Rules of Thumb
United States Department of Agriculture, Forest Service Southern Region, February 1989; Technical Publication R8-TP 11.
- Obtain and use latest weather and smoke management forecasts.
- Relative humidity will roughly halve with each 20oF 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 tress 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.
- Don't 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 1/2 mile with 1 1/2 miles of the fire.
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