Search Fire Behavior Field Reference Guide, PMS 437

Text (indexed):

Once estimates of Rate of Spread and Flame Length have been obtained, use these classifications to evaluate the level of concern they generate.

Fire Behavior ClassRate of Spread (ch/hr)Flame Length (ft)Tactical Interpretation
Very Low0-20-1Direct, Hand
Low2-51-4Direct, Hand
Moderate5-204-8Direct, Equip
High20-508-12Indirect
Very High50-15012-25Indirect
Extreme150+25+Indirect

Surface Fire Behavior Characteristics Chart provides means for classifying and comparing individual fire events using spread rate and flame length (Fireline Intensity) or Heat per Unit Area.

 

Text (indexed):
  1. Surface Fire Shape
  2. Surface Fire Area Estimation from Point Source Fire, in Acres
  3. Surface Fire Perimeter Estimation from Point Source Fire, in Chains

These tools are intended for use with initiating fires only. Use the known ignition time and the number of hours after that you are interested in. Consider using:

  • Number of hours from ignition until the end of the expected burn period.
  • Number of hours until you arrive at the fire.

Surface Fire Shape

Remember that effective windspeed, as referenced here, is based on midflame windspeed.

Length to Width Ratio in parenthesis

Surface Fire Elliptical Shape - These elliptical fire shapes demonstrate the effect of midflame windspeed and point out the length to width ratio in each case.

Surface Fire Area Estimation from Point Source Fire, in Acres

Table 1, Spread Distance 1–50 Chains

Surface Fire Area, Table 1 (1—50 chains spread distance).

Table 2, Spread Distance 52–165 Chains

Surface Fire Area, Table 2 (52-165 chains spread distance).

Surface Fire Perimeter Estimation from Point Source Fire, in Chains

Table 3, Spread Distance 1–50 Chains

Surface Fire Perimeter, Table 3 (1—50 chains spread distance).

Table 4, Spread Distance 52–165 Chains

Surface Fire Perimeter, Table 4 (52-165 chains spread distance).

 

 

Text (indexed):

Andrews, P. L., Heinsch, F. A., Schelvan, L., How to generate and interpret fire characteristics charts for surface and crown fire behaviorU.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, 2011.

Rothermel, R.C., How to predict the spread and intensity of forest and range fires, USDA Forest Service General Technical Report, INT-143, 1983.

Rothermel, Richard C., Fire behavior nomograms. Appendix A excerpted from How to Predict the Spread and Intensity of Forest and Range Fuels, PMS 436-3, NFES 2220, National Wildfire Coordinating Group, 1992.

Scott, Joe H., Burgan, Robert E., Standard fire behavior fuel models: a comprehensive set for use with Rothermel’s surface fire spread model, Gen. Tech. Rep. RMRS-GTR-153, U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, 2005.

Scott, Joe H.,  Nomographs for estimating surface fire behavior characteristics, Gen. Tech. Rep. RMRS-GTR-192, U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, 2007.

 

 

Text (indexed):
  1. Evaluating Spotting Behavior
  2. Estimating Maximum Spotting Distance
  3. Spotting Likelihood/Probability
  4. Integrating Spotting Spread into Fire Growth Projections
  5. Comparing Spotting Estimation Tools

Evaluating spotting fire behavior requires the integration of three factors:

  • The source, size, and number of firebrands.
  • The distance the firebrand is carried downwind.
  • The probability of igniting a new fire at the downwind location.

Spotting distance factors include the spotting source, the lofting height of the ember, the downwind transport, and the time it takes the ember to burn out.

Short-Range Spotting is not generally considered as significant in the growth of wildfires, because the advancing fire usually overruns the developing spot fire. Based on that, the assumption is that short-range spotting is typically accounted for in the spread model outputs.

Long-Range Spotting is differentiated from short-range spotting, primarily because firebrands are being lofted by a convection column and carried beyond the immediate fire area.

Estimating Maximum Spotting Distance

Both the included Spotting Distance Nomograms, shown here, and BehavePlus provide methods for estimating the Maximum Spotting Distance from a Torching Tree, or trees.

The maximum spotting distance model requires identification of tree species, height, and DBH (Diameter at Breast Height) of the torching tree to estimate the flame height and duration from the torching tree that will initiate the lofting of the ember into the windfield.

