Not only are aerosols needed for clouds and liquid precipitation to form, they are also required for the formation of ice. Some of the ice that forms in a cloud starts out as liquid and then freezes. Much of the ice, such as snow, forms when water vapor deposits directly onto aerosols. Clouds that are saturated with respect to a plane surface of water are supersaturated with respect to a plane surface of ice. This is because the concentration of water vapor on a surface of ice would be less than the concentration on a surface of water. But, as in the case of liquid droplets, freezing without the appropriate nuclei present rarely occurs. For this reason, water vapor and water droplets can exist at sub-freezing temperatures as cold as -40 degrees Celsius (-40 C is equal to -40 F). Not until reaching this temperature will either water droplets or water vapor freeze to other water vapor molecules in the absence of the right kind of nuclei. Water droplets that exist at sub-freezing temperatures are called supercooled droplets. The nuclei that initiate freezing by contact with water droplets or water vapor are appropriately called ice forming nuclei or IFN. Ironically, most of those aerosols that are good ice forming nuclei generally are not good cloud condensation nuclei or CCN. Ice forming nuclei are unique from most other nuclei in that they all have a six-sided structure. Like the ice crystals that form in their presence, they are hexagonal. Freezing, due to contact with IFN, can begin at temperatures as "warm " as -4 degrees Celsius (24.8 F).
Unlike CCN, which are numerous in the atmosphere, IFN are very scarce. The table below makes an interesting comparison of typical atmospheric concentrations of CN, CCN, and IFN. Common IFN include clay minerals such as kaolinite and vermiculite, volcanic ash, bacteria in leaf mold, and some pure substances such as silver iodide (AgI), lead iodide (PbI2), and ice itself. Incidently, the latter three have been used to seed cold clouds in order to reduce the size of hail stones the cloud will produce. By increasing the number of IFN in the cloud, the hailstones are more numerous but smaller.
| Nuclei Type | Average Atmospheric Concentration |
CN | 10,000 - 30,000 /cm3 | CCN | 1,000 - 5,000 /cm3 | IFN | 0.1 - 1.0 /liter |
|---|
By now, you may be wondering how we ever get so much frozen precipitation if there are so few ice forming nuclei. Well, there are several theories, one of which is the ice multiplication process. When water freezes, it expands. You may have noticed this phenomena when making ice cubes in the freezer. When a water droplet makes contact with a IFN, it freezes on the outside first. The expansion creates a build up of pressure on the inside and the partially frozen droplet explodes, splintering off tiny ice particles which can serve as additional IFN. Thus, as this process continues, it multiplies the number of IFN present.
The Shodor
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