Introduction to Fire Weather Index (FWI)
Notes from Understanding the Fire Weather Index(FWI) System course
- Fine Fuel Moisture Code (FFMC)
- Duff Moisture Code (DMC)
- Drought Code (DC)
- Initial Spread Index (ISI)
- Buildup Index (BUI)
- Fire Weather Index (FWI)
FWI answers:
- what is the likelihood of a fire starting
- how big could the fire get
- how difficult will the fire be to control
- will the fire continue to smolder after passage of the flame Front
Fire Hazard: describes fire behaviour potential of a given fuel type, without regard to burning conditions. Based on physical characteristics of fuel complex (load, size, distribution, etc.). E.g. cured grass.
Fire Risk: the probability of fires starting, determined by the potential number of ignition sources. High fire risk because of human activity.
Fire Danger: hazard & risk are components of danger. Elements of fire environment that influence ignition, behaviour, difficulty of control, & impact of forest fires.
- includes variable factors such as weather, fuel moisture, season, & lesser vegetation condition.
- includes constant factors like fuels, topography, risk, access, & values at risk.
FWI Assumptions & Limitations
- FWI only accounts for weather
- uses standard fuel type (different fuel types affect fire behaviour)
- assumes level terrain
- daily standard values represent mid-afternoon peak of fire danger (based on single observation)
- higher values represent more severe fire behaviour. Numerical values are relative - e.g., 40 is not necessarily twice as severe as 20. There are no units
- approx. 20% minimum change in value required to see recognizeable difference in fire behaviour
FWI Advantages
- forecasted weather can be used to forecast codes & indexes
FWI Applications
- prevention
- preparedness
- detection
- suppression
Required Observations:
- air or dry bulb temperature
- relative humidity
- 10-minute average wind speed
- rainfall in last 24 hours (from noon the previous day)
Standard weather observation time is noon (±15 minutes)
Precision standards:
- temperature: ±0.5°C
- relative humidity (RH): to nearest whole percentage point
- rainfall: to nearest 0.2 mm. Centrimetres of snow reported as mm of rain - e.g., 2.4 cm of snow is reported as 2.4 mm of rainfall
- windspeed: to nearest 1 km/h, averaged over a 10-minute period
Rule of Three 10s: FWI uses 10-minute average wind reading taken at 10m height in an opening with a diameter 10 times the height of the surrounding trees
Starting Conditions:
Regions with snow cover begin calculations on 3rd day after snow is gone
Regions without snow cover begin calculations on 3rd successive day with noon temperatures ≥12°C
Use starting values of FFMC = 85, DMC = 6, DC = 15 (but not for late starting stations)
A continuous daily weather record is essential. Must estimate missing data. Continuity of records is key to accuracy.
FWI is based on a single fuel type: either mature jack pine (within BC) or lodgepole pine
Forests contain crown fuels, live & dead understory vegetation, downed woody debris, & forest floor organic matter
FWI tracks moisture content of 3 types of forest floor fuels with different drying rates
. . . . . . . . . . . . . surface litter . . . . . . . . . . . .
- - - - - - loosely compacted duff - - - - - -
///////// deep compact organic layers /////////
Drying is not a constant process - less moisture is lost during each successive hour/day in constant conditions. If conditions remain constant, fuel moisture content eventually reaches equilibrium with the relative humidity of the atmosphere
Time lag: length of time for fuels to lose 2/3 of available moisture between any point & equilibrium level
Moisture codes simulate wetting & drying over 24 hours in standard fuel. Codes reflect past & current weather on fuel moisture
Designed to predict daily peak fire conditions
Rates moisture content of surface litter layers (approx. 1-2 cm depth)
Affected by temp, RH, wind speed, rainfall
Takes ~1 mm rain to raise moisture content of fine fuels above ignition threshold
Higher FFMC = lower % moisture content
Scale 0 - 101
| FFMC | Description |
|---|---|
| 74 | Ignition threshold (25-30% MC) |
| 80 | Continuous surface spread |
| 90 | Spot fire potential |
| 92 | Extreme fire behaviour |
Closely related to ease of ignition - most fires ignite with fine fuels.
FFMC can only go <73 with precipitation: 24-hr rainfall must reach ≥0.6mm before it's considered to have any reducing effect.
Default starting value in spring is 85.
FFMC fuels dry quickly. Starting at saturated state (~250% MC), fine fuels lose 2/3 of available moisture in ~2/3 of a day under standard conditions (21.5°C, 45% RH, 13 km/h wind at noon in July). After 3-5 days of constant drying, FFMC levels off - no more moisture to lose. --> FFMC has an effective memory of a few days.
Lowest possible moisture content in fine fuels is approx. 2%.
If RH increases, equilibrium MC increases, & fine fuels absorb moisture from atmosphere until reaching the new equilibrium in 3-5 days.
FFMC peaks around 4 pm, is lowest at dawn.
Rating of dryness of forest floor layers approx. 7cm deep. Corresponds to fermentation layer of soil.
Fully saturated duff layer holds approx. 15cm water. MC ~300%. Duff involved in combustion zone when DMC is 20 in many fuel types.