Further, the open windspeed is used to suggest how far the firebrand will be transported as it falls back to the ground, and the nomogram because it assumes level ground uses the surface (20ft) windspeed and direction.

The downwind Canopy, or Tree Cover, Height (reduced by half for open canopies) is used to factor out embers intercepted by the canopy before reaching surface fuels.

The graphic here depicts additional inputs to the BehavePlus spotting module. In mountainous terrain, ridge top winds are used if wind is blowing across valleys as shown. The shape of the valley is considered with inputs for Ridge-to-Valley distance and elevation difference.

Spotting distance terrain factors include the position of the torching tree, the ridge to valley elevation horizontal distance, and the ridge to valley elevation change downwind from the source.

Western Tree Species Quick Reference Lookup Table

This table assumes three torching trees 50 ft tall and 10-inch DBH with downwind cover and an open stand of 50 ft tall trees. 

Maximum Spotting Distance, in miles
20 ft Windspeed, in mph
Tree Species05101520253035404550
Balsam Fir00.10.30.40.60.70.811.11.31.4
Grand Fir00.10.30.40.60.70.811.11.31.4
Subalpine Fir00.10.20.40.50.60.70.911.11.2
Lodgepole Pine00.10.20.30.50.60.70.80.911.1
Engelmann Spruce00.10.30.40.50.70.80.911.21.3
Ponderosa Pine00.10.20.30.50.60.70.80.911.1
Douglas-Fir00.10.20.40.50.60.70.911.11.2

Southern Pine Species Quick Reference Lookup Table

This table assumes three torching trees 50 ft tall and 10-inch DBH with downwind cover and an open stand of 50 ft tall trees.

Maximum Spotting Distance, in miles
20 ft Windspeed, in mph
Tree Species05101520253035404550
Shortleaf Pine00.10.20.20.30.40.50.60.60.70.8
Slash Pine00.10.20.30.30.40.50.60.70.80.8
Longleaf Pine00.10.20.30.30.40.50.60.70.80.8
Pond Pine00.10.20.20.30.40.50.60.60.70.8
Loblolly Pine00.10.20.20.30.40.50.60.60.70.8

Nomogram Instructions

This process and the nomograms that integrate the factors do not factor in terrain features as discussed above.

Inputs Required:
Torching Tree: Species, Height, DBH.
Open 20 ft Windspeed.
Downwind Average Tree Height (Divide by 2 for open stands).

Nomogram 1 (Flame Height) & Nomogram 2 (Flame Duration)
Start with input DBH, draw a vertical line to interest curve for input torching tree species, turn and draw a horizontal line to determine flame height in Nomogram 1 and flame duration in Nomogram 2.

Nomogram 3 (Firebrand Lofting):
Divide Flame Height (Nom 1) by the input torching tree height and use that value to select the curve in Nom 3. Using the flame duration (Nom 2), draw a vertical line from the bottom axis to intersect the selected curve. From that intersection, draw a horizontal line to determine the ratio for calculating firebrand height. Multiply ratio from Nom 3 by flame height to determine firebrand height.

Nomogram 4 (Maximum Spotting Distance):
Using the estimated firebrand height, draw a vertical line from the bottom axis on right to intersect curve for selected downwind tree height. From intersection draw a horizontal line to line for input windspeed, then down to spot distance.

Nomogram Worksheet

Follow 1 to 5, left to right on each line.

Maximum Spotting Distance Nomogram Worksheet.

NOM 1. Maximum Spotting Distance (Flame Height)

Spotting Distance Nomogram 1: Flame Height as an indicator of convective forcing.

NOM 2. Maximum Spotting Distance (Flame Duration)

Spotting Distance Nomogram 2: Flame Duration as an indicator of convective forcing.

NOM 3. Maximum Spotting Distance (Firebrand Lofting)

Spotting Distance Nomogram 3: Firebrand lofting factors including flame duration and flame height.

NOM 4. Maximum Spotting Distance (Maximum Spotting Distance)

Spotting Distance Nomogram 4: Maximum Spotting Distance factors including firebrand height, downwind tree cover height, and treetop windspeed.

