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Fire Weather Index Definitions

FFMC - Fine Fuel Moisture Code

Material: Cured (dead) small material e.g. pine needles, dead grass

Indices: Indicates the relative ease of ignition and flammability of fine fuels

How Does it Change: FFMC changes by the hour and is affected by as little as 0.6mm of rainfall

Points of Interest on Graph:

  • 75 - Ignition can occur
  • 85 - Easy ignition
  • 92+ - Extremely easy to ignite

DMC - Duff Moisture Code

Material: Medium material e.g. branches, small trees, scrub, and forest duff layer

Indices: Indicates the depth a fire will burn and how much medium fuel will be consumed

How Does it Change: DMC changes by the day, it is affected by >= 1.5mm of rainfall

Points of Interest on Graph:

  • 20 - Some burning will occur
  • 40+ - Deep burning will occur

DC - Drought Code

Material: Large Material e.g. logs, subsurface fuels, and peat soils

Indices: Indicates depth of burning & smouldering in large material and seasonal drought effects in forest fuels

How Does it Change: DC changes by the day and is affected by >= 2.9mm of rainfall

Points of Interest on Graph:

  • 200 - Large material will start to burn and some deep smouldering will occur
  • 300+ - All large and sub surface material will burn easily

ISI - Initial Spread Index

What does it Tell Us: Determines the expected rate of fire spread shortly after ignition

How: Combines the effects of wind and Fine Fuel Moisture Code (FFMC) to determine the rate of spread

What is the Scale it is Measured on: zero to infinity

Points of Interest on Graph:

  • 10 - Fast rate of spread
  • 16+ - Extremely fast rate of spread

BUI - Build Up Index

What does it Tell Us: Amount of fuel available to burn and the level of difficulty in controlling a fire

How: Total amount of fuel available for combustion is measured by calculating the Duff Moisture Code and Drought Code

What is the Scale it is Measured on: zero to infinity

Points of Interest on Graph:

  • 40 - High level of fuel becomes available for combustion
  • 60+ - All fuels available to burn and extremely difficult fire to control

FWI - Fire Weather Index

What does it Tell Us: Indicates the likely intensity a fire will burn at and is used to indicate fire danger levels

How: Combines Initial Spread Index and Build Up Index

What is the Scale it is Measured on: zero to infinity

Points of Interest on Graph:

  • 0-5 - Low
  • 6-12 - Moderate
  • 13-20 - High
  • 21-29 - Very High
  • 30+ - Extreme

Notifications for Wairarapa Rural Fire District