Using relative humidity to predict spotfire probability on prescribed burns

Abstract

Spotfires have and always will be a problem that burn bosses and fire crews will have to contend with on prescribed burns. Weather factors (temperature, wind speed and relative humidity) are the main variables burn bosses can use to predict and monitor prescribed fire behavior. At the Oklahoma State University Research Range, prescribed burns are conducted during different seasons of the year and in different fuel types all over the state of Oklahoma. Since 1996 records have been kept to track spotfires that occur on these prescribed burns. Most of these spotfires were due to firebrands caused by crowning eastern redcedar (Juniperus virginiana). Leaves from oak (Quercus spp.) trees or tallgrasses floating or blowing across the fireline and smoke or fire whirls were responsible for most of the other spotfires. When burn data were reviewed, one weather variable stood out as the main cause of spotfires, low relative humidity. Of the 99 burns conducted there were 21 spotfires. From the data, the probability of a spotfire occurring on any prescribed burn with the relative humidity between 20 and 80 percent was determined to be 21.2 percent or approximately one out of five burns. Looking at the 40 percent relative humidity threshold, there is 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. 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 and equipment needed. Most of all, inexperienced burn bosses can use this to help reduce risk (liability) and increase safety for their crews.

Parent Publication

• Proceedings: Shrubland dynamics -- fire and water

Citation

Weir, John R. 2007. Using relative humidity to predict spotfire probability on prescribed burns. In: Sosebee, Ronald E.; Wester, David B.; Britton, Carlton M.; McArthur, E. Durant; Kitchen, Stanley G., comps. Proceedings: Shrubland dynamics -- fire and water; 2004 August 10-12; Lubbock, TX. Proceedings RMRS-P-47. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. p. 69-72.