How aerosol particles act as nuclei for ice and other crystalline phases like hydrates, and how this can quantitatively be described in models, are among the major open questions in atmospheric cloud and climate research. From an ice-microphysical perspective, atmospheric clouds may be subdivided into the following classes, according to their cloud top temperature or region of occurence:

• warm clouds (above 0°C)
• mixed-phase clouds (0°C to -35°C)
• cirrus clouds (below -35°C)
• polar stratospheric clouds (PSCs)
• noctilucent clouds (NLCs).

Except the warm clouds which involve only the gaseous and liquid water phases, these clouds are influenced by a variety of ice nucleation modes induced by soluble and insoluble aerosol particles of different origin and nature.