Fire, Weather and Climate

High winds, high temperatures and low humidities encourage the spread and intenisfication 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 suppression.

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 40^oC, 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.

In the 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– October.

cThe 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.