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Spotting Fire Behavior

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 Factors

Short-Range Spotting is not generally considered as significant in the growth if wildfires, because the advancing fire usually overruns the developing spot fire.

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 (with the associated worksheet in Section 5.5) 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 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 fire brand will be transported as it falls back to the ground.  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.

Terrain Factors

Maximum Spotting Nomogram Worksheet 

(follow 1 to 5, left to right on each line)

Spotting Nomogram Worksheet 

NOM 1. Maximum Spotting Distance (Flame Height)

Spotting Nomogram 1

NOM 2. Maximum Spotting Distance (Flame Duration)

Spotting Nomogram 2

NOM 3. Maximum Spotting Distance (Firebrand Lofting)

Spotting Nomogram 3

NOM 4. Maximum Spotting Distance (Maximum Spotting Distance) 

Spotting Nomogram 4

Comparing Maximum Spotting Distance Tools

Spotting Likelihood/Probability

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

Tonja Opperman suggested a method for applying the WFDSS spotting models to isolate the potential probability of spotting across significant barriers using FSPro analysis.

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.