5th Symposium on Fire and Forest Meteorology and the 2nd International Wildland Fire Ecology and Fire Management Congress

Wednesday, 19 November 2003: 4:00 PM
Probability of Spotfires During Prescribed Burns
John R. Weir, Oklahoma State University, Stillwater, OK
Spotfires have and always will be a problem that burn bosses and fire crews will have too contend with on prescribed burns. Spotfires can cause mental and physical stress on burn bosses and crews if they occur or not. If a spotfire does occur it can then cause personal injury or even loss of life, as well as, the loss of public support and monetary damages. From interviewing many private and public land managers in Oklahoma, spotfires or risk of escape (liability) is the main reason many of them do not conduct prescribed fires. Weather factors (temperature, wind speed and relative humidity) are the main variables burn bosses can use to predict and monitor prescribed fire behavior. Burning above 40 percent relative humidity has been shown to slow rates of spread significantly (Lindenmuth and Davis 1973), and reduce danger from firebrands (Green 1977). Bunting and Wright (1974) determined that danger from firebrands was lower if the ambient air temperature was below 60F (15C) when burning. Wind speeds of 8 mi/h (13km/h) are needed to ignite and burn standing fuels (Britton and Wright 1971), but with winds over 20 mi/h (32km/h) firebrands and other debris become problems (Wright and Bailey 1982). Since 1996 we have been keeping track of spotfires that occur on our prescribed burns. A spotfire is considered any fire that occurs outside the burn unit no matter what size it is or what caused it. Most of these spotfires were due to firebrands caused by crowning eastern redcedar (Juniperus virginiana). While leaves from oak trees or tallgrasses floating or blowing across the fireline and smoke or fire whirls were responsible for most of the other spotfires. Of the 99 burns conducted there were 21 spotfires. Most all of our spotfires occurred at or below 40 percent relative humidity. If you look at the 40 percent relative humidity threshold, from this data there was a probability of 41.3 percent for a spotfire occurring below 40 percent and 3.8 percent above 40 percent. There also appears to be another threshold at less than 25 percent relative humidity. At this point there is a 100 percent probability of a spotfire occurring. So below 25 percent relative humidity burn bosses should be prepared, because from the data there is a 100 percent probability of a spotfire occurring. With this information burn bosses can determine spotfire potential when considering burn units or burn days. It can also assist them when considering crew size, equipment needed and possibly reduce anxiety when burning below 40 percent relative humidity. Most of all inexperienced burn bosses can use this to help reduce risk (liability) and increase safety for their crews.

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