Spotting Likelihood/Probability

Though tables for Probability of Ignition are provided in the Fuel Moisture section, they describe only the likelihood that an ember will ignite a fire in receptive fuels. Wildland Fire Decision Support System (WFDSS) spatial analyses integrate the potential frequency and distance for spotting fire behavior, but frequency information is hard to isolate.

Combining Maximum Spotting Distance with Probability of Ignition

Spotting Threat can be displayed as a combination of ignition potential and spotting distance.

Integrating Spotting Spread into Fire Growth Projections

FARSITE, FLAMMAP, and FSPro attempt to integrate the estimate of the number of embers and the distribution of distances they travel into the fire growth projection. Estimating maximum spotting distance from nomograms or BehavePlus only suggests an outer perimeter for spotting potential.

Isolating Spotting Spread Potential with FSPro

A suggested method for applying the WFDSS spotting models is to isolate the potential probability of spotting across significant barriers using FSPro analysis. FSPro is normally used to apply probabilities associated with windspeed and direction combined with day to day variability in fuel moisture scenarios. The MTT spotting spread model includes monte carlo probability assessments associated with embers lofted from crown fires.

Consider these adaptations to the FSPro Inputs:

  • Assume a one-day analysis with forecasted ERC, Windspeed and direction favorable to spotting spread.
  • Assume a number of fires (several hundred to several thousand) using the one-day scenario from above.

This method assumes that the scenario is favorable for torching and spotting behavior. Ensure that the analysis will produce at least passive crown fire. The number of fires assumed for the analysis will drive the variability in:

  • Ember source location from user applied probabilities.
  • Ember size and, therefore, distance.

Probability contours produced by the analysis reflect the locations for possible spotting and the relative probabilities for their occurrence. Tonja Opperman demonstrated this technique on the Chakina fire near McCarthy, Alaska in 2010.  Output map is shown here.

This image shows how the analyst can utilze FSPro with specialized inputs to characterize where spotting behavior is most likely to breach barriers like rivers and roads.

 

Comparing Spotting Estimation Tools

Tonja Opperman, Fire Applications Specialist, Wildland Fire Management RD&A, assembled these tables based on contributions and discussions among many fire behavior researchers, programmers, and practitioners, including: Pat Andrews, Mark Finney, and Chuck McHugh at the Missoula Fire Lab; Brian Sorbel from the Alaska Region of the NPS; Mitch Burgard and Erin Noonan-Wright from the Wildland Fire Management RD&A; Stu Brittain with Systems for Environmental Management in Missoula; Joe Scott with Pyrologix in Missoula; and Rick Stratton from the Pacific Northwest Region of the USFS. Corrections can be forwarded to Tonja Opperman at tonja_opperman@firenet.gov.

Maximum Spotting Distance (Non-Spatial)

SystemMaximum Spotting Distance from Torching Trees NomogramsBehavePlus v.5.0.5
Inputs
  • Torching tree height.
  • Torching tree species.
  • Torching tree DBH.
  • Average tree height.
  • 20-foot wind speed.
  • Torching tree height.
  • Torching tree species.
  • Torching tree DBH.
  • Downwind canopy height.
  • 20-ft wind speed.
  • Number of torching trees.
  • Ridge/Valley elevation difference.
  • Ridge/Valley horizontal distance.
  • Spotting source location.
Spotting Process
  • Nomograms published in Rothermel (1983) are based on the model published by Albini (1979).
  • Predictive mathematical model to calculate the maximum distance an ember will travel from a single firebrand is lofted from a torching tree to calculate maximum distance.
  • Spotting from torching trees is based on nomograms with options for multiple torching trees and terrain adjustment (Albini 1979, Chase 1981, Rothermel 1983).
  • BehavePlus also calculates spotting from wind-driven surface fire (Albini 1983, Chase 1984, Morris 1987) and spotting from burning piles (Albini 1981).
  • Spotting from active crown fire will be added in BehavePlus v6.0 (Albini et al. 2012).
Outputs
  • Maximum spotting distance from a single torching tree on flat ground is read from a nomogram.
  • Maximum spotting distance from torching trees (single or multiple) is displayed in a table and graph.
  • Probability of ignition is calculated separately.
Assumptions & Limitations
  • Gives maximum distance only.
  • Assumes level terrain; single torching tree.
  • Does not account for likelihood of trees torching, firebrand material availability, the number of spot fires, or the probability of ignition for that firebrand.
  • Gives maximum distance only.
  • Accounts for terrain and number of torching trees.
  • Number of torching trees is used to calculate firebrand lofting height; higher firebrands travel further, all else equal.
  • Does not account for likelihood of trees torching, firebrand material availability, or the number of spot fires, or the probability of ignition for that firebrand.

