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Nucleus richness of cloudrainability of clouds*

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A. K. ROY

12A , M " J.i'" 1I"ldilf Street, Cfllrlll/f1-26(Recri"ed 19 J Illy 1.9( 8 )

Aru:iTRACT. Consklered thcorctleallv, ('ull\"c"(,1ivc domLt formed i ll highl.r polluted air And. M . nch. in air over -rh-hin ite condeesanon nuclei eontont, "n'; It~ Ita"l., to precipitate than ..tuulee clouds forming in uorma lly delLII air .Tb.ig Ilo.pooto{rolati\·omi nim'MII elfcloud... formed und er dilft'ring nucteus ete te of air has bcon (':taminetl by eomperetlve. tuny of preelpltet lon behe vioue of I hundr -e r.lullll::l tluring pre-tnou soou months over C'a.!r.ulta. grouped und er t hreeclassoa as dlsenssed in tho p...por.

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1. IntroducUon

In discussing infurmully~ amongst t hemselves,about certain anomalo us feat ures of rainfall inrelation to Borne specified meteorological sit uut ions,meteorologists sometimes speak of 'hard' and'goff zones, a'i they refer respect ively to areas,(a) where, after several clays of rainless and nearlyoloncUess weather, a well defined change in synopt iosit untion, favouring ra infall , occurs hut. despit edevelopment of adequate clouds of right. type,very lit tle or no rain falls and (b) where, f,,!Iowingact ive weather development and some rain ona day, precipitation activity continues un thegame, or eve n 0 11 a more pronounced scale on thenext 011e or two days , although not ing certain im­pro vement in thc broad meteorological s ituation insome of these cases, not much rain wn..~ expectedthere again.

Whilo no satisfactory explanation could begi\"'en to account for such anomalies in the preci­pitat iou behaviour of cloud. in the two situationsmentioned, on t he basis only of the macro-physical,or meteorologial, state of tho atmosphere, researchesin recent yoars on cloud and rain physics, givingdue consideration also to the associated micro­physical featu res of cloud. in relation to ra infonnation in them, have led to some plausible andinte resting' conclusions in th is regard, Thefinding which may be considered directly rele­vant to the featu re. under examination is thatover-abundance of nucleus population in cloud airtends to produce a negati ve cffect on rain forma­tion in clouds , moro particular ly in clouds of con­vectivo type.

Verification directly of the t rend, as above, ofrain activity of cloud. under differont micro­physical condi tions, but in nearly the same orvery similar moteorological situation, is not OlI8y,

because (i) necessa ry datu relati ng to nuclei con­centrnt ion arc ordinarily not avn iluhle, and aIMbecause (ii) it is not II simple matter to judgeobjectively the samenei.... or near ident ity of theli}> llopt ic situation in two instances. .A.II attemptha. accurdingly been made to estimate th e pro­bsbilities of rain occurrence and relat ive yield ofrain from clouds in different nucleus states of air,1\.. judged on the basis of certain indirect cr iteria,

2. Nucleus .sta.te of air and ItsinOuence on precipitation growthIn elouds-Evldence In support of tbe hypothesis

A cloud consisting of uniform snudl droplets isessent ially stable, and would not precipitateunless this unfavourable micro-phys ical state ofthe cloud could he modified suitably, by inducingformation in the cloud of a few droplets substan­t ially larger than tho average small ones, Whilethis first essent ial condition of a certain width ofthe spectrum of droplet sizes.for rain format ion ina cloud was postu lated by cloud physicists at thefirst phase On their enquiries int o and researches OIl

the subject, leading to the present attemptedtechniques of cloud modification by method.of cloud seeding, further studies in this field andactual measurements within clouds of dropletsizes and their concentration have since laid atres,•on yet another important condition of a suitablemiero-strncture ofc loud, namely, that the mediansizes of cloud droplets should be above a certainminimum limit . This is important because, to ensurequ ick enough rain drop growth during the ratherlimited life period of convective cloud, the collec­t ion efficiency of the relatively large cloud droplets ,on collision with average small ones, should bereasonably high. According to theoretical deter­minations of the collection efficiency valu es fordroplets of different sizes, it is now accepted gene­rally that the minimum limit of the avera gesize ofdroplets should be at least 7 microns radius. In this

.Rosoorohscheme on Cloud and RAinstorm Studies in Bengel and Al8am