Parts of a fire
Bushfires can be divided into heading, backing and flanking fires, which each have different characteristics.
2 May 2008 | Updated 14 October 2011
When a bushfire spreads from a point it forms a roughly elliptical shape, aligned in the direction of the prevailing wind.
The perimeter of the fire can be divided into three main parts:
- the head
- the flank
- the back fire.
Each part is related to the orientation of the edge of the fire with respect to the wind direction and their speed of travel.
Each part of the burning fire perimeter has characteristic behaviour so it is useful to define the types of fire most frequently observed.
The heading fire
A heading fire is one where the flames are blown towards unburnt fuel.
The fuel bed is ignited at the top and burns progressively down into the lower layers.
A heading fire, particularly under extreme conditions, can be quite inefficient in its combustion, resulting in thick black smoke and partially burnt fuel.
Large envelopes of burning gas can be quite often seen as flashes of flame well above the average flame height.
The backing fire
A backing fire is one which moves into the wind. The flames lean over already burnt ground and ignite the fuel at the bottom of the fuel bed.
A heading fire results in thick black smoke and partially burnt fuel.
The rate of spread of a backing fire is quite slow and independent of the wind speed.
Combustion is often very efficient and complete, resulting in less smoke than a heading fire and, in some fuel types, a fine white ash residue.
The flanking fire
The edge of a flanking fire is generally aligned parallel to the direction of the wind. Flames more or less lean along the flank.
Due to the ever changing nature of the wind, slight changes in wind direction means that the flank will become, by turns, a heading fire and a backing fire in response to the changes in wind direction.
Therefore a flank may exhibit the high flames and black smoke of a heading fire one moment, and the low flames and little smoke of a backing fire the next.
At different times in the development of a fire, heading fires, backing fires and flanking fires may occur at any location around the fire perimeter, depending on the fluctuations in the direction of the local wind at the fire edge.
The local wind results from the interaction between the prevailing wind and the convective updraft of the whole fire.
| Click on the image to display a larger diagram. |
During a lull in the prevailing wind, the convection from the fire may draw wind towards the fire centre so that a backing fire may occur around the entire perimeter.
The parts of the fire described above are useful indicators of the likely location of an observer relative to the fire.
If you come across a fire and see high flames and thick smoke you may be close to the head of the fire. But if a moment later there is a slight change in the direction of the wind and the flames become low you may be on the flank.
Observation of the fire over a period of time (say ten minutes) is important when making observations of a fire, especially in fast burning fuels such as grass, because the fire can respond quite quickly to changes in the wind.
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