Airborne measurements of cloud condensation nuclei (CCN) were made in the vicinity of a planned burn of natural forest products. The concentrations of CCN, active at 0.5% supersaturation, reached a peak value of 5000 cm−3 about 38 km downwind of the fire, compared to concentrations of 250–500 cm−3 upwind. It is estimated that 6 × 1010 CCN were produced per gram of wood consumed.On another occasion the microstructure of small cumulus clouds were measured in the vicinity of a natural forest fire. The droplet size distributions were narrower and the concentrations of droplets higher in two clouds situated in the plume from the fire than in two clouds upwind of the plume. Abstract Airborne measurements of cloud condensation nuclei (CCN) were made in the vicinity of a planned burn of natural forest products. The concentrations of CCN, active at 0.5% supersaturation, reached a peak value of 5000 cm−3 about 38 km downwind of the fire, compared to concentrations of 250–500 cm−3 upwind. It is estimated that 6 × 1010 CCN were produced per gram of wood consumed.On another occasion the microstructure of small cumulus clouds were measured in the vicinity of a natural forest fire. The droplet size distributions were narrower and the concentrations of droplets higher in two clouds situated in the plume from the fire than in two clouds upwind of the plume.