WFDSS Spotting Spread (Spatial)

In all geospatial systems, embers are only generated from passive and active crown fires, not from surface fires, fire whirls, burning piles, or structures. Spotting can be turned off or set to zero in all tools.

SystemShort Term Fire Behavior (STFB)
FLAMMAP MTT similar
Near Term Fire Behavior (NTFB)
FARSITE similar
Fire Spread Probability (FSPro)
Inputs
  • Canopy characteristics from spatial layers (LCP).
  • Analyst specifies foliar moisture content.
  • Spotting tree species is always grand fir with a DBH of 20 cm (7.9 inches).
  • Analyst sets spotting probability. Can be higher than allowed for NTFB, generally less than 0.25.
  • Wind speed/direction is constant for entire burn period, but can use gridded winds that are modified based on topography.
  • Weather is static, though dead fuel moistures can be conditioned.
  • Canopy characteristics from spatial layers (LCP).
  • Analyst specifies foliar moisture content. 
  • Spotting tree species is always grand fir with DBH of 20 cm.
  • Analyst sets spotting probability. Maximum setting needs to be less than 0.15.
  • Distance and perimeter resolutions are determined from the LCP resolution; timestep is 60 minutes. 
  • Wind speed/direction input can be changed hourly. Forecast wind speed and direction are for every 3 hours. Gridded winds are not yet available. 
  • Canopy characteristics are from spatial layers (LCP).
  • Foliar moisture content always 100%.
  • Spotting tree species is always grand fir with a DBH of 20 cm.
  • User sets spotting probability for each fire danger (ERC) bin. See STFB.
  • Winds can be probabilistic, forecast or a combination.
  • Weather can be probabilistic or forecast or combination. No fuel moisture conditioning.
  • Ignition delay is optional.
Spotting Process
  • Fire behavior is calculated for each cell. Nodes are on fixed grid equal to LCP spatial resolution.
  • User-set spotting probability determines which predicted crown fire cells (and associated nodes) can produce spots. Those nodes generate a single ember with random distance from zero to the max for that node. For cells predicted to have active or passive crown, 16 incrementally-sized embers are lofted.
  • Max ember distance & azimuth are calculated using canopy cover, crown fraction burned, elevation, and all available wind information.
  • Embers landing on unburnable or already burned substrate do not ignite. Embers landing on burnable substrate always ignite (Finney 2002). Similar to spotting in FlamMap 5.0 and FSPro.
  • Vertices loft 16 incrementally sized embers. The number of vertices depends on perimeter and distance resolutions & timestep. Each ember goes through a random draw process based on user-set spotting probability.
  • Ember distances and azimuth are based on canopy cover, crown fraction burned, elevation, winds, and species/DBH. Embers are tracked until they burn out or land.
  • Ignores all embers that land within one cell from where the ember originated, as the main fire would eventually burn over these spot fires.
  • A spotting grid overlaid onto the LCP allows the first spot that lands in a burnable fuel model to “seize” that cell so no other spots can land in that particular cell.
  • Same process as STFB.
Outputs
  • Models fire behavior for every cell simultaneously for a single scenario, and uses MTT to calculate fastest fire travel paths. Embers produced only with passive and active crown fire.
  • Randomly lofts a single ember from a node if the predicted fire type is passive or active crown fire.
  • Simulates lofting and downwind travel of individual embers of different sizes from each vertex that exhibits passive or active crown fire.
  • Fire probability surface output that may or may not distinguish spot fire activity.
Limitations & Assumptions
  • Spotting only occurs when passive or active crown fire is modeled. Finney and Scott & Reinhardt methods are available for crown fire; each calculates crown fraction burned (CFB) differently. CFB and canopy cover are used to determine “number of torching trees” (1-10) used in firebrand lofting height.
  • More embers will be lofted at finer landscape resolutions. Faster ROS will encounter more nodes, but the absolute number of nodes is static. One ember per node; less chance than in NTFB/FARSITE that an ember will travel the maximum distance.
  • NOTE: Users will probably want to set spotting probability higher in STFB than for NTFB tools.
  • Grand fir is used as the spotting tree species for the entire landscape. Distance resolution, perimeter resolution, and timestep are automated.
  • The minimum spotting distance (set to the landscape resolution) essentially skips the first cell. For example, on a 60-meter landscape, no spots occur in the first 60 meters from the perimeter, but any viable embers that land beyond 60 meters can produce spot fires.
  • NOTE: Users will probably want to set spotting probability lower in NTFB than in systems using MTT (STFB, FSPro).
  • Same as STFB. Finney and Scott & Reinhardt crown fire methods are available; each calculates crown fraction burned (CFB) differently. CFB and canopy cover are used to determine the number of torching trees used in firebrand lofting height.
  • NOTE: Users will probably want to set spotting probability higher in FSPro than for NTFB tools. Calibrate FSPro with STFB utilizes consistent spotting methods.
Text (indexed):
  1. Definitions
  2. Active Crown Fire Rate of Spread and Flame Length
  3. Estimating Active Crown Fire Spread Rate With Surface Shrub Models

