Bushfire basics

Types of bushfires

Australians generally consider that the word bushfire means a fire in a forest, bush, crops and grass: anywhere in the natural environment. Technically, however, bushfires are classified into three types: forest fires, grass fires or ground fires.

Forest fires:

Forest fires, not surprisingly, burn among trees. They tend to burn more slowly than grassfires because of the shelter against wind offered by the canopies. Also, the density of trees slows down the speed at which the fire travels. The more litter and undergrowth there is, the more intensely (hot) the fire burns.

There are three types of forest fires: surface fires where only the ground litter and low shrubs are burning; dependent crown fires occur when the fire reaches the canopies of the trees adding to the danger and intensity of the fire considerably; and lastly a running crown fire which is the most dangerous of all: the fire jumps, or runs from one crown to the next causing burning leaves, bark and twigs to start new spotfires where they land. A spotting running crown fire is practically uncontrollable, and causes high winds that further help new spotfires start.

Grass fires:

Grass fires burn faster than forest fires because of exposure to the full force of wind. The height of the flames is approximately 5 times that of the grass, and grassfires can cause spotfires within 100 meters of the fire front. While grassfires may be individually small, the indroughts of multiple spotfires may cause them to draw together and form a large, fast-moving fire front.

Ground fires:

Ground fires burn slowly and with little flame under the ground in coal seams and in peat beds in swamps. They are extremely rare in Australia, and beyond the scope of this site.

Factors affecting bushfires

There are a number of factors that affect the ignition and burning of wildfires:

Fuel:

Anything that burns is fuel for the fire: litter in the ground (leaves, twigs, rubbish), undergrowth (shrubs, grass, seedlings), trees and other vegetation, structures (such as your house) and any miscellaneous stuff laying about; gas bottles, piles of firewood, tyres, etc. Ladder fuels are low growing (30 cm to 2 meters) vegetation that offers a ladder for the fire to rise to the canopies of trees.

Weather:

Weather is a major contributor to bushfires. The hotter and dryer, the more likely it is for a bushfire to start. High winds will reduce humidity, and cause an already started bushfire to spread more rapidly. Most bushfires start in the afternoon, when it is driest and hottest.

Lightning strikes from thunderstorms and electric storms can create tens or hundreds of spotfires over hundreds of kilometres from a single storm front without providing much rain. Bushfires in Northern Territory are predominantly ignited by lighting strikes from electric storms.

Topography:

The topography of the terrain is a major factor in bushfire behaviour. Generally the fire spreads faster uphill. Each 10 degree increase in slope doubles the speed of bushfire. Conversely fire going downhill advances more slowly. The superheated air is pushed in front of the fire drying and pre-warming the fuel for ignition. When a fire progressing downhill hits the flat at the bottom of the hill, the height of the flame can quadruple, when the fire hits the up slope opposite, the height may quadruple again. In other words, 1 meter flames going downhill can turn into 4 meter flames at the bottom of the hill, and to 16 meter flames starting to climb the next hill. While the height of the flame depends mainly on the height of the fuel, the former stands as a reminder that an innocent looking small bushfire can rapidly change into a life threatening fire.

Fire breaking out of bushland into open grassland can explode out into the grass due to wind behaviour in these situations.

The human factor:

Unfortunately, we are often put at risk by the negligent or malicious behaviour of individuals that become arsonists by "accident" or design.