From Slide Rule to Sign Post: The History of Australia's Fire Danger Meter

If you've ever driven through regional Australia in summer, chances are you've seen one - a roadside sign with a needle pointing to a colour-coded scale ranging from low to catastrophic. The Fire Danger Meter is one of Australia's most recognisable pieces of public safety infrastructure. But behind that simple roadside sign lies a remarkably rich history of science, tragedy, and ongoing adaptation.

A general view showing Greta Bushfire Brigade Station in the background, and a front view of a bushfire danger sign, 7 November 1987 [Museum of Fire Collection]

The Man Behind the Meter

The story begins not with a sign, but with a circular slide rule and a scientist with a penchant for lighting fires, deliberately, in the name of research.

The Forest Fire Danger Index (FFDI) was originally developed by the legendary pioneer of Australian bushfire science, Alan Grant McArthur, during the 1950s and 1960s. McArthur's path to becoming one of Australia's most consequential scientists was a gradual one. After studying forest science at the University of Sydney in 1945, and later the Australian Forestry School in Canberra, McArthur worked first in softwood plantations in the Tumut and Orange districts. In 1951, he was appointed the first full-time fire control officer of the Snowy Mountains, and in 1953 transferred to the Commonwealth Forestry and Timber Bureau in Canberra as a fire researcher. Five years later he was appointed Principal Research Officer in the newly created Division of Forest Research within CSIRO.

McArthur was a hands-on scientist in the truest sense. To gather data, he deliberately lit over 450 experimental fires between 1956 and 1961 under a range of low to moderate weather conditions in the Kowen Forest and Bulls Head Creek area around Canberra, with additional test fires near Traralgon and the Wombat Forest near Daylesford. He made thousands of detailed observations of wind speed, relative humidity, temperature, cloud cover, rainfall, fuel moisture content, flame height, fire intensity, spotting distance, rate of spread and fuel quantity, as well as subjective assessments of fire suppression difficulty.

Because extreme fire weather was too dangerous to recreate experimentally, his subsequent fire danger equations needed to be extrapolated. To set the upper limit of his scale, McArthur turned to the worst fire disaster in Australian history at the time, the Black Friday fires of 1939, which he used as his benchmark for a rating of 100.

A. G. McArthur’s Mk3 Forest Fire Danger Meter, 1963 [Museum of Fire Collection]

How the Meter Works

The physical tool McArthur designed to do these calculations - the Forest Fire Danger Meter (FFDM) - was itself an elegant piece of engineering. The slide-rule design was a circular analogue tool that facilitated practical application by combining inputs like drought factor, temperature, relative humidity, and wind speed to output fire danger ratings, rate of spread, and spotting potential, to produce a Fire Danger Index in a range of 0 to 100. The result is a number that communicates far more than just weather, it describes a fire's likely behaviour in totality. The index is related to the chances of a fire starting, its rate of spread, its intensity, and its difficulty of suppression. An index of 1 means a fire will not burn, or will burn so slowly that control presents little difficulty. An index of 100 means fires will burn so fast and hot that control is virtually impossible.

The FFDM first appeared in operational use in 1967 as the Mk 4, and brought together the results of over 800 experimental fires and wildfire observations into an easy-to-use system. It was designed for general forecasting purposes and is based on the expected behaviour of fires burning for an extended period in high eucalypt forest carrying a fine fuel load of 12.5 tonnes per hectare and travelling over level to undulating topography.

McArthur also developed a separate Grassland Fire Danger Meter to account for the different fuel types found across Australia's vast open landscapes. The Grassland Fire Danger Meter combines temperature, relative humidity, wind speed and degree of curing to produce an index of the degree of difficulty of suppressing fire in a standard average pasture carrying four tonnes per hectare.

Reaching the Public: The Fire Danger Sign

The Fire Danger Meter as a roadside sign, the one most Australians actually encounter, grew out of McArthur's work as fire agencies sought to communicate risk to the public. The signs have been a feature of rural and peri-urban Australia since the 1960s, translating that 0–100 index into a visual scale that could be understood at a glance. In its classic five-category form, the scale ran from Low (a fire would probably not start) through Moderate, High, and Very High to Extreme (a fire would be virtually uncontrollable).

Most successful firefighting, and fuel reduction burning, occurs when the FFDI is in the Moderate range between 5 and 12. The FFDI rises to High between 12 and 24, and Very High between 24 and 50. A day with an index exceeding 50 is considered Extreme.

When the Scale Wasn't Enough

For decades, McArthur's system remained the standard. But it was designed with limits, and Australian bushfires, driven by changing climate conditions, eventually exceeded them.

The remains of a house in Glenburn, Victoria, 2009. [Courtesy of Fire and Rescue NSW]

The FFDI went off the scale on both Ash Wednesday in 1983 and Black Saturday in 2009. Under these extreme or catastrophic bushfire conditions, the weather rather than fuel load or arrangement becomes the dominant factor influencing fire behaviour. When the index recorded well over 200 during the Black Saturday fires in Victoria, conditions McArthur's worst-case benchmark could not have anticipated, the gap in the rating system became impossible to ignore. A new category, Catastrophic (or Code Red), was added for conditions exceeding an index of 100.

The McArthur system also had other acknowledged limitations. It was only designed for use in forests and grasslands. Australia's Mallee heath, woodlands and open savanna were not well forecast. It did not account for conditions like wind changes and atmospheric stability, and the model began to break down at the extreme end of the scale, where small changes to temperature, humidity and wind speed could have a disproportionate influence on the danger index.

A National Overhaul: The Australian Fire Danger Rating System

The introduction of the Australian Fire Danger Rating System (AFDRS) in 2022 was the most significant change to the forecasting of fire danger in Australia in the last 50 years. It replaced the patchwork of state-based systems, each with slightly different rating categories and thresholds, with a single nationally consistent framework.

The new system introduced two distinct tools for measuring risk. The Fire Behaviour Index is a scale of fire danger that takes the latest fire science and produces outputs across eight different fuel types, compared to the existing two. Once calculated, a simpler Fire Danger Rating is determined for an area, using broad bands designed to quickly communicate to the public the expected level of danger and to describe actions they should take to ensure their personal safety.

The four national ratings are now: Moderate (plan and prepare), High (be ready to act), Extreme (take action now to protect life and property), and Catastrophic (for your survival, leave bushfire risk areas).

The planning, development and implementation of the AFDRS required national collaboration of a scale never before seen in Australia's emergency management sector. Its rollout across all states and territories from 2022 marked the point at which Australia's roadside signs were finally unified. The same colours, the same language, the same thresholds from Darwin to Hobart.

An Enduring Legacy

McArthur retired from the CSIRO in 1978 and died later that year. He didn't live to see the Catastrophic rating or the national overhaul his work eventually necessitated — but the system he built from hundreds of controlled burns, handwritten field notes, and a circular slide rule underpinned Australia's approach to fire danger for over half a century.

The humble roadside sign, with its needle and colour-coded arc, is perhaps the most visible legacy of that work — a daily reminder, in summer at least, of just how much science sits behind a simple warning.

-Blog by Acting Curator Ella Terry Murtagh