ULPBA partners with John Weir and his associates at OSU, the Oklahoma Prescribed Burn Association and the Great Plains Fire Science Exchange as they gather and compile data from prescribed burns across several states.  Weir is the superintendent of the Oklahoma State University Research Range and prescribed burning instructor in the Rangeland Ecology and Management program, Plant and Soil Science Department, Oklahoma State University, Stillwater, OK.   

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"If we can narrow spot fire causes down to a single main weather factor, burn bosses might focus on that variable, possibly reducing the chance of spot fires."     John Weir   

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PROBABILITY OF SPOT FIRES DURING PRESCRIBED BURNS
By John R. Weir

Published in Fire Management Today

Spot fires have always been a problem on prescribed burns.  Just the possibility of a spot fire can cause mental and physical stress on burn bosses and crews. Actual spot fires can cause personal injury or even loss of life, as well as costly damages and loss of public support for prescribed fire programs.

Many private and public land managers in Oklahoma have told me that they avoid prescribed burning for fear of spot fires and escaped fires. Many have the resources needed to conduct prescribed fires, but lack the experience or knowledge to deal with spot fires. A simple guideline or rule-of-thumb might help.

 

Variables Affecting Fire Behavior
Weather factors are the main variables that burn bosses can use to predict and monitor prescribed fire behavior.  In general, there are three main weather factors:
•  Relative humidity.

Burning when relative humidity exceeds 40 percent significantly slows rates of spread (Lindenmuth and Davis 1973) and reduces danger from firebrands (Green 1977).
•  Temperature.

Bunting and Wright (1974) found that danger from firebrands was lower if the ambient air temperature is below 60 ºF (15 ºC) when burning.
•   Windspeed.

Windspeeds of at least 8 miles (13 km) per hour are needed to ignite and burn standing fuels (Britton and Wright 1971). However, wind-speeds of more than 20 miles (32 km) per hour can create problems with firebrands and other blowing debris (Wright and Bailey 1982).
If we can narrow spot fire causes down to a single main weather factor, burn bosses might focus on that variable, possibly reducing the chance of spot fires.
Key Variable
At the Oklahoma State University Research Range (OSURR), we conduct prescribed burns during different seasons all over Oklahoma. Fuels include tallgrass prairie (NFES fuel models 1 and 3—see Anderson 1982), post oak–blackjack oak (fuel models 3, 8, and 9), eroded mixed prairie (fuel models 1 and 3), sandsage grassland (fuel model 4), and oak–pine (fuel models 3, 8, 9, and 11). Since 1996, we have been keeping track of spot fires on our prescribed burns. We consider a spot fire to be any fire outside the burn unit, no matter what the size or cause.

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. . . Read Weir's complete article, which includes the 40 Per Cent Threshold and Spotfire Probability