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<br />that shallow upslope cloud systems as defined in this study do not <br />contribute the major portion of natupal precipitation during the cold <br />season. The low correlation may be important~ however~ during years <br />when the cold season precipitation is low~ since seedable upslope <br />cloud systems may be present when the major precipitation-producing <br />systems fail to produce normal amounts. Upslope cloud systems may <br />thus constitute an important potenti a 1 source of a.xotificial pre- <br />cipitation. <br /> <br />4.3 Cloud Characteristics <br />The seedability criteria for choosing the data to be analyzed set <br />limits on the values of certain cloud characteristics. Remember that <br />some limits (e.g.~ moist layer top temperatures warmer than -15oC) <br />will affect statistics that describe other characteristics of the <br />upslope cloud systems (e.g.~ the variation in thickness of the moist <br />layer from north to south). <br />4. 3. 1. Cloud base height <br />Figure 11 shows the mean height (in meters) of the base of the <br />moist layers~ as measured from ground level ~ at each of the 14 High <br />Plains rawinsonde stations. As previously indicated.~ this can be <br />considered as the height of the cloud base. Other things being equal ~ <br />a low base height would be advantageous for cloud seeding~ since <br />precipitation would reach the ground with smaller losses from evapora- <br />tion and sublimation. Although the base heights vary little throughout <br />the High Plains~ two rlistributional characteristics are significant: <br /> <br />16 <br />