Definitions

Crown Fraction Burned (CFB) is a theoretical concept that is used to model and classify crown fire. It may be observable after the fact in burn severity assessments.

This graph compares Crown Fire spread rates utilizing several surface shrub fuel models and compares them to the Rothermel Crown Fire Spread Model.

Passive Crown Fire (Intermittent or Persistent Torching) occurs where surface fire intensity is sufficient to ignite tree crowns, individually or in groups, but winds are not sufficient to support propagation from tree to tree. CFB between 0.10 and 0.90.

Active Crown Fire occurs where surface and crown fire energy are linked. Surface intensity is sufficient to ignite tree crowns, and fire spread and intensity in the tree crowns encourages surface fire spread and intensity. CFB at least 0.90.

Independent Crown Fire occurs (rarely) where tree crown loading and flammability is sufficient to carry fire without surface fire contribution under ambient weather and wind conditions. CFB generally approaching 1.0.

Isolated Tree Torching should not be considered crown fire, though it may be an indicator of potential later in the burn period. It usually occurs due to anomalies in surface fire behavior due to jackpots of surface fuel, isolated terrain features, or brief wind gusts. CFB is less than 0.10.

Active Crown Fire Rate of Spread and Flame Length

After the 1988 fire season, Rothermel (1991) developed an empirical model for estimating crown fire spread rates and fireline intensities, referencing several fires from the Rocky Mountains in its development. Based on fire behavior in Fuel Model 10 (FB10), the calculation is essentially:

ROSActiveCrownFire = 3.34*ROSFuelModel10

(Assuming MFWS = 20ft windspeed*0.4)

These graphs, using season, slope, and 20ft windspeed, provide rough estimates of active crown fire spread rates using the Rothermel Crown Fire Spread model.

No Slope

Using the season of the year and the 20-ft windspeed, this graph helps the analyst estimate crown fire spread rate for fires on generally level or low slope landscapes.

50% Slope

Using the season of the year and the 20-ft windspeed, this graph helps the analyst estimate crown fire spread rate for fires on steep slopes of approximately 50%.

100% Slope

Using the season of the year and the 20-ft windspeed, this graph helps the analyst estimate crown fire spread rate for fires on steep slopes of approximately 100%.

Estimating Active Crown Fire Spread Rate with Surface Shrub Models

In fireline assessments, it may be necessary to make quick estimates of crown fire spread based on simple inputs.  Simple lookup tables or graphs like those above provide rough estimates. Anderson (1982), when describing the original 13 surface fuel models, identified several shrub models as representative of crown fire behavior in several classic types:

  • FM4 (Chaparral) for New Jersey Pine Barrens and Lake States Jack Pine.
  • FM6 (Dormant Brush) for Alaska Spruce Taiga.
  • FM7 (Southern Rough) for Alaska Black Spruce/Shrub Communities.

