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Moisture doesn’t have as much of an effect on wildfires during the dormant season as people might hope, according to J. D. Carlson, fire meteorologist in the department of biosystems and agricultural engineering at Oklahoma State University, and program director of OK-FIRE


Oklahoma has two seasons for wildfire: the dormant season, which is generally November through April, depending on geographical location within the state; and the growing season, generally May through October. 


Due to the predominance of dead fuels during the dormant season as well as weather conditions conducive to wildfire, those months usually see the greatest wildfire activity, both in terms of numbers of fires and acres burned. Historically, March is the peak month for wildfire activity, followed by April.


“Most Oklahoma fuels that time of year consist of dead grass, and they respond quickly to hour-to-hour weather changes,” Carlson said. “It’s not the amount of rain that matters, it’s the hour-to-hour relative humidity and wind speeds.”


There are two types of fuels available to wildfires: dead and live. Examples of dead fuels are dead leaves, dead grasses and dead wood on the ground. Live fuels about March include vegetation that has any degree of greenness such as evergreen eastern redcedars or cool-season grasses.


The moisture content of dead fuels is controlled by hourly weather changes. Dead fuels are further categorized into four classes according to their diameters: one-hour, 10-hour, 100-hour and 1,000-hour fuels. The dead grasses and other dead fuels that cover much of Oklahoma during the dormant season are the state’s primary fuel before spring green-up.


“The moisture content of one-hour fuels, like dead grasses, is very responsive to any wet or dry period. So when it rains or the relative humidity rises, their moisture goes sky high, but also can quickly dry out,” Carlson said. “That’s because they are so thin and can respond to the changing weather conditions.”


Moisture during the dormant season can help increase the moisture levels of the larger 100- and 1,000-hour dead fuels, as well as the live woody vegetation such as eastern redcedar.  However, it has minimal effect on the one- and 10-hour dead fuels, which are critical to the start and maintenance of wildfires.


“Because these smaller-diameter fuels can dry out quickly, fire danger can be high during conducive weather conditions and will remain so until spring green-up occurs and live green grasses serve to mitigate the fire danger,” Carlson said.





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