The addition of air particles to hydrated drops (snow, rain, sleet, etc.) by coagulation as the drops fall through the sky.
[Air Pollutants and Their Effects on the Terrestrial Ecosystem; v18 - Advances in Environmental Science and Technology Series; Hidy, ed.; pages 239, 240, 243; 1986; Wiley; New York.] [Journal of Geology; v102n3; pages 283-296; 1994.]
Source: Atmospheric Chemistry Glossary