Similar particles may nucleate at somewhat different temperatures (a few degrees) depending on the process. Observations of natural freezing nuclei indicate that there is normally present in the atmosphere
a large variety of such particles with varying activation temperatures (temperatures at which they become effective nucleators). Certain bacteria from vegetation (pseudomonas syringae) nucleate ice
at temperatures as high as -2°C; mineral particles (e.g., clays: kaolinite and montmorillonite) at -10° to -20°C; artificial nuclei (e.g., silver iodide
, lead iodide, and metaldehyde), as smoke
, can be found to nucleate at intermediate temperatures, i.e., -5° to -10°C. The origin, distribution, and composition of these particles is highly variable; some are composed of a mixture with a hygroscopic
component that dilutes prior to nucleation
of the water by the freezing nucleus.
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