Once a cloud extends to altitudes where the temperature is below 0 °C, ice crystals may form by homogeneous or heterogeneous ice nucleation, as will be discussed here. Heterogeneous ice nucleation takes place with the help of ice nucleating particles (INPs) and can occur via different pathways, including, for example immersion freezing and pore condensation and freezing, depending on the temperature and ice supersaturation. By contrast, homogeneous ice nucleation usually occurs without an INP at temperatures below −38 °C. Here, we discuss these different ice formation pathways and make use of the Classical Nucleation Theory and the Gibbs free energy, to discuss the energy barriers that are associated with these. Dependences of ice formation on environmental parameters, as well as INP properties, are discussed.

Once nucleated, the ice particles can grow by diffusion from the vapor phase, leading to crystals with different habits. This chapter further discusses ice particle growth by other mechanisms like aggregation and riming. Finally, we discuss ice multiplication (secondary ice production) mechanisms, including the Hallett–Mossop process, and melting of ice particles.