How a “dirty burn” affects wildfire containment percentage

Wildfire perimeters aren’t always easy to define. When drawn on a public map, they are often depicted as one simple line to describe the outermost bounds of the area that is burning. However, fire may not burn evenly throughout the interior of that fire, so things might look quite different on the ground. The reasons for the manner of fire spread are numerous (e.g., weather, fuel loads, suppression actions). The Ojo de los Casos Fire falls into this category.

This fire is made up of many patches of burned and unburned vegetation. In cases like this, lingering heat can hide within unburned fuels close to the ground. This heat may not be recognized until weather and fuel conditions are just right for ignition. Once ignited, these areas could burn hot and fast. Firefighters call this a “dirty” or “scabby” burn. Complete “containment” is the ultimate goal for the fire management team in command of fire suppression activities. To reach containment, fire managers must attain a high level of confidence that the fire will not spread beyond a defined boundary and that unburned fuels within the perimeter and in the surrounding landscape are not at risk of igniting (spot fires).  Until firefighters have a high level of confidence that no significant heat resides adjacent to the physical containment line, containment percentage estimates remain conservative.