Prescribed Burning Rules of Thumb

Below, the most important guidelines from planning, preparing, executing, and evaluating a prescribed burn are summarized.

• Obtain and use latest weather and smoke management forecasts.
• Relative humidity will roughly halve with each 20^oF rise in temperature and double with each 20^oF drop in temperature in a given air mass.
• Expect increased spotting when relative humidity drops below 30 percent. Do not burn when the relative humidity is below 25 percent.
• Burn when mixing height is above 1,650 feet [500 meters].
• Do not burn under temperature inversions.
• Burn areas with low fuel loadings and large-sized trees on marginal days at the high end of the prescription window.
• Never underburn during a drought. Soil moisture is needed to protect tree roots and lower litter.
• Dont burn on organic soils unless the water table is very close to the surface.
• Heading fires produce about three times more particulate than backing fires.
• Burn when fuels are dry, but not too dry. Wet fuels produce substantially more particulate than do dry fuels.
• Start burning logging debris by midmorning.
• Site prep burning behind chopping or other mechanical treatment gives best results if done 10 to 15 days after treatment.
• Windrows are the most polluting of all southern fuel types.
• Broadcast burn scattered debris if possible.
• Do not pile when either ground or debris is wet.
• Dirt in piled debris will increase the amount of smoke produced by up to four times. Shake out dirt while piling; "bump" piles while burning, and repile as necessary.
• Use a smoke management plan. Consider smoke sensitive areas. Look several miles downwind and down-drainage for potential targets.
• Estimate background smoke concentration [micrograms per cubic meter] in the absence of high humidities by dividing 500 by the visibility in miles.
• If nighttime Dispersion Index forecast is poor or very poor [less than 13], stop burning by 3 p.m. ST.
• Doubling the Dispersion Index implies a doubling of the atmospheric capacity to disperse smoke within a 1,000 square mile area.
• Assuming 1 ton of fuel per acre is being consumed by smoldering combustion during poor nighttime dispersion conditions, expect visibility in the smoke to be less than a half mile within 11/2 miles of the fire.