Fire
John's Weather-Watcher
Christchurch, New Zealand

Fire Weather Index

If you're intending to light a fire click here for official advice first.

NOTE: The fire danger information displayed on this page is updated daily at noon NZST (1:00pm if we are in NZDT), and is based on the current weather at the time. The readings may be higher or lower than the overall fire danger in the Selwyn - Christchurch region.

Low
0 - 7
Mod
8 - 16
High
17 - 25
V. High
26 - 31
Extreme
32+


Daily Observation: 23/10/2020 1:00pm
Temperature (°C)   16.9   FFMC   85.4
Humidity (%)   66   DMC   30.9
WindDir   NE   DC   97.2
Speed (kmh)   18.8   ISI   5.8
Rain (mm)   0.0   BUI   34.5
        FWI   12.1

Last 4 weeks (in reverse date order)
(Click a plot point on the graph to see its Fire Weather component parts above)


How it works

In New Zealand, fire danger is assessed using the Canadian Fire Weather Index (FWI) System, which we adopted in 1980.

The FWI System uses 24 hour rainfall, air temperature, relative humidity, and wind velocity measured at noon local standard time to estimate the moisture content of fuels from three levels of the forest floor.

Each of the parameters interacts with the others to produce the Fire Weather Index

The FFMC (Fine Fuel Moisture Code) is a numerical rating of the moisture content of surface litter and other cured fine fuels. FFMC shows the relative ease of ignition and flammability of fine fuels. The FFMC rating is on a scale of 0 to 99. Any figure above 70 is high, and above 90 is extreme.

The DMC (Duff Moisture Code) is a numerical rating of the average moisture content of loosely compacted organic layers of moderate depth, and indicates the depth that fire will burn. The Duff Layer is decomposing organic material, decomposed to the point at which there are no identifiable organic materials (pine straw, leaves, twigs, etc).

Duff layers take longer than surface fuels to dry out. A DMC rating of more than 30 is dry, and above 40 indicates that intensive burning will occur in the duff and medium fuels.

The DC (Drought Code) is a useful indicator of seasonal drought and shows the likelihood of fire involving the deep duff layers and large logs. A long period of dry weather is needed to dry out these fuels and affect the Drought Code. A DC rating of 200 is high, and 300 or more is extreme.

ISI and BUI combine to produce the Fire Weather Index

The ISI (Initial Spread Index) indicates the rate at which the fire will spread in its early stages. It is calculated from the current day's FFMC rating and the wind speed. The ISI scale is open ended and starts at zero. A rating of 10 indicates high rate of spread shortly after ignition. A rating of 16 or more indicates an extremely rapid rate of spread.

The BUI (Build Up Index) shows the amount of fuel available for combustion, indicating how the fire will develop after initial spread. It is calculated from the DMC and the DC. The BUI scale starts at zero and is open-ended. A rating above 40 is high, above 60 is extreme.

FWI (Fire Weather Index). You will no doubt have seen signboards like this one, particularly when driving in country areas.

The FWI is divided into fire danger classes:

  • Low 0 - 7
  • Moderate 8 - 16
  • High 17 - 25
  • Very High 26 - 31
  • Extreme 32+

Click here for other locations and a more in-depth look from NIWA.