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<br />Other atmospheric particles are known as ice nuclei (IN) , <br />particles upon which, if found in condensed water droplets, enhance <br />droplet freezing. Ice crystals also may form directly from water <br />vapor upon ice nuclei. Despite the atmospheric abundance of CCN, <br />there is a relative scarcity of IN particles. <br /> <br />Clquds are made up of unfrozen water droplets, ice crystals or <br />a combination of both. Within a convective cloud having a portion <br />colder than freezing, some of those water droplets below freezing <br />remain in a liquid state, or "supercooled". They create a condition <br />in which both unfrozen water droplets and ice crystals co-exist. <br />This supercooled cloud volume is critlcal to tHe formation of rain <br />and hail. Supercooled wat.er can remain unfrozen to -40 C (~ E) . <br />before spontaneously changing to ice. When spontaneous freezing <br />occurs, it is termed homogeneous nucleation. <br /> <br />Those supercooled water droplets containing ice nuclei freeze <br />first. The speed at which supercooled water droplets convert into <br />ice crystals increases as cloud temperatures decrease as the clouds <br />grow higher. The process of vapor deposition starts to have a <br />significant effect within clouds when ice crystals and supercooled <br />water exist in the same medium. Surface pressures over ice crystals <br />are lower than those over water droplets creating a pressure <br />gradient between them. This gradient causes liquid to flow from the <br />droplets to the ice crystals thereby growing at the expense of the <br />droplets. Once ice crystals develop, they continue growing rapidly <br />by using up surrounding water vapor and cloud water from nearby <br />water droplets. Continuous unequal movements of water droplets and <br />ice particles inside convective clouds also insure ice and water <br />droplet collisions. The collisions promote the processes of <br />coalescence, accretion and aggregation. Coalescence is a process in <br />which the unfrozen water droplets collect other water droplets by <br />impact, with freezing occurring after the impact. Accretion, or <br />riming, 'occurs when droplets freeze. upon impact with cloud ice <br />particles. Aggregation is the process in which ice particles collect <br />other ice particles. In advanced stages of cloud growth, ice <br />particles will shatter, coalesce, grow larger and repeti ti vely <br />collide in a complex manner through the processes just mentioned. <br />When the various sizes of ice particles eventually fallout of the <br />cloud and drop below the freezing level, they begin melting. If <br />melting is not complete, hail, graupel or snow is the precipitation <br />instead of rainfall. <br /> <br />Sizes, concentrations, chemical and the electrical properties <br />of all nuclei present in the atmosphere combine in important ways <br />to determine how efficiently a cloud system can produce <br />precipitation. Although there are massive amounts of water vapor <br />in the atmosphere at any time, precipitation won't occur if certain <br />conditions required for the formation of precipitation are absent. <br /> <br />Two cloud types produce all precipitation: "warm clouds" and <br />"cold clouds". <br /> <br />4 <br />