Assuming that fuels are mostly continuous in the fire illustrated in figure 2, what are the primary factors that will affect rate of spread? Well, they normally are fine fuel moisture, windspeed, fuel loading, steepness of slope, and the occurrence of spotting. In later units, you will see the extent to which these factors affect rate of spread and how these inputs are used to calculate fire spread.
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In figure 2, we diagrammed the perimeter and area relationship to rate of spread from a point source. Page 8, figure 3, illustrates some variations in the elliptical or egg-shaped patterns of fire spread, depending on windspeeds. As windspeeds increase, the fire shapes elongate. Note that these patterns are not drawn to scale, and merely illustrate fire shapes. The fire shapes in figure 3 can be used for planning purposes, particularly on the initial attack of fires. These shapes may not fit your fire situation, as conditions in the field can cause variations in fire shapes. These conditions are the following: heterogenous fuel complexes that produce fingering, barriers that stop or partially stop the spread, the effect of slope that reduces or increases fire spread at the head or flanks, and spotting ahead of or downslope from a fire. All of these serve to complicate the task of estimating where the fire perimeter will be after times number o£ hours. Together with ignition and rate of spread, fire intensity is a third feature of wildfires of great concern to firefighters. Under item D, note these factors that fire intensity is dependent upon: Fuel loading, compactness or arrangement of fuels, moisture content of fuels, and atmospheric instability. These are important factors to remember, and you will be expected to know them later. The listing of these factors at this point in the course is intended primarily to acquaint you with the variables that must be analyzed and considered when making fire behavior predictions. Near the end of this unit, we will pull together the most important fire environmental factors and show what inputs are required to make fire behavior calculations and predictions. To better understand when and how ignition and combustion occur in a wildfire, we need to discuss the physical processes involved. The next part of this unit, starting on page 9, will deal with heat transfer and how the various methods of heat transfer affect fire behavior. First of all, note that heat transfer refers to the physical processes by which heat energy moves to and through unburned fuel. The three common methods of heat transfer should be listed under item E. The first is conduction. Conduction is the transfer of heat from one molecule of matter to another. An example of this is fire smoldering through a solid piece of fuel. Since wood is generally a poor conductor of heat, conduction is the least important method of the three.
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