Fire Danger Rating

Category	Fire Danger Index
Catastrophic (Code Red)	100 +
Extreme	75 – 100
Severe	50 – 75
Very high	25 - 50
High	12 – 25
Low to moderate	0 - 12

The McArthur Forest Fire Danger Index (FFDI) was developed in the 1960s by CSIRO scientist A.G. McArthur to measure the degree of danger of fire in Australian forests. The index combines a record of dryness, based on rainfall and evaporation, with meteorological variables for windspeed, temperature and humidity.

A similar approach was adopted by McArthur for Grassland areas. Luke and McArthur details the early development and use of these indices.

In 1980 Noble et al produced algorithms based on mathematical equations of best fit. These algorithms have been used since the early 1980's to enable easy computation. However they are often used with input values outside McArthur's intended design, resulting in FFDI values in excess of 100. The power function nature of the algorithm, together with limits of precision in measuring the input variables may result in a large range of uncertainty in the calculated FFDI. For example in conditions that produce an FFDI of 100, small fluctuations in wind speed and temperature would suggest a +/- 20% range in FFDI.

A fire danger rating of between 12 and 25 on the index is considered a "high" degree of danger, while a day having a danger rating of over 50 is considered an "Severe" fire danger rating, over 75 as "Extreme" and over 100 as "Catastrophic" (In Victoria the alternate rating name of "Code Red" has been adopted).

McArthur used the conditions of the Black Friday

Black Friday (1939)

The Black Friday fires of 13 January 1939, in Victoria, Australia, were considered one of the worst natural bushfires in the world, and certainly the single worst in Australian history as a measure of land affected...

 fires of 1939 as his example of a 100 rating.

The FFDI on Black Saturday, 7th of February, 2009, reached much higher than this, although at such extremes it is meaningless to specify a particular value of FFDI.