Understanding the risks that lead to bushfire and how bushfires behave will help you to understand how to prepare your property. A more intense fire will generate more heat, be harder to control and cause more damage. Fire intensity depends on three main factors: vegetation, weather and topography.
Vegetation is a fire's main fuel source. Fine fuel (leaves, bark, twigs, grass) burn rapidly and give off heat fast. They provide much of the heat energy and are largely consumed by the front of the fire. Heavier fuels (branches, logs, trees) burn more slowly and give off heat more slowly, continuing to burn for hours, if not days, after the front has passed.
Fine fuel and dense undergrowth increase the risk and intensity of bushfires. Shrubs, low hanging branches and fibrous dry bark can catch fire and preheat fuel above them, assisting fire to climb higher in trees adding to both the height of flames and the heat of the fire. Fire in treetops (crown fire) moves the fastest and is most dangerous.
Weather conditions such as extreme heat, low humidity and strong winds dramatically increase the likelihood of fires, their intensity and their spread, once started. Winds have a significant influence as they dry out the bush and fan flames. They increase the intensity of the fire by providing more oxygen and throw embers ahead of the fire front causing new fires (spot fires) to ignite. Strong winds can carry larger items such as branches, roof tiles or outdoor furniture which can break windows or remove parts of roof and walls thereby letting embers into the house.
Fires burn faster uphill because the flames can reach more unburnt fuel and because the heat radiating from the fire will preheat more fuel on the slope above the fire. Generally, for every 10° of uphill slope the fire doubles its rate of spread. Conversely, for every 10° of downhill slope, the fire will halve its speed. The most dangerous home sites are on ridge tops and steep slopes. North facing slopes also receive more direct sunlight which dries out vegetation and can result in more intense fires.
Rugged terrain can set air eddying, fanning flames more erratically and making a fire harder to control. Topography can also affect access, with narrow roads making it difficult for residents to get out quickly or fire trucks to get in.