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Active Crown Fire Behavior

  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.

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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.

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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

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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

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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

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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%.

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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.  

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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.

NWCG Latest Announcements

Updated NWCG Guide for Wildland Fire Modules, PMS 431

Date: May 14, 2025
Questions?  Please contact:
Fire Use Subcommittee

The updated NWCG Guide for Wildland Fire Modules, PMS 431 serves as a supplemental resource for wildland fire module operations, providing information frequently used in the field.

PMS 431 defines the organizational structure, staffing, and qualifications for Type 1 and Type 2 Wildland Fire Modules, outlining their roles in wildland fire suppression, prescribed fire, and resource management. The NWCG Guide for Wildland Fire Modules, PMS 431, should be used in conjunction with the NWCG Standards for Wildland Fire Module Operations, PMS 430.

References:

NWCG Guide for Wildland Fire Modules, PMS 431

NWCG Standards for Wildland Fire Module Operations, PMS 430

Updated NWCG Standards for Wildland Fire Resource Typing, PMS 200 Now Includes Off-Highway Vehicle Typing Standards

Date: May 13, 2025
Questions?  Please contact:
Mobile Fire Equipment Subcommittee 

The NWCG Standards for Wildland Fire Resource Typing, PMS 200 has been updated to include newly developed Off-Highway Vehicle (OHV) typing standards. These standards organize OHV information into logical categories and outline key safety requirements for occupants.

NWCG Standards for Wildland Fire Resource Typing, PMS 200 establishes minimum typing standards for aircraft, crews, and equipment used in national mobilization to wildland fire incidents. Typing designations are assigned based on measurable capability and performance criteria.

References:

NWCG Standards for Wildland Fire Resource Typing, PMS 200

NEW! S-204, Interpersonal and Critical Thinking Skills for Safety Officers Now Available

Date: May 12, 2025
Questions?  Please contact:
Risk Management Committee

The S-204, Interpersonal and Critical Thinking Skills for Safety Officers self-directed online course is now available! Developed through the Incident Performance and Training Modernization (IPTM) effort, this training supports individuals working towards Safety Officer, Field (SOFF) incident qualifications.

S-204, Interpersonal and Critical Thinking Skills for Safety Officers focuses on facilitating hazard mitigation conversations and building trust to influence safety-related decisions. Individuals can self-enroll through the Wildland Fire Learning Portal (WFLP).

Any changes to qualification pathways will take effect with the next update of the NWCG Standards for Wildland Fire Position Qualifications, PMS 310-1.

References:

S-204, Interpersonal and Critical Thinking Skills for Safety Officers 

NWCG Safety Officer, Field (SOFF) Position Page

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Great Basin Cache Upgrading to New Inventory System

Date: May 7, 2025
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The Great Basin Cache (GBK) is transitioning to a new inventory system to better serve the wildland fire community. During this upgrade, GBK will be unable to process standard orders from Part 1: Fire Supplies and Equipment and Part 2: Publications between May 8-20, 2025, with exceptions made for emergency fire orders. Orders will be accepted through close of business May 7.

To browse the latest available items, please refer to the National Fire Equipment System (NFES) catalogs. 

References:

NWCG NFES Catalog-Part 1: Fire Supplies and Equipment, PMS 449-1

NWCG NFES Catalog-Part 2: Publications, PMS 449-2