Bishop (2010), in developing the Fireline Assessment Method (FLAME), averaged spread rates for fuel models 5, 6, and 7 to estimate crown fire spread.

Fuel Models sh5 (145) and sh7 (147) have been used in the same manner in spatial modeling in different situations.

This graphic demonstrates the similarity in spread rates produced by the Rothermel Crown Fire Spread Rate (crown) and several surface shrub fuel models.  

This graph compares Crown Fire spread rates utilizing several surface shrub fuel models and compares them to the Rothermel Crown Fire Spread Model.

Caution: Using surface fuel models to represent crown fire behavior may not accurately provide for the calculation of Crown Fraction Burned (CFB) or the modeling of increasing spread due to passive crown fire (torching and spotting) behavior in spatial fire analyses. It may also over-estimate fire spread and intensity under moderated environmental conditions.

Text (indexed):
  1. Introduction
  2. Crown Fire Initiation
  3. Active Crown Fire Propagation and Crowning Index (CI)
  4. Finney, Scott/Reinhardt, and Van Wagner Approaches to Crown Fire Modeling

Introduction

In the publication “Conditions for the start and spread of crown fire”, C.E. Van Wagner (1977) identified that crown fire is the interaction between separate fuel layers in forested areas.

Further, he described two processes and defined models for estimating their potential:

  • Crown Fire Initiation is an indicator of the potential for surface fire to ignite tree crown and produce either passive or active crown fire. Inputs include the gap between the surface fuels and the tree crowns (Canopy Base Height – CBH), the foliar moisture content (FMC) of the tree crowns. The result is a threshold surface fire intensity required to produce some crown fire.
  • Active Crown Fire Propagation (Crown Spread) is an indicator of the potential for continuous spread through the tree crowns. Inputs include only a characterization of the canopy fuel density in a single number. The result is a threshold rate of spread required to sustain a “solid crown flame…with associated horizontal spread.”

These models are very coarse due to the way they represent highly variable characteristics, canopy base height and canopy fuel density. Because they are so variable, their inputs represent grand averages and may require adjustment in modeling efforts.

As shown in this matrix, the Crown Fire Initiation and Active Crown Fire Propagation models work together to estimate when fires will remain as surface, when they will produce torching, or passive crown fire, behavior in the canopy, and when they will progress to active crown fires.

Linking Surface and Crown Fire Behavior

(Scott & Reinhardt, 2001)

Linking Surface and Crown Fire Behavior. Essentially describes criteria for distinguishing the range of fire behavior types where forest canopies are flammable.

Use the model results from sections Crown Fire Initiation and Crown Fire Propagation to compare against estimates of surface and crown fire spread produced using anticipated environmental factors.

Crown Fire Initiation

These two graphs identify the height to live crowns (CBH) and the canopy foliar moisture content (FMC) as critical factors, resulting in the threshold surface fire intensity or flame length for evaluating of crown fire initiation. Use either of them to estimate minimum surface fireline intensity (FLI) or flame length (FL) that will support at least passive crown fire.

Threshold Evaluation

Use either of the two graphs below. This assessment only determines whether surface fire behavior is sufficient to initiate crown combustion. Both passive and active crown fire are possible if this threshold is met. See the criteria for active crown fire above under Linking Surface and Crown Fire Behavior to differentiate those conditions.

  • Determine the current and/or expected surface intensity (FLI or FL) for that landscape.
  • Estimate the CBH and FMC for the landscape you are evaluating for crown fire potential.
  • Lookup the threshold surface intensity from either graph here.
  • Compare the two intensities. If the projected intensity is greater than the threshold value, crown fire is expected. A ratio of projected over threshold provides a confidence value.

 Crown Fire Initiation; characterized as threshold surface fire intensity. Input factors are canopy base height and foliar moisture content.

 Crown Fire Initiation; characterized as threshold surface flame length. Input factors are canopy base height and foliar moisture content.

Active Crown Fire Propagation and Crowning Index (CI)

According to Van Wagner (1977), minimum threshold values for canopy fuel/bulk density (CBD) are necessary to sustain active crown fire at given spread rates. Since there is only a single crown fire fuel model, that threshold spread rate can be converted to a threshold windspeed or “crowning index” (CI).

