Microscopic atmospheric particles that attract water droplets, that may then coalesce to form a raindrop. Condensation nuclei come from pollen, salt from sea spray, dust from volcanic eruptions and soil erosion, and particulate air pollution (C. Park 2001). Many small condensation nuclei cause many small droplets to remain liquid well below normal freezing point, and small ice nuclei may increase the rainfall rate from single clouds, which is the principle of cloud seeding (Graf (2004) Science 303, 5662).