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<br />, '1'. <br /> <br />VII. EXPERIMENTAL UNIT DECLARATION CRITERIA <br /> <br />For the purpose of randomization, the experimental units are classi- <br />fied into two categories of cumulus congest us clouds, depending on <br />the likelihood that the clouds will precipitate naturally. The first <br />category, Class A clouds, are those clouds which are expected to have <br />a low probability of precipitating naturally. They constitute the <br />experimental units for the primary randomized experiment for which <br />physical hypotheses specifying each step in the chain of events <br />leading to the altered precipitation are being stated in advance. It <br />is anticipated that seeding will result in an increase in both the <br />probability and amount of precipitation from the Class A clouds. The <br />second category, Class B clouds, are those which are considered to <br />have a relatively high probability of precipitating naturally. The <br />experiment on Class B clouds will be a more elementay'y exploratory <br />experiment. There will be no a priori statement of hypotheses for <br />these clouds, although it is felt that the physical hypotheses for <br />the Class A clouds are probably applicable. There is insufficient <br />confidence in these or any other hypothesis for predicting an increase <br />in rainfall from a cloud that is already raining or will soon rain <br />naturally. Randomization will be used primarily to minimize bias in <br />cloud selection and to gain confidence in the a posteriori statistical <br />and physical evaluations. <br /> <br />The Class A clouds are further divided into two subclasses, each <br />subject to a separate randomization. The first subclass, Class A-I <br />clouds, are small cumulus congestus for which the largest difference <br />between natural and seede!d clouds should occur. An attempt will be <br />made to select clouds with sufficient liquid water and lifetime for <br />precipitation formation, yet with insufficient ice. Ice does not <br />usually appear in signifjicant amounts in these clouds until their <br />tops are colder than -10 .C. The second subclass, Class A-2 clouds, <br />is a special subset of cumulus congestus clouds that often occur <br />after the passage of a Canadian cold front, Pacific cold front, or <br />strong surface low. These clouds appear to be more thermodynamically <br />compl ex. Secondary ice-~}enerat ion processes by Ha 11 ett-Mossop ice <br />multiplication have been found to exist in these clouds. The physical <br />hypotheses are, however, expected to be applicable to these clouds <br />also. <br /> <br />" <br /> <br />The experimental units for the three randomized experiments (cloud <br />Classes A-I, A-2, and B) will first be selected on the basis of <br />forecasts and airborne visual appearance. Final selection will, <br />however, be based on the result of key measurements made by the <br />primary cloud physics aircraft (University of Wyoming King Air 200) <br />during a pretreatment pass through the cloud. These measurements <br />will be evaluated in flight by the onboard computer and compared <br />against the following cy'iteria to determine whether the subject cloud <br />is a candidate experimental unit and to which category it belongs. <br /> <br />13 <br />