Threshold Evaluation

The table and the graph below it provide threshold values for both ROS(active) and open 20 ft windspeed.

For a given CBD, if observed, or forecast 20 ft wind or projected ROS(active) are larger than these threshold values, sustained active crown fire is expected. A ratio of estimate/threshold provides a confidence value.

Threshold for Active Crown Fire. Characterized as the crown fire spread rate needed to sustain an active crown fire, the inputs are canopy bulk density and season severity.

Crowning Index: Because there is only one crown fire ‘fuel Model’ wind is the primary environmental factor for determining the threshold.

Finney, Scott/Reinhardt, and Van Wagner Approaches to Crown Fire Modeling

All approaches identify the threshold for predicting crown fire initiation and active crown fire spread using the same criteria, based on the Van Wagner Crown Fire Initiation and Propagation models. They diverge in the way they estimate:

  • Final spread rates for passive and active crown fire.
  • Crown Fraction Burned (CFB).
  • Final Fireline Intensity.
  • Fire Type (Surface, Passive, Active).

Both US and CFFDRS systems implement the initiation and active crown fire criteria where crown fire is anticipated. This table compare the different ways the criteria are implemented.

Model UsedCFFBP Van Wagner (1977) Modeling SystemSurface Fire Control Finney (1998) Modeling SystemCrown Fire Control Scott and Reinhardt (2001) Modeling System
Surface FireVan Wagner (1977) Integrated, empirical modelRothermel (1972)Rothermel (1972)
Crown Fire SpreadVan Wagner (1977) Integrated, empirical modelRothermel (1991)Rothermel (1991)
Crown Fire (Torching) Initiation Threshold Spread RateVan Wagner (1977)Van Wagner (1977)Van Wagner (1977)
Active Crown Fire Propagation Threshold Spread RateVan Wagner (1977)Van Wagner (1977)Van Wagner (1977)
Methods AppliedCFFBP Van Wagner (1977)Surface Fire Control Finney (1998)Crown Fire Control Scott and Reinhardt (2001)
Crown Fraction BurnedNatural Log function based on the difference between estimated spread rate and Initiation Threshold spread rate. 90% when estimated spread rate is 10m/min greater than Initiation Threshold spread rate.Natural log function based on the difference between the estimated surface spread rate and the Initiation Threshold rate. 0.9 when surface spread rate reaches 90% of difference between Initiation Threshold and Active Propagation threshold.Proportionally intermediate between 0 (surface fire) and 1.0 (active Crown Fire) based on input windspeed and where it falls between windspeeds at Initiation Threshold and Active Propagation Threshold spread rates.
Passive Crown Fire SpreadIntegrated within basic spread model for conifer & mixedwood fuel types. Fire type designated as Passive based on estimated Crown fraction Burned (see below).Surface Fire spread rate plus any spotting spread.Based on Crown Fraction Burned (CFB). Proportionally intermediate between surface spread rate and Active Propagation threshold, based on crown fraction burned plus any spotting spread.
Final Spread Rate with Active Crown FireIntegrated within basic spread model for conifer & mixedwood fuel types Fire type designated as Passive based on estimated Crown fraction Burned (see below).Proportionally intermediate between surface spread rate and Rothermel 1991 Model based on Crown Fraction Burned. Generally, less than half of Rothermel model rate.Spread Rate estimated directly from Rothermel 1991 crown fire spread model.
Fire IntensityByram (1959)Byram (1959) for Surface
Thomas (1963) for Crown
Byram (1959) for Surface
Thomas (1963) for Crown
Fire Type:
Surface
Passive
Active
Based on Crown Fraction Burned
<0.1 = surface
0.1 to <0.9 = passive
.9 to 1.0 = active
Based on Crown Fire Initiation and Propagation thresholds (see above)
Surface if ROS < Init. Criteria
Active if ROS > Active Crit.
Passive Between
Based on Crown Fire Initiation and Propagation thresholds (see above)
Surface if ROS < Init. Criteria
Active if ROS > Active Crit.
Passive Between

 

 

Text (indexed):

Interpreting Expected Crown Fire Behavior

Crown Fire Behavior Characteristics Chart provides means for classifying and comparing individual fire events using spread rate and flame length (Fireline Intensity) or Heat per Unit Area where crown fire is anticipated or observed.