DMC scale 0 - &infty;. At DMC 150-200, duff has lost most available moisture.
| DMC | Description |
|---|---|
| 20 | Fuel layer available for combustion; lightning fire ignition threshold |
| 40 | Layer aids in spread & intensity of fire; fire behaviour noticeably increases |
| 60 | Onset of extreme fire behaviour |
| 150 | Not much variation |
^ This table applies to most forest types
Duff consumption is main source of energy produced by moving flame front.
Default starting value is 6.
Weather factors: temperature, RH, rainfall.
Rainfall is the only weather factor that can reduce DMC from the previous day. Threshold of 1.5mm rainfall before any reducing effect on DMC.
Duff dries more slowly & has longer memory than fine fuels. Saturated duff takes ~15 standard drying days to remove 2/3 of available moisture.
Amount of drying varies with day length. Monthly drying factor applied in DMC.
- Long-term drought indicator.
- Represents moisture content of deep compact organic layers ~18cm thick
- Deep organic layers can hold ≤100mm water, = 400% MC
- Scale 0 - &infty;. Deep duff has lost most available moisture when DC = 800
| DC | Description |
|---|---|
| 15 | Startup value - 3 drying days after saturation |
| 300 | Onset of smoldering combustion |
| 500 | Sustained smoldering combustion; indicator of mop-up & extinguishment difficulty |
Deep duff not usually consumed in moving fire front - doesn't contribute to frontal fire intensity. Smoldering combustion can occur following lightning ignition or passage of flame front.
Indicator of effect of seasonal drought on forest fuels, total fuel consumption, & smoldering in deep organic layers & large fallen logs.
Driven by temperature and rainfall.
If overwinter precipitation < 200mm, deep duff is not recharged - drought effect carried over into next year. Starting value next spring must be adjusted.
DC value can only be reduced from previous day's value by rainfall - reducing threshold is 2.9mm in 24 hours.
DC levels off after 3-5 constant drying days.
Time lag of 53 days. Long memory compared to FFMC & DMC.
When fully saturated, takes ~65 standard drying days for MC = 100%, which means DC = 500 (minimum required for smoldering combustion).
Length of day affects drying. Monthly drying factor applied to DC.
Commonly, top surface is drier than deep soil. But when light rains follow period of drought (usually early fall), get moisture reversal - deep duff is drier than surface. DC will continue to indicate smoldering potential because of its long memory.
- numerical rating of relative fire spread without the effect of slope or fuel consumption
- well correlated with spread rates in all sizes of fires across many fuel types
- key to predicting rate of fire spread
- factors: FFMC & wind speed -- can be updated at any time
- ISI increases exponentially with wind speed. Doubles with every increase of 14 km/h in wind speed
- scale 0 - &infty;
| ISI | Description |
|---|---|
| 10 | Threshold for onset of crowning in most coniferous fuel types |
| 20 | Extreme fire behaviour |
| 70 | Severe fire behaviour conditions; rarely exceeds this |
- numerical rating of amount of fuel available for combustion
- scale 0 - &infty;
| BUI | Description |
|---|---|
| <30 | Low intensity surface litter fires |
| 30 | Deeper, heavier fuels becoming involved in combustion with resultant increase in fire behaviour due to available fuel |
| 60 | Threshold for continuous/extreme fire behaviour; safety concerns exist; mop-up problems likely |
| 90 | Severe fire behaviour; more erratic; most fires will escape initial attack |
DMC & DC are combined so upper duff layers control BUI.
- if DMC = 0, BUI = 0 regardless of DC
- if DMC > 0, DC will cause BUI > DMC
- max possible effect of DC is to raise BUI equal to twice the DMC
DC tends to increase over the summer, so BUI is typically higher in summer than in spring, even for the same DMC. There are exceptions to this.
Fire behaviour varies dramatically in different fuel types at same BUI.
Graph of BUI more closely matches shape of DMC graph than of DC graph.
- combines BUI & ISI into 1 numerical rating of fire intensity
- scale 0 - &infty;
| FWI | Description |
|---|---|
| 3 | Threshold for sustained combustion & fire growth |
| 25 - 30 | Onset of crowning; extreme fire behaviour |
| > 50 | Severe fire behaviour; erratic; most disaster fires in this range |
Individual BUI & ISI values indicate relative importance of fuel consumption rate or rate of spread to overall fire intensity for that day.
- e.g., BUI = 200, ISI = 5 ====> FWI = 24
- e.g., BUI = 24, ISI = 18 ====> FWI = 24
Fire behaviour varies dramatically in different fuel types at same FWI.
FWI system only considers influences of weather on fire potential. FWI is best related to fire potential in mature jack pine & lodgepole pine forests.
Day-to-day decrease in FWI due to rainfall and/or lack of wind (ISI).
Shape of FWI graph more closely matches shape of ISI graph than of BUI graph.
Byram's fire intensity equation: I = Hwr
where I ≡ FWI (fire intensity), H ≡ heat yield of fuel (constant value), w ≡ BUI, r ≡ ISI