FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

Managing smoke from wild or prescribed fires requires a daily prediction
of smoke accumulations and whether they will reach unacceptable
levels. Choice of suppression strategies and tactics must include a
consideration of smoke effects on safety and visibility.
a. Avoidance. Avoidance is a strategy that considers meteorological
conditions when scheduling burns to avoid incursions of smoke into
sensitive areas. Burning should occur on days when weather conditions
allow the transport of smoke away from populated areas. Smoke may
not be such a limiting factor in sparsely populated areas, but any
downwind effects should be considered when burning. The wind
direction during both the active burning period (flaming stage) and the
smoldering period must be considered. At night, downslope winds can
carry smoke toward smoke sensitive areas, or allow valley bottoms to fill
with smoke. Residual smoke emitted during the smoldering stage is
especially critical.
b. Dilution. The dilution strategy controls the amount of emissions or
schedules the rate of burning to limit the concentration of smoke in
sensitive areas. The concentration of smoke can be reduced by diluting
it through a greater volume of air, either by scheduling during good
dispersion periods or burning at slower rates (burning narrow strips or
smaller areas). However, burning at a slower rate may mean that
burning continues into the late afternoon or evening when atmospheric
conditions may become more stable. Burn when weather systems are
unstable, but not at extremes of instability. The time of day at which
ignition occurs is also important. Consider early morning ignitions to
take advantage of weather conditions where improved mixing will occur
as atmospheric heating takes place. Avoid days with low morning
transport wind speed, less than 4 miles per hour (6.5 km/hr). Use firing
methods to rapidly build a smoke column to vent smoke up to the
transport wind and larger volumes of air. Using mass-ignition or rapid
ignition will loft the column up and away from the unit, allowing for better
dispersion and reduced emissions during the smoldering phase.
Generally, a burn early in the day encounters improving ventilation; an
evening burn encounters deteriorating ventilation.
c. Emission reduction. Emission reduction is an effective control
strategy for decreasing the amount of regional haze and avoiding
smoke intrusions into Designated Areas (DA's) (Sandberg et al. 1985).
It reduces the smoke output per unit area, and is a concept applicable
in both forest and rangeland areas. Most emission reduction techniques