Nucleate:
Nucleation is the first step in the formation of either a new thermodynamic phase or a new structure via self-assembly or self-organization. Nucleation is typically defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appears. Nucleation is often found to be very sensitive to impurities in the system. Because of this, it is often important to distinguish between heterogeneous nucleation and homogeneous nucleation. Heterogeneous nucleation occurs at nucleation sites on surfaces in the system.[1] Homogeneous nucleation occurs away from a surface. [Wikipedia]
Noted in Dark Matter And The Dinosaurs, Lisa Randall, p 177:
Cosmic rays and supernovae as well as cosmic impacts have been suggested as possible culprits with the potential to be relevant on longer time scales. Cosmic rays can affect cloud cover in several ways. One is by ionizing atoms in the troposphere so that water droplets can nucleate nearby. This influence could enhance cloud formation, which would in turn affect the Earth’s weather. [typos mine]
