homogeneous ice nucleation

Homogeneous ice nucleation is the spontaneous formation of ice crystals from water vapor without the influence of foreign particles. This process requires significant supersaturation or supercooling, meaning the air must contain much more water vapor than normal, or the temperature must be well below the freezing point.12

In the atmosphere, this typically occurs at temperatures at or below -39°C (-38°F). Unlike heterogeneous nucleation, which uses aerosols like dust as a starting point, homogeneous nucleation relies on water molecules randomly arranging themselves into a crystalline structure. This is a less common mechanism for cloud formation compared to heterogeneous nucleation, which is facilitated by condensation nuclei.21