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<br />\ ' <br /> <br />There are other special kinds of atmospheric particles known <br />as ice nuclei (IN)---particles upon which, if found in condensed <br />water droplets, enhance droplet freezin~. Ice crystals also may <br />form directly from water vapor upon J.ce nuclei. _ Despite the <br />abundance of condensation nuclei found in the atmosphere, there is <br />a relative scarcity of ice nuclei by comparison. <br /> <br />I <br />I <br /> <br />Clouds can be made up of unfrozen water droplets or ice <br />crystals or a combination of them. Within a cloud that has some <br />part of it colder than freezing (32 F, or 0 C), some of the water <br />droplets remain in a liquid state and are termed "supercooled", a <br />condition in which both unfrozen water droplets and ice crystals <br />are found existing together. This supercooled cloud volume is <br />critical to the formation of rain and hail. Supercooled water can <br />remain unfrozen as low as -40C (or -40 F) before spontaneously <br />changing to ice; when freezing happens spontaneously, it is called <br />homogeneous nucleation. <br /> <br />I <br />I <br />I <br />I <br /> <br />Those supercooled water droplets containing ice nuclei are <br />first to begin freezing. The speed that supercooled water droplets <br />convert into ice crystals generally increases as cloud temperatures <br />decrease during cloud growth. An important process called vapor <br />deposition starts to have a significant effect within clouds when <br />ice crystals and supercooled water exist in the same medium. <br />Surface pressures over ice crystals are lower than those over water <br />droplets creating a pressure gradient between them. This gradient <br />causes water molecules to flow from the droplets to the crystals <br />and ice crystals to grow larger at the expense of the droplets. <br />Eventually, the supercooled water is used up as crystals grow <br />larger. Similarly, in a supercooled cloud environment, once ice <br />crystals develop they continue growing rapidly using up surrounding <br />water vapor and cloud water from nearby water droplets. Once ice <br />particles develop inside growing clouds the continuous unequal <br />movement of water droplets and ice particles also insure ice- <br />droplet collisions. These collisions create a chain reaction <br />process known as coalescence, a process in which the unfrozen water <br />droplets collect other water droplets by impact and freezing or by <br />a second process known as accretion---one in which droplets grow <br />rapidly and freeze upon impact with cloud ice particles. Ice <br />particles then may shatter, grow ,larger and repetitively collide in <br />a complex manner. When the various sizes of ice particles <br />eventually fall from the cloud below the freezing level, they begin <br />melting. If melting is not complete, hail, graupel or snow results <br />instead of rainfall. <br /> <br />Sizes, concentrations, and chemical properties of all nuclei <br />present in the atmosphere combine in important ways to determine <br />how efficiently a cloud system can produce precipitation. Al though <br />there are massive amounts of water vapor in the atmosphere at any <br />time, precipitation won't occur if certain conditions required for <br />the formation of precipitation are absent. <br /> <br />6 <br />