New particle formation (NPF) in fire smoke is thought to be unlikely due to large condensation and coagulation sinks that scavenge molecular clusters. We analyze aircraft measurements over the Amazon and find that fires significantly enhance NPF and ultrafine particle (UFP < 50 nm diameter) numbers compared to background conditions, contrary to previous understanding. We identify that the nucleation of dimethylamine with sulfuric acid, which is aided by the formation of extremely low volatility organics in biomass-burning smoke, can overcome the large condensation and coagulation sinks and explain aircraft observations. We show that freshly formed clusters rapidly grow to UFP sizes through biomass-burning secondary organic aerosol formation, leading to a 10-fold increase in UFP number concentrations. We find a contrasting effect of UFPs on deep convective clouds compared to the larger particles from primary emissions for the case investigated here. UFPs intensify the deep convective clouds and precipitation due to increased condensational heating, while larger particles delay and reduce precipitation. Display omitted •Aircraft measurements in vegetation fires show abundant ultrafine particles (UFPs)•Previous model formulations greatly underpredict the observed fire UFPs•Nucleation of dimethylamines from fires with sulfuric acid explains observed UFPs•UFPs from fires intensify deep convective clouds and precipitation Fine particles in wildfire smoke can lower air quality and harm human health. Smoke can also influence weather and climate by modifying cloud formation and changing how much of the sun’s energy is reflected or absorbed by the atmosphere. Compared to larger particles directly emitted from fires, the formation and presence of ultrafine particles (UFPs) have previously been overlooked, as it was thought that they were quickly “scavenged” by the larger particles. However, we found that UFPs were abundant in aircraft measurements of smoke from vegetation fires in the Amazon, and their formation and survival were favored. Furthermore, high-resolution modeling showed that these UFPs may intensify cloud convection and heavy rain. This research deepens our understanding of how vegetation fires impact weather and climate change. Ultrafine particles (UFPs) in biomass-burning smoke are formed by efficient nucleation mechanisms, including dimethylamines and sulfuric acid, and nucleated clusters grow by the condensation of oxidized organic vapors. UFPs may cause a stronger storm with a larger anvil and heavier rain, while larger particles directly emitted by fires delay and suppress rain.