1. (Sometimes incorrectly called coagulation. ) In cloud physics, usually the growth of an ice hydrometeor by collision with supercooled cloud drops that freeze wholly or partially upon contact. May also refer to the collection of smaller ice particles. This has been called a form of agglomeration and is analogous to coalescence, in which liquid drops collect other liquid drops. See ice accretion; compare coagulation. 2. In cloud modeling, the collection of cloud drops by drizzle drops and raindrops. This nomenclature is used along with autoconversion and self collection to distinguish among three subprocesses, evident from numerical results, responsible for the growth of the drop-size distribution by the collision–coalescence process.