Fires behave differently. Some burn slowly and evenly; others are extremely hot, burning fiercely and quickly. Different fires have different coloured flames. Some fires start easily; others don’t. Some fires produce deadly gases that could kill you if not ventilated.

The behaviour of the fire often depends on the fuel. Other factors or variables may include where the fuel is situated and how near it is to other fuels, the weather (especially wind and relative humidity), oxygen concentration and, in the case of outside fires, the shape of the terrain.

Fuel

Type: Different fuels catch fire at different temperatures. It takes a certain amount of heat energy to change any particular material into a gas (if it is not already). Then it takes more heat energy to trigger the reaction with oxygen. The amount of heat produced depends on the molecules that make up the fuel. The most flammable fuels are hydrocarbons (contain carbon and hydrogen) that recombine with oxygen quite easily to form carbon dioxide, water and other gases.

Size: How quickly a fuel catches fire and burns relates to the surface area or the size of the fuel. For example, large pieces of wood take a lot longer to absorb heat energy to ignition temperature. A twig catches fire easily because it heats up easily.

Surface area: The bigger the area of the surface of the fuel, the more oxygen molecules can collide with the surface. The more oxygen molecules that collide per second with the fuel, the faster the combustion reaction is.

You can increase the surface area of a solid by breaking it up into smaller pieces. If you chop or break up wood into small pieces, it will ignite and burn more quickly than larger pieces of wood. People often start fires with kindling (small pieces of wood) that they criss-cross to allow greater surface area and lots of oxygen getting in and around.

A powder has the largest surface area and will have the fastest reaction rate.

Heat produced: How much energy is released in the reaction and how quickly the fuel burns depends on what the fuel is made up of. Different compounds react with oxygen differently – some contain lots of heat energy while others produce a smaller amount. The reaction with the oxygen may happen very quickly or more slowly.

Amount: The amount of fuel available to burn is known as the fuel load. The bigger the fuel load, the more intense the fire will be in terms of heat energy output.

Moisture content: If the fuel isn’t dry enough, it won’t burn. The less moisture in the fuel, the more likely it will ignite and burn.

Oxygen availability

The amount of oxygen available will affect the rate of burning. A low concentration of oxygen will slow the burning right down.

An example of dangerous fire behaviour that can occur in a situation where there is a low concentration of oxygen is called backdraught. This is when an enclosed fire has used up most of the oxygen and is just smouldering. If there is a sudden influx of oxygen (like someone opening a door or window), the fire will immediately explode into flame.

Weather

Relative humidity: This reflects the amount of moisture in the air. If relative humidity is low, it will contribute to the drying of fuels. If it is high, fuels will absorb moisture from the air, making ignition more difficult.

Wind: This is a major factor in determining fire spread. Wind affects the rate of oxygen supply to the burning fuel (controlling combustion) and it tilts the flame forward so that unburned fuel receives energy by radiation and convection at an increased speed. Wind can also dry out the fuel.

Rainfall: This also has an effect on wetting fuels, but absorption of moisture is dependent on fuel size. Fine fuels absorb moisture more quickly than coarse fuels. Lack of rain (precipitation) is the biggest factor affecting the drying process of fuels.

Increased temperatures: These will dry out potential fuel so that there will be less preheating of fuels to reach ignition temperature.

Terrain

The terrain (shape of the land) has significant influence on wildfire behaviour. Steep slopes may increase fire speed because fuels (scrub and vegetation) are preheated ahead of the fire through convection and radiation.

Rugged terrain with narrow valleys, sharp ridges and irregular slopes affect the direction and rate of fire spread. For example, narrow valleys can funnel winds, increasing the rate of spread of a fire due to convection.

The direction a slope is facing will depend on how much sun it gets. This will affect the amount of drying the fuels get. The drier the fuel, the faster it will burn.

    Published 19 November 2009