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Recent moisture effects on wildfires

March 11, 2013

Recent rains may little affect on wildfires in Oklahoma.

STILLWATER, Okla. – About this time every year, Oklahomans watch thousands upon thousands of acres of land burn up in wildfires. However, this fire season has been better than usual due to the low grassy fuel loads caused by drought during last year’s growing season.
While we have seen some recent moisture across much of the state, how big of an effect does it have on wildfire this time of year?
Not as much as people might think, 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 (http://okfire.mesonet.org).
“Most Oklahoma fuels this time of year consist of dead grass and respond quickly to hour-to-hour weather changes,” said Carlson. “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 surface. Live fuels include any vegetation that has any degree of greenness, which is essentially limited this time of year to evergreens, such as eastern redcedar.
The moisture content of dead fuels is controlled exclusively by hour-to-hour weather changes. Dead fuels are further categorized into four classes according to their diameters: 1-hour, 10-hour, 100-hour and 1,000-hour fuels.
The dead grasses and other dead fuels that now cover much of Oklahoma, falling in the 1-hour and 10-hour classes, are the state’s primary fuel before spring greenup. 
“The moisture content of 1-hour fuels, like dead grasses, is very responsive to any little moisture peak 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.”
The recent moisture experienced by the state over the last 30 days has helped the moisture levels of the larger 100- and 1000-hour dead fuels, as well as the live woody vegetation out there such as eastern redcedar.  However, it has had minimal effect on the 1- 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 appropriate weather conditions and will remain so until spring greenup occurs and the live green grasses serve to mitigate the fire danger,” Carlson said.

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