Relative humidity will roughly halve with each 20 F rise in temperature and double with each 20 F 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 areas with low fuel loading 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.
Don t burn on organic soils unless the water table is very close to the surface. Heading fires produce about three times more particulates 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.