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The "snowtorms" or "snowing" or "whitout" is caused by a special set of circumstances that all must exist for this event to occur. As already mentioned, the event is a precipitatiion of Calcium Carbonate ( CaCO3 ) from the water column. The solubility of CaCO3 in seawater is around 500 PPM at around 27 degrees C (80.6 F). When in the presence of Magnesium and a few other ions (and the corresponding sulfate ion), the concentration of Calcium can exceed its max solubility by a small percentage. As long as the conditions stay the same (temperature, pressure, ion speciation and concentration), the water column will stay at this "supersaturated" state. Any change in the conditions will cause the calcium to exceed its equilibrium rate between iionized/soluble and unionized/insoluble. Due to the low solubility of Calcium as the carbonate, and as the carbonate is the most reactive specie in seawater for calcium, a small increase in concentration (as when evaporation occurs) or a small increase in temperature, or if both of these are really close to the point of causing precipitation, a change in barometric presure will cause the supersaturated solution to find some starting point that initiates the formation of the precipitate, and the entire solution will precipitate out the amount of calcium carbonate necessary to bring the ionized solution back to equibrium with the solid unionized phase of CaCO3 under the existing conditions. Although sulfates and phosphates of Calcium could occur, they are not significant in the concentrations they exhibit in seawater, as the sulfate salt is quite soluble in comparison to the carbonate, and the phosphate levels are not high enough to precipitate out
at its normal concentration in seawater
This is the quick and dirty model, I took a few liberties in describing the phenomenon. Interestingly enough, most calcium salts exhibit the peculiar solubility trait in that they are more soluble in cold solutions than they are in warm ones. Save that for your next trivial persuits game...
