How a Bushfire Behaves
What Creates a Bushfire Risk?
A bushfire will burn faster going uphill. The reason is the flames are able to easily reach more unburnt fuel in front of the fire. Radiant heat pre-heats the fuel in front of the fire, making the fuel even more flammable.
For every 10 degree slope, the bushfire will double its speed. For example, if a fire is travelling at 5km per hour along flat ground and it hits a 10 degree slope, it will double in speed to 10km per hour up the hill. The faster the bushfire, the higher the intensity which means it becomes hotter.
The opposite applies to a fire which travells downhill. The fire reaches less fuel and less radiant heat pre-heats the fuel in front of the fire. For every 10 degrees of downhill slpoe, the fire will halve its speed. Fires tend to move more slowly as the slope decreases.
What Fuels a Bushfire?
- Grass can burn easily and quickly on days that are hot, dry and windy
- Branches, twigs and leaves dropped from shrubs and trees become fine fuels, which burn easily. These can give off far more heat when they burn
- Fibrous and dry tree bark can carry fire to treetops. The fire can break away and spread further
- Dry brances, twigs, leaves and other fine fuels on the ground can also easily burn
Unfortunately bushfires are unpredictable and often vary greatly according to weather conditions. They often start on days that are hot, dry and windy.
Where there has been consecutive hot and dry days, the vegetation dehydrates making it easier to burn and made even worse by lack of rainfall. The drier the vegetation, the easier it will burn. A bushfire will spread as a result of burning embers, radiant heat and direct flame contact.
A change in wind direction is one of the most dangerous influences on fire behaviour. Death by bushfire is often caused by a wind change.
The Effects of an Ember Attack
- Burning twigs, leaves and pieces of debris
- Ember attack occurs when twigs and leaves are carried by the wind and land on or around houses
- Is the most commen way that houses can catch fire during bushfires
- Are able to land on top of debris in your gutters and set fire to your house
- Can happen before, during and after the bushfire.