Any particle immersed within supercooled water, initiating the growth of an ice crystal to be compared with particles nucleating directly from the vapor phase (deposition nucleus). Similar particles m
ay 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 -2C; mineral particles (e.g., clays: kaolinite and montmorillonite) at -10 to -20C; artificial nuclei (e.g., silver iodide, lead iodide, and metaldehyde), as smoke, can be found to nucleate at intermediate temperatures, i.e., -5 to -10C. 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.
The changing of phase from water to Ice in soil or rock. The temperature at which ground freezing starts may be lower than 0C as a result of freezing-point depression through changes in the chemical c
omposition of the water or soil (e.g. the presence of salt).
(Also called apparent freezing point.) The temperature at which a liquid solidifies under any given set of conditions. It may or may not be the same as the melting point or the more rigidly defined tr
ue freezing point or (for water) ice point. It is not an equilibrium property of a substance; it applies to the liquid phase only. The freezing point is somewhat dependent upon the purity of the liquid; the volume and shape of the liquid mass; the availability of freezing nuclei; and the pressure acting upon the liquid. The freezing point is a colligative property of a solution and becomes proportionately lower with an increasing amount of dissolved matter. Therefore, since natural water almost invariably contains some solutes, its freezing point is found to be slightly below 0C. For example, bulk samples of normal seawater freeze at about -1.9C (28.6F).
The temperature at which a substance begins to solidify (see melting point). The freezing point of water is 273.15 K at an ambient pressure of 1013.25 hpa.
Any form of liquid precipitation that freezes upon impact with the ground or exposed objects, that is, freezing rain, freezing drizzle, or freezing fog.
The positive pressure developed at ice-water interfaces in a soil as it freezes (freezing of in situ pore water - 9% expansion). It is also known to result in a heaving pressure or frost-heave pressur
e that is responsible for the heaving of utilities, foundations and pavements. Ice-water interfaces occur at the contact between Ice lenses and the Frozen Fringe.