Spontaneous condensation or freezing of water in the atmosphere in the absence of substances to act as nuclei. This is most likely in supercooled air below −40 °C.
Geography
The spontaneous freezing of water droplets at around −40 °C, as clusters of water molecules within a droplet settle, by chance, into lattice formation of ice, making the entire droplet freeze. See Easter and Peters (1994) J. Appl. Met. 33, 7.