Amazon forest fires produce large amounts of smoke particles that allow more surfaces for water vapor condensation in tropical clouds. With more droplets forming, each droplet accumulates less water, so the resulting droplet size is reduced (Andreae et al., 2004). Smaller droplets are carried higher into the troposphere or even lower stratosphere before either evaporating and losing latent heat or growing to precipitation size. Evaporating droplets in the stratosphere release both water vapor and smoke particles at a level that significantly extends residence times and impacts the regional and global radiation balance (absorption of solar radiation by particles, absorption of longwave radiation by water vapor). Droplets that do grow to precipitation size do so at higher levels and lead to more intense storms, some including large hail.
Fires in the Amazon region, largely a result of deforestation and associated agricultural practices, may be having a significant impact on local and regional, and perhaps global, weather and climate.
Reference
Andreae, M.O., D. Rosenfeld, P. Artaxo, A.A. Costa, G.P. Frank, K.M. Longo, and M.A.F. Silva-Dias, 2004: Smoking rain clouds over the Amazon. Science, 303, 1337-1342.