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<br />~ <br /> <br />J <br /> <br />Supercooled water droplets containing ice nuclei begin freezing <br />first. The speed of droplets converting ,from supercooled water into <br />ice generally increases as cloud temperatures decrease <clouds grow in <br />height) and the physical properties of ice and water start interact- <br />ing. An important process known as vapor deposition begins having a <br />significant effect within clouds when ice crystals and supercooled <br />water exist in the same environmental conditions, Since surface <br />.pressures over ice crystals are lower than those over water droplets, <br />the pressure gradient between them causes water molecules to flow from <br />the 'droplets to the ice crystals. Ice crystals then grow larger at the <br />expense of the water droplets, eventually using up all the supercooled <br />water and producing larger ice crystals. Likewise, after ice crystals <br />develop within a supercooled cloud they continue growing rapidly using <br />up surrounding water vapor and cloud water from nearby supercooled <br />water droplets, Inside growing clouds the unequal but continuous <br />movement of water droplets and ice particles insure inevitable <br />collisions. These collisions soon create a sort of chain reaction <br />process, called coalescence, in which the unfrozen water droplets <br />collect other water droplets and eventually freeze or they can grow <br />rapidly and freeze upon impact with cloud ice particles in a process <br />called accretion. The ice particles may also shatter, grow larger and <br />again collide in a complex repeating manner. Eventually, the various <br />sizes of ice particles fallout of cloud suspension, warm up and melt <br />while falling to the ground. If melting isn't complete, hail, graupel <br />or snow results instead of rainfall. One idealized conception of how <br />the complex precipitation process evolves is shown in the flow-chart <br />in Fig, 2. <br /> <br />COlli,iOn <br />breakup <br /> <br />FREEZING NUCLEI <br /> <br />I <br />WARM AAI~Htltroqtntou' <br />r r, t t J 1I\l~ <br />I . <br />COllection FROZEN DROPS <br />Condln10lion I <br />I ACcrtlion--, <br />I 51 <br />DRIZZLE GRAUPEL ' <br /> <br />DEPOSITION HUCLEI. <br />t <br />Nucltolion <br />Dtpo,il,on <br />. <br />o"o.;,J CRY5TT <br /> <br />Aq9rtqollol <br />AcertHon <br />I <br /> <br />, <br />.wt=1 <br />HAIL SNOW <br />1 I <br />_._............_.......n_. Me III "9-.---.---........ <br />1 <br />Recirculation <br /> <br />Collecllon <br />coo'rnoo <br /> <br /> <br />BROAD SPECTRUM <br />1 <br />Corllhn1oHon <br />CollHllon <br />I <br />CLOVO BASE SPECTRUM <br />..................."f......................._........................... ...-.....-.......-............cloud bOI' <br /> <br />coo'T'noo j j j <br /> <br />CeN. VAPOR GRAUPEL HAIL COLD RAIN <br /> <br />WARM RAIN <br /> <br />The Precipitation Process <br /> <br />(Braham) <br /> <br />Fig. 2 <br />