Answer:
Tiny particles in air
Explanation:
Tiny particles in air act as condensation nuclei. The condensation nuclei usually have size of 0.2 µm. The continous accumulation of condensation nuclei results in the formation of clouds. When abundantly present in the air, water droplets condense on the surface of these particles and leads to rainfall. For example, carbon aerosols can also act as condensation nuclei and this is the reason that cities with high emission of carbon aerosols occasionally see clouds formation and rainfall.
