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    An interactive on rural fire risk – environmental components.

    Scientists are getting a better understanding of fire behaviour and possible fire risk in rural areas. In this interactive scientist Stuart Anderson talks about some of the environmental components that contribute to fire risk in the outdoors.



    The fire environment consists of three parts or three components, and that is the fuel, so the vegetation – and included in that is how dry it is, how much of it is there, the fuel load – and then there is the weather component, which is all those things like relative humidity and rainfall and wind speed and how they influence fire, and then the third component is the topography, which is the likes of slope and terrain and barriers – things like that.

    Fuel loads

    Fuel loads are really important in understanding the fire potential or how a fire is going to burn, how hot it’s going to burn. When we talk of fuel loads we are talking about the amount of vegetation that is there and is available to burn. It has a major impact in terms of the intensity of the fire, and that is important because that determines how difficult that fire is going to be to control or potentially how dangerous it could be as well.


    Topography, with rural fires being out in the open on the landscape, it does have a major impact. There’s a number of different parts of topography or terrain that are important, slope being a really critical one. And the reason why slope is so important is that it allows fires to burn and spread faster. Other factors related to terrain are the likes of ridgelines and tops of hills where fires can race up to and then they can either throw embers over the ridgeline onto the other side. You also have things like the wind starting to interact with terrain, so eddying and swirling of winds in hill country, which again makes fire a little bit more unpredictable and a little bit more dangerous as well.


    Wind is really important as one of the main variables affecting fire. Obviously, it needs to be dry enough but then wind is really critical in that it really drives and spreads fires, so it will determine the direction the fire spreads in and also how fast it will burn, which is really important in terms of how hot the fire gets, how difficult it might be to control or to contain it, and in New Zealand, that’s… often a lot of our fires are driven by wind.


    Moisture is really important because obviously if a fuel isn't dry enough, it won't burn, or it won’t ignite even, and the drier it gets, the worse conditions can become. When you have a really hot dry summer day, or a series of days like that or a long period, and the fuels become really dry, they really become very susceptible to being ignited and will burn rapidly, and fire will consume them easily.


    Temperature is of less importance for it compared to relative humidity, which is the amount of moisture in the air, but it is also very closely linked to relative humidity. So temperature is important in that it influences the amount of water vapour that the air can hold, and obviously during the summer months, the hotter the air gets or the higher the temperature is, often the drier the air will be as well. Things start to dry out more.

    Relative humidity

    Relative humidity is the amount of water vapour that the air can hold. A low relative humidity means that the air is getting very dry, and fuels will start to lose moisture, they will become drier as well. And then when they become dry, that is when they are prone or susceptible to being ignited and fires starting. So generally, the drier the air becomes, the lower the relative humidity, that is when the fire risk or the fire danger will start to increase.


    If a fire is burning up a slope, up a hill, what actually happens is because of the angle of the slope, the vegetation above where the fire is burning, above the flame front, is in a way, really being tilted, I guess, towards the fire. The flames still go straight up but because you've got slope, they are actually making more contact with the vegetation ahead of the flame front and because they are making more contact with the vegetation up hill, they heat quicker and they then ignite and they burn faster. So a fire burning up a slope can burn a lot faster than a fire on level terrain.


    99% of our wildfires are started by people. This can include wildfire developing from campfires, cigarettes, burn-offs, sparks from vehicles, trains and powerlines, and arsonists.

    Acknowledgement: Galyna Andrushko, licenced through 123RF limited


    Weather is a key component affecting wildfire risk. Scientists endeavour to understand climatic factors (such as wind, temperature, relative humidity and rainfall) to provide fire managers with better fire risk information. Research into climate change and future fire risk has shown that fire danger is likely to increase, particularly in the eastern areas of New Zealand.

    Acknowledgement: Globalphoto, licenced through 123RF limited


    Rainfall (precipitation) wets the fuel – or vegetation      . Its effect depends on the fuel size. Fine fuels absorb moisture more quickly than coarse fuels. Lack of rain is the single biggest factor contributing to the drying out of moisture in fuels.

    Acknowledgement: Georgios Alexandris, licenced through 123RF limited

    Related content

    Learn more about Fire behaviour in the outdoors, the Fire danger sign and Bernie that were developed to keep the public informed about fire risk, how we are Managing fire behaviour in the outdoors and meet a rural fire researcher – Stuart Anderson.

    Activity idea

    Fire risk assessment is a student activity designed to work with this interactive. The activity will enable students to identify and define exterior and interior fire risks and the similarities and differences between fire risks inside and outside.


    Rights: University of Waikato Published 3 November 2009, Updated 29 August 2017 Size: 420 KB Referencing Hub media