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Unit 2: Fuels Classification

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Size and Shape

The figure below illustrates the relationship between fuel size and surface area. The cube or block of fuel on the left is 1 foot on each side and contains 1 cubic foot. The surface area of this cube is 6 square feet. If that same cube is divided up into 16 pieces as shown on the right, we have the same volume of fuel, but now there is much more surface area to the 16 pieces. Calculations will show 18 square feet for the same cubic foot of fuel. This is three times the surface area of the cube on the left.

Fuel Size and Surface Area Relationships

Why is this important to fire behavior? We know from our experiences in starting campfires, wood stoves, or fireplaces that small fuels ignite and sustain combustion easier than large blocks of fuel. Less heat is required to ignite the small particles.

Shapes of Fuels Influences Spotting

Fuel shape is a significant factor in the problem of spotting. The images below illustrate some fuels that are likely candidates as aerial firebrands. Each of these has been found to have traveled distances of 10 miles or more downwind from a large, raging forest fire. In these cases, their flatness and greater surface-area-to-volume ratios have increased the aerodynamic qualities of the particles, thus making it easier for convection columns to lift them to greater altitudes.

Pine Cones BarkPlates Pine Needles

The shape of fuels is also important to spotting downslope by rolling fire-brands. Pine cones, round logs, and round yucca plants are particularly troublesome in their respective areas.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, October 05). Unit 2: Fuels Classification. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: This work is licensed under a Creative Commons License Creative Commons License