire danger rating” and ‘fire danger index” advice to the community mean and how should it be used? An issue touched on in my 16 February (click here) posting that I’ll come back to in a future posting, as I believe that in the manner it’s currently being used by the CFA to warn the public can be misleading, instil panic and is potentially dangerous.

The FDI/fire danger rating is not static or “one-size-fits-all” and can vary upwards according to a dynamic weather environment as the recorded weather data on my 16 February posting illustrates. And can vary downwards where land is subjected to onshore wind in coastal environments or the blocking influence of high ground.

To conclude this posting, the default FDI used by the CFA in its approach to dealing with Bushfire Management Overlay Bushfire Management Statements is FDI 120, which is based on weather recorded at the Bureau of Meteorology Recording Station at Melbourne Airport during the afternoon of the 1983 “Ash Wednesday Fires: 41degrees; Relative Humidity 5 per cent ; wind velocity of 45 k/hr and a drought factor of 10 (0 to 10).

One example of an on-line Forest Fire Danger Index calculator (click here).

Key inputs:

• Temperature

• Relative humidity

• Wind speed

• Drought factor