homogeneous nucleation

In this process, a water droplet or ice crystal forms without a condensation or freezing nucleus, respectively. For water droplet formation, a sufficient number of water vapor molecules spontaneously come together and condense to form a liquid droplet embryo of critical size such that the water droplet continues to grow in a saturated environment. For ice crystal formation, a sufficient number of liquid water molecules spontaneously aggregate inside a water droplet to form an ice embryo of critical size such that the entire droplet turns to ice. Homogeneous nucleation of water droplets does not occur in nature because a relative humidity of 200 percent is required. Homogeneous nucleation of ice crystals does, however, occur in nature and is more likely for lower air temperatures and smaller droplet volumes.