The forest fire danger meter was a series of calibrated cardboard discs which allowed one to see the effects of air temperature, relative humidity, windspeed in the open (at 10m height) and drought on the forest fire danger index (FFDI) (McArthur 1967). The FFDI scale had one hundred units, the bigger the number the greater the danger. The meter now is more likely to be plastic or in the form of equations (Noble et al. 1980). FFDI values can be grouped together to form the ratings we see on the billboards and hear on the media – low, moderate, high, very high and extreme.
Peter Moore, a colleague, and I have calculated the FFDI for Canberra for the period of weather record (Gill and Moore 1990). In Figure 1 you can see how the average daily 3pm FFDI varies during the year (the lowest line); similarly, you can see how the average maximum for each month varies (middle line) and the estimated record value for each month (top line). Look at the record values and note the ‘extreme’ (FFDI greater than 50) fire danger ratings from September through April.
The second graph (Figure 2) shows a measure of how FFDI has varied through time. It is as you might expect. There is lots of variation, some years have seasons with ‘extreme’ values of FFDI (greater than 50), others do not. It is interesting to try to assess whether or not it is worth tracking current values of FFDI against the average maximums of past years (i.e. the middle line on Figure 1) to see if this provides a guide to the likely FFDI on the days ahead. Possibly, the plotting of the current drought component of FFDI (the Drought Factor, not shown here) against the historical average for the month would be better? Certainly there are wide fluctuations of FFDI during a day and from day to day (Gill and Moore 1990) but the chances of an ‘extreme’ day are higher when a drought condition prevails.
The Forest Fire Danger Index has been a valuable weather index for decades. It is not the answer to all our questions but it has weathered the ravages of time very well.
1 December 1999