High winds, high temperatures and low humidities
encourage the spread and intensification of fires.
"Fire weather" is a term used to describe the prevailing weather
conditions as they relate to fire behaviour. The primary weather
variables that determine fire behaviour are atmospheric humidity,
air temperature and the strength of the winds. Variation in these
factors in northern Australia is governed by the arrival and
departure of the monsoon, hence there is strong seasonal variation
in fire weather.
Fire weather variables, in conjunction with fuel variables, can
be incorporated into indices that express fire danger. Fire danger
is the product of the factors that determine chance of ignition,
tendency to spread and ease of suppression. Summaries of fire
weather and fire danger are given in the books by Luke and
McArthur, and Cheney and Sullivan (see Further reading).
The two main fire danger indices used in Australia were
developed by A. G. McArthur in the 1960s— the Forest Fire
Danger Index (FFDI) and the Grassland Fire Danger Index (GFDI).
FFDI includes a drought factor while GFDI includes a fuel-curing
factor. Fire danger rating systems allow public fire weather
forecasts to be made by the Bureau of Meteorology, and provide fire
management agencies (or other land managers) with up-to date,
reliable information for purposes of fire control or
Both FFDI and GFDI rise as temperatures and wind speed increase
and as relative humidity and soil moisture drop. The indices range
from 0 to 100, with values above 50 considered extreme. Luke and
McArthur indicate that an index of 100 represents the ‘near
worst possible fire weather conditions that are likely to be
experienced in Australia’, although Cheney and Sullivan
indicate that such values have been exceeded on several occasions
since 1966. A value of 100 would be produced by air temperatures of
40oC, relative humidities of 15%, wind speeds of 55
km/h, an extended period of drought of 6–8 weeks or more and
abundant fuel. Fires that burn under extreme conditions are
virtually impossible to put out without massive effort. Both
indices can be calculated easily using fire danger meters.
savannas of northern Australia, fire danger is calculated using a
modified Grassland Fire Danger Meter (Figure 3.4c) because savanna
fires, even in woodlands, behave more like grass fires than forest
fires. There are five operational classes of fire danger: Low
(<2.5); Moderate (2.5–7.5); High (7.5–20); Very High
(20–50) and Extreme (>50). Calculations are made by the
Bureau of Meteorology with appropriate operational responses taken
by the responsible fire management agencies (both rural and urban)
and other land managers.
The indices may be used to compare seasonal variation in fire
weather but there have been few systematic studies of this in the
savannas. One such study, from Jabiru in the NT, examined seasonal
variation in FFDI and GFDI based on 12 years of records .
The Forest Fire Danger Index increases as the
dry sesaon progresses, as temperatures and wind sepped rise and
relative humidity drops. Based on 3pm weather data from 12 years'
records at Jabiru, NT
The patterns of afternoon (3 p.m.) FFDI show strong seasonality
(at right). The average daily FFDI is below five during the peak
monsoon period of January to early March, and the vegetation will
generally not burn. The wet season ends abruptly, and both
atmospheric and soil moisture drop from late March onwards. Despite
this, average daily FFDI in the early dry season (May–June)
remains below 20, steadily increasing to average daily values of
around 20 in the September–October period. However, the
average maximum FFDI during this period of peak fire weather
averages about 40— Very High. The highest value recorded was
60, and values >40 occurred in all months from June–
The Grassland Fire Danger Index for 100% dried
grass fuels. GFDI alos increases as the dry sesaon progresses as
temperatures and wind speeds rise and realtive humidity drops. Data
same as figure above.
These extreme values, despite being in the High–Very High
classes, are below peak levels of 100 that can occur on extreme
days in southern Australia, such as on Ash Wednesday in 1983. The
FFDI declines again with the onset of the wet season, but the
maximum can still be around 20–30 on some days in
November–December. Under such conditions, fire spread is
possible, allowing wet season burning to be undertaken.