A picture of a fire danger sign indicating the levels of danger from low to extreme.

Fire danger

Daily fire danger warnings during the fire season help authorities distribute firefighting resources and help the community to protect lives and property.

  • 30 April 2008 | Updated 14 October 2011

Every day during the fire season the Bureau of Meteorology calculates the maximum fire danger of the day.

These fire danger warnings are published in newspapers with the daily weather information, broadcast on radio and television, and shown on roadside fire danger signs.

Fire authorities in Australia use the forecasts of fire danger to warn the public of bad fire weather and to determine the level of suppression preparedness needed, that is the number of men and equipment needed to suppress fires under the prevailing weather conditions.

Because most firefighters in rural Australia are either volunteers from rural communities or employees of land management agencies, a good fire danger rating system minimises the disruption to their normal daily activities by indicating just how many resources are required for firefighting each day during summer.

Fire danger rating systems

There are two fire danger rating systems in Australia - one for forest country and one for grassland and pastoral areas.

At a fire danger of Extreme fires start very easily from sources which, under milder conditions, normally do not start fires.

The fire danger classes of Low, Moderate, High, Very High and Extreme are a rating of the difficulty of suppression of a well-developed fire in each fuel type.

At a fire danger of Low, fires either will not burn or will spread so slowly that they are very easy to extinguish.

At a fire danger of Extreme fires start very easily from sources which, under milder conditions, normally do not start fires, (e.g. from the hot molten metal produced when powerlines clash together or from the incandescent carbon particles produced by faulty engine exhausts) and spread so rapidly and fiercely that they are impossible to extinguish unless they are attacked within a few minutes of starting.

Under these conditions, a large number of firefighters are placed on standby ready to be dispatched immediately a fire breaks out so they have a chance of suppressing it before it has developed to its full potential.

The fire danger rating systems provide a measure of suppression difficulty based on the prevailing weather and the seasonal conditions of the fuel.

Fire danger factors

The following factors are combined to produce a numerical index:

  • seasonal dryness - this is indicated by a drought index or a soil dryness index for forests or by the degree of curing in grasslands
  • amount and duration of rainfall
  • temperature and relative humidity of the air
  • wind speed.

Rainfall is not used in calculating a grassland fire danger index because the effect of recent rain in open grasslands may evaporate in two or three hours.

In regions where there are tracts of both forest and grassland, fire authorities calculate both the forest and grassland fire danger and set their fire danger signs according to the system that calculates the highest index.

At times, particularly early in the season, these signs may indicate a High fire danger when the surrounding grasslands are still green and obviously will not burn.

This is because the forest fuels have already dried out and are quite flammable.

Total fire bans

When the fire danger rating reaches Extreme, fire authorities declare a day of total fire ban, when lighting of any fire in the open is prohibited by law.

A day of total fire ban normally stays in place for 24 hours but it may be extended for longer periods if firefighters are already tied up fighting large fires in the region.

In some states, declaration of a total fire ban also prohibits other activities that may start fires such as harvesting operations, welding in the open, operation of chainsaws, etc.

One of the frustrating aspects for the general public is when a day of total fire ban is declared but the weather turns out to be cool and wet.

This often occurs because the hot northerly winds that cause extreme fire weather precede a cold frontal change, and the change moved across the state faster than forecast.

When this happens, spare a thought for the embattled firefighters who are getting welcome relief from a hot and dangerous job, and postpone the barbecue until the next day.

Learn more about our bushfire research in Bushfires