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 gases1 that could kill you if not ventilated.
The behaviour of the fire often depends on the fuel2. Other factors or variables may include where the fuel is situated and how near it is to other fuels, the weather3 (especially wind and relative humidity4), oxygen5 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 energy6 to change any particular material into a gas (if it is not already). Then it takes more heat7 energy to trigger the reaction with oxygen. The amount of heat produced depends on the molecules8 that make up the fuel. The most flammable9 fuels are hydrocarbons (contain carbon10 and hydrogen11) that recombine with oxygen quite easily to form carbon dioxide12, water and other gases.
Size: How quickly a fuel catches fire and burns relates to the surface area13 or the size of the fuel. For example, large pieces of wood take a lot longer to absorb heat energy14 to ignition temperature15. 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 collide16 with the surface. The more oxygen molecules that collide per second with the fuel, the faster the combustion17 reaction is.
You can increase the surface area18 of a solid by breaking it up into smaller pieces. If you chop or break up wood into small pieces, it will ignite19 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 energy20 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 load21. 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 backdraught22. 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 humidity23 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 radiation24 and convection25 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 (precipitation26) 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 temperature27.
Terrain
The terrain (shape of the land) has significant influence on wildfire28 behaviour. Steep slopes may increase fire speed because fuels (scrub and vegetation29) 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.
Related content
Explore the science concepts around Heat energy – heat is needed to keep a fire going. It involves the flow of energy between matter30 in three ways – convection, conduction31 and radiation.
Slowing the burning – researchers are investigating ways of slowing down fire in fabrics and foams in our furniture.
A truck and a tunnel – fire engineers are using computational modelling32 to predict fire behaviour – what might happen if a truck caught fire in a tunnel?
Fire behaviour in the outdoors – Project FuSE33 involved research in the field where scientists about fire behaviour in the outdoors, particularly interactions of fire on slopes, wind and the moisture content of fuels.
Managing fire risk in the outdoors – scientists are getting a better understanding of fire behaviour. They use the information to make assessments of possible fire risk, then warn fire managers who warn the public.
Activity ideas
- Fire risk assessment – use an interactive to identify and define rural fire risks
- Fire safety – using scenarios involving fire risks and safety plans
- Fire in Antarctica – a literacy activity using an online article
- Exploding flour – observing combustion.
- gases: The state of matter distinguished from the solid and liquid states. Gases have the ability to diffuse readily and to become distributed uniformly throughout any container.
- fuel: 1. A combustible substance that provides energy. 2. A body fuel such as fat, carbohydrates and protein that supplies energy for animals’ activities.
- weather: Daily or short-term conditions like temperature, cloud cover, precipitation and wind affecting a certain area.
- relative humidity: The ratio of the amount of water vapour in a given parcel of air, at a set temperature and pressure, to the maximum amount that the parcel of air could hold under the same conditions. It is expressed as a percentage.
- oxygen: A non-metal – symbol O, atomic number 8. Oxygen is a gas found in the air. It is needed for aerobic cellular respiration in cells.
- heat energy (heat): Heat energy: the transfer of energy in materials from the random movement of the particles in that material. The greater the random movement of particles the more heat energy the material has. Temperature is a measure of the heat energy of a material.
Heat: the flow of energy from a warm object to a cooler object. - heat energy (heat): Heat energy: the transfer of energy in materials from the random movement of the particles in that material. The greater the random movement of particles the more heat energy the material has. Temperature is a measure of the heat energy of a material.
Heat: the flow of energy from a warm object to a cooler object. - molecule: Two or more atoms bonded together. The molecule of an element has all its atoms the same. The molecule of a compound has two or more different atoms.
- flammable: Easily ignited or set on fire.
- carbon: A non-metal element (C). It is a key component of living things.
- hydrogen: First element on the periodic table – symbol H, with the atomic number of 1, meaning that it has a single proton in its nucleus.
- carbon dioxide: CO2 is a colourless, odourless, incombustible gas. It is a product of cellular respiration and combustion and is an essential component in photosynthesis.
- surface area: The total area of an object or surface.
- heat energy (heat): Heat energy: the transfer of energy in materials from the random movement of the particles in that material. The greater the random movement of particles the more heat energy the material has. Temperature is a measure of the heat energy of a material.
Heat: the flow of energy from a warm object to a cooler object. - ignition temperature: The lowest temperature at which combustion can begin and continue, for example, the ignition temperature for wood is when its volatile gases reach about 260 °C.
- collide: When two or more objects, including particles, briefly come into contact with each other.
- combustion: A chemical reaction that involves the process of burning.
- surface area: The total area of an object or surface.
- ignite: To set on fire or cause to start burning.
- heat energy (heat): Heat energy: the transfer of energy in materials from the random movement of the particles in that material. The greater the random movement of particles the more heat energy the material has. Temperature is a measure of the heat energy of a material.
Heat: the flow of energy from a warm object to a cooler object. - fuel load: The amount of fuel (combustible substances) available for burning.
- backdraught: An explosive surge in a fire produced by the sudden mixing of air with other combustible gases. Also spelt as backdraft.
- humidity: The amount of water vapour in the air.
- radiation: Energy that is transmitted (radiates) from a source in the form of rays or waves or particles.
- convection: A method of moving heat energy. Found only in gases and liquids. Convection currents move heat from one location to another.
- precipitation: 1. The formation of an insoluble solid (precipitate) from a given solution by altering either its temperature, concentration or chemical composition. 2. In meteorology, this term describes the formation of rain, hail, snow or ice in the atmosphere.
- temperature: A measure of the degree of hotness or coldness of an object or substance. Temperature is measured with a thermometer calibrated in one or more temperature scales. Kelvin scale temperature is a measure of the average energy of the molecules of a body.
- wildfire: An uncontrolled, raging, rapidly spreading fire. It occurs in vegetation, bush, forest or peat.
- vegetation: Plant life.
- matter: The basic structural component of all things that have mass and volume.
- conduction: 1. Heat flow or transfer through a substance from a higher to a lower temperature. 2. Flow of electric charges through a material in response to an electric field.
- computational modelling: Using a computer to make scientific models for testing. This is particularly useful if it is difficult to test the science in reality (for example, in testing fire behaviour).
- fuse: 1. A safety device made of a short length of resistance wire that will melt if more than the allowed current is present in the circuit. 2. To join together.
