Technical Article

Fire Weather Index (FWI):

A firefighter's perspective

The National Fire Weather Index (FWI) is more than just a coloured sign on the side of forest roads, it is an invaluable tool in vegetation fire preplanning and incident management.

 

The New Zealand system was developed from that used in Canada, called the Canadian Forest Fire Weather Index System. It has a long-standing history of providing fire risk planners and wildland fire intervention teams with vital information about the probability of a fire starting, fire development and likely fire behaviour.

 

The fire danger rating system measures the variable elements which cause day to day changes in wildland fire risk and interprets the information to provide:

  • Fire season status

  • Appropriate fire prevention measures

  • The likelihood of fire occurring

  • Fire suppression response and resources

  • Inform the public

  • Make decisions to close areas at high risk

  • Issue or cancel fire permits

  •  Plan and conduct controlled burns

 

The information required to calculate the FWI is based on natural weather components, the wind speed and direction, temperature, relative humidity and rainfall over the last 24 hours.

 

The system does not however consider differences in fuel type or topography. It provides a uniform method of rating fire danger throughout New Zealand.

 

The six components that make up the FWI system provide numerical ratings of relative wildland fire potential.

 

The first three components are fuel moisture codes that follow daily changes in the moisture content of three classes of forest fuel with different drying rates, arranged so the higher values represent lower moisture content and hence greater flammability.

 

The final three components are fire behaviour indices, representing rate of fire spread, amount of available fuel, and fire intensity. Their values increase as fire weather severity worsens.

 

FFMC: Fine Fuel Moisture Code
A numerical rating of the moisture content of litter and other cured fuels. This code is an indicator of the relative ease of ignition and flammability of fine fuel.

 

DMC: Duff Moisture Code
A numerical rating of the average moisture content of loosely compacted organic layers of moderate depth. This code gives an indication of fuel consumption in moderate duff layers and medium sized woody material.

 

DC: Drought Code
A numerical rating of the average moisture content of deep, compact, organic layers. This code is a useful indicator of seasonal drought effects on forest fuels, and the amount of smouldering in deep duff layers and large logs.

 

ISI: Initial Spread Index
A numerical rating of the expected rate of fire spread. It combines the effects of wind and FFMC on rate of spread without the influence of variable quantities of fuel.

 

BUI  Build Up Index
A numerical rating of the total amount of fuel available for combustion that combines DMC and DC

 

FWI: Fire Weather Index
A numerical rating of fire intensity that combines ISI and BUI. It is suitable as a general index of fire danger throughout the forested and rural areas of New Zealand.

 

Good understanding of the FWI System
A good understanding of the FWI system will equip an officer with vital information on fire development and likely intervention measures including appropriate resources.
The indices can be obtained from communication centres en-route to incidents or by maintaining an up-to-date log on station. Ensure you have a means of deciphering the codes on the appliance.

 

For more information on the FWI visit the National Rural Fire Authority website www.nrfa.org.nz or the RURALnet. - Should it link to this.. http://frfanz.org.nz/??

NZFBI Field Day: Understanding the Rural Environment

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