Extreme Fire Behavior

Extreme fire behavior is a level of fire behavior that goes beyond human methods of fire control and prediction. The erratic nature of extreme fire behavior presents too serious a risk to the lives of ground crews to perform direct attack. Extreme fire behavior is unpredictable even by complex fire behavior models (Albini 1976, Hough and Albini 1978, Rothermel 1983). Compared to western regions of the U.S., extreme fire behavior in the Southeast is uncommon and generally only occurs during periods of severe drought. Under these conditions, communities where extreme fire behavior is most likely to occur include pocosins, palmetto-gallberry flatwoods, Table Mountain pine, and sand pine scrub.

Although extreme fire situations are few in the Southeastern U.S., they account for most of the fire-related injuries and fatalities. Below, the characteristics of large/mass fires are described and the fuel, weather, and topographical factors that create extreme fire behavior are explained. These concepts are then combined in two cases studies of extreme fire behavior that occurred in the Southeast: the Air Force Bombing Range Fire of 1971 and the Florida wildfires of 1998.

Characteristics of extreme fire behavior include:

• very high to extreme rates of spread;
• prolific crowning and torching;
• fire whirls;
• tall, well-developed convection columns;
• long flame lengths; and
• excessive spotting (National Wildfire Coordinating Group 1981).

High rates of spread and long flame lengths are the most common types of extreme fire behavior. Rates of spread are dramatically altered by long distance spotting. A fire may begin exhibiting the characteristics of extreme fire behavior during an event known as a blowup, which is a sudden increase in the intensity and rate of spread of a fire. Blowup occurs when windspeed increases dramatically and/or an area of atmospheric instability settles over the fire. Blow-ups are difficult to control and have taken the lives of wildland firefighters. They generally subside with a change in fuel or weather conditions.

One of the more fascinating manifestations of extreme fire behavior is fire whirls. Fire whirls are spinning vertical vortices of hot air and gases rising up from a fire that carry smoke, embers, debris, and flame. They can range in size from less than one foot to more than 500 feet in diameter. Large fire whirls have the intensity of a small tornado and can rapidly spread embers and new fire spots beyond the perimeter of the main fire.

Torching and crowning are also spectacular examples of extreme fire behavior. Torching occurs when an entire tree, or clump of trees, goes up in flames, but the fire does not spread to the adjacent crowns. In contrast, a crown fire occurs when fire spreads from crown to crown in the overstory. Potential for torching and crown fire initiation increases as the height to the base of the crown decreases or ladder fuels increase. Ladder fuels (vines, small trees, large shrubs) provide vertical continuity between surface fuels and tree canopy fuels.

Excessive spotting is also indicative of extreme fire behavior and can be linked to strong winds, the presence of fire whirls or a convection column. Spotting occurs when sparks, embers, and/or firebrands are carried up into the convection column and dropped outside of the perimeter of the original fire, causing an entirely new fire. Embers are small pieces of burning material that usually go out before they spread more than ¼ mile from the ignition zone (National Wildfire Coordinating Group 1981). Firebrands are larger materials like branches or palm fronds, which can be carried much further while still burning and may cause new fires more than ¼ mile from the original fire (National Wildfire Coordinating Group 1981). In 1983, Albini published mathematical equations that predicted the maximum height and drift of firebrands. These equations were simplified for easy field calculations and were further refined in a publication by Chase in 1984.

Extreme fire behavior results from combinations of fuel and weather conditions that include:

• abundant available fuels,
• drought,
• low relative humidity,
• strong winds,
• atmospheric instability, and
• topography (in mountainous areas).

Fuels

High loading of available fuel affects the ability of wildfire to develop into extreme behavior. The availability of fine dead fuels and live foliage depends largely on their moisture content. Very low fuel moisture content allows wildfires to consume more of the fuel and therefore release more heat. Larger dead fuels will also cure under drought conditions, and can become important heat sources during extreme fire conditions.