Text (indexed):
  1. Crown Fire Area Estimation for Point Source Fires, in Acres
  2. Crown Fire Perimeter Estimation for Point Source Fires, in Miles
  3. Crown Fire Length to Width Ratio

Crown Fire Area Estimation for Point Source Fires, in Acres

Area estimation from crown fire spread, initiating from point source fires, in acres.

Crown Fire Perimeter Estimation for Point Source Fires, in Miles

 Perimeter estimation from crown fire spread, initiating from point source fires, in acres.

Crown Fire Length to Width Ratio

20ft Windspeed (mph)Length to Width (x:1)
102.2
152.9
203.5
254.1
304.8
355.4
406
456.6
507.3
557.9
608.5
659.1
Text (indexed):

Crown Fire References

Alexander, M. E. 1988. Help with making crown fire hazard assessments. Pages 147-156 in Fischer, W. C.; Arno, S. F. (compilers). Protecting People and Homes from Wildfire in the Interior West: Proceedings of the Symposium and Workshop. General Technical Report GTR-INT-251. Ogden, UT: USDA Forest Service, Intermountain Research Station.

Anderson, H. E. 1982. Aids to determining fuel models for estimating fire behavior. Gen. Tech. Rep. INT-122. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 22 p.

Andrews, Patricia L.; Heinsch, Faith Ann; Schelvan, Luke. 2011. How to generate and interpret fire characteristics charts for surface and crown fire behavior. Gen. Tech. Rep. RMRSGTR-253. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 40 p.

Bishop, Jim 2007. Technical background of the FireLine Assessment MEthod (FLAME). In: Butler, Bret W.; Cook, Wayne, comps. The fire environment--innovations, management, and policy; conference proceedings. 26-30 March 2007; Destin, FL. Proceedings RMRS-P-46CD. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. CD-ROM. p. 27-74

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Rothermel, Richard C. 1991. Predicting behavior and size of crown fires in the northern Rocky Mountains. Res. Pap. INT-RP-438. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 46 p.

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Text (indexed):
  1. Before an Assignment
  2. Assigned and Enroute
  3. On Scene Fire Assessment
  4. Determine Decision Thresholds to Ensure LCES
  5. Document Your Assessment

What Makes a Good Analyst (Mark Finney...FBSC YouTube Video)
https://www.youtube.com/embed/UIS27uMdfG8

Before an Assignment

  • Evaluate Weather Forecasts and Outlooks
  • Consider local climatology and critical fire weather patterns
  • Review area Pocket Cards and current season severity
  • Review yesterday’s fire activity and notable fire behavior

Assigned and Enroute

  • Get on scene weather reports from yesterday, overnight and current conditions
  • Assess maps and photos of the fire area with current perimeters and recent activity
  • Ask for Spot Forecast and confer with fire weather forecaster
  • Interpret sky and smoke conditions for stability, wind speed and direction, and burning intensity.

On Scene Fire Assessment

  • Request current weather observation and validate your forecast. Is your fireline exposed to or sheltered from the expected winds?
  • Get a picture of current fire activity level.
  • Anticipate today’s next big changes. Do you anticipate changes? When?
  • Characterize fuels (fuel types, loadings, moistures) adjacent to your fire, especially where folks are working and where fire could move.
  • Inventory of significant terrain features ahead of the fire. Will it burn upslope or down?
  • Continue to monitor the sky for cloud and smoke indicators.
  • Estimate the fire behavior you anticipate in view of the current situation and the expected changes. What spread rates do you anticipate? What flame lengths? Do you anticipate crown fire? Spotting across your lines or long range?

Determine Decision Thresholds to Ensure LCES

  • Determine time frames for escape to safety and escape routes. What windspeeds or changes in fire behavior will render those time frames insufficient?
  • Identify best locations and methods for lookout to monitor and validate your assessment.
  • Ensure that weather and fire behavior observations are communicated to the entire crew.
  • Will fatigue and logistics factors impact these decisions?

Document Your Assessment

  • Record your observations and assumptions.
  • Use worksheets and include notes for each assessment.
  • Include assessments and decisions in personal logs.
  • Remember: If you’re not keeping score, it’s just practice.