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.