Some fuels are more prone to long-range spotting: medium to heavy palmetto-gallberry, dry swamps, and canopies with crown closure greater than 75%. Certain fuel types also increase the incidence of extreme fire behavior. For example, the presence of foliage with a profusion of volatile oils such as gallberry (Ilex glabra) can increase the incidence of torching and crowning into the canopy.

Weather factors

Extreme fire behavior is likely to ensue if ignition occurs when fine fuel moisture content is below 5% and the relative humidity is below 20% (National Wildlife Coordinating Group 1989). Wind speeds above 20 mph can produce extraordinary rates of spread under droughty or dry conditions.

Atmospheric instability can play a role in the evolution of extreme fire behavior. Atmospheric instability contributes to the development of a convection column by allowing rapid vertical lift in the rising column of hot gases and debris (see Fire and Plumes). The influx of air at the base of the convection column causes erratic winds and rapid fire growth. The Haines Index is used to measure the degree of atmospheric instability and dryness of the air over a fire. An index of 6 indicates that there is high potential for the type of fire growth that is observed in blow-ups. Another indicator in the South is the dispersion index which measures the process by which smoke mixes with the atmosphere and is carried away. A daytime dispersion index greater than 60 is an indication that wildfires will be difficult to control; values less than 30 will present smoke and visibility issues, but not erratic fire behavior. Dispersion index values in excess of 100 were reported during the 1998 fires in Florida.

Drought conditionshave a role in promoting extreme fire behavior by drying out and curing fuels. The Keetch-Byram drought index (KBDI) estimates the dryness and thus the flammability of the duff and soil horizons (Pyne et al. 1996). It takes into account the net effect over time of precipitation and evapotranspiration on the amount of moisture in these horizons. A value of 0 indicates that the duff and soil horizons are completely saturated with water while a value of 800 indicates that it would take eight inches of rain to lower the KBDI to zero (or saturate the soil). The index increases every day without rain and decreases when it rains. The amount it increases and decreases depends on the types of soils found in a region. Indirectly, the KBDI is also related to the gradual drying of large dead fuels, which contribute significantly to extreme fire behavior.

Air Force Bombing Range Fire

A well-documented wildfire exhibiting extreme fire behavior in the South was the Air Force Bombing Range Fire which occurred in eastern North Carolina on March 22, 1971 (Wade and Ward 1973). The fire crowned through 15,000 acres of pond pine with pocosin understory during the first 4 hours and a total of 29,300 acres in four days. It had an average rate of spread of 2 mph and a peak of 5 mph. Low relative humidity for four of the five days preceding the start of the fire cured the fuels. A second long run occurred the first night of the fire during the passage of a dry cold front, with wind gusts up to 40 mph and low relative humidity. The fire died down only when its flaming fronts ran into wet, marshy areas (Wade and Ward 1973).

1998 Florida Fires

Florida was affected by an unusually strong ENSO (El Nino-Southern Oscillation) in the year prior to the 1998 wildfires, which helps to account for their severity. ENSO is composed of three phases: El Nino, La Nina, and neutral. In Florida, the El Nino phase produces higher than normal levels of rainfall while the La Nina phase produces lower than normal rainfall. ENSO was in an El Nino phase during the winter of 1997. As a result of the increased rainfall, there was an abundance of new plant growth in many ecosystems. However, these plants dried up in the spring of 1998 when ENSO switched suddenly to a La Nina phase, creating a severe drought. During the 1998 Florida wildfires, the Keetch-Byram drought index (KBDI) reached an all-time high of 780. The curing of the new plant growth contributed to the already hazardous fuel loads found in many forests in 1998. Fuel loads over 10 tons/acre are considered hazardous (Wade 1989). With high fuel loads and droughty conditions, the state was susceptible to both arson and summer lightning storms. Approximately 500,000 acres in Florida burned in more than 2000 wildfires in May and June 1998 (Butty et al. 2001).