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<br />.,. <br /> <br />..., <br /> <br />. <br /> <br />r . <br />~ <br /> <br />, <br />, <br />I; <br /> <br />. <br /> <br />f .. <br />I <br />I <br />I <br /> <br />r <br />!" <br /> <br />;. <br /> <br />produce significantly difFerent amounts of precipitation. Precipitation formation processes <br />within clouds and storms can also cllifer dramatically in time. This was particularly evident <br />in the Bethlehem region of South Africa. Although Krauss et al.(1987) concluded that the <br />dominant precipitation formation mechanism was the ice process with no evidence that <br />the coalescence process was active in any of the clouds studied, a case study presented <br />in a later chapter documents an active coalescence process in combination with an active <br />ice multiplication process in the same area. The period of the experiment described in <br />Krauss et ale was characterized by far below normal summer rainf8.ll amounts. The case <br />study documented in a later chapter actually occurred in a wet summer season. The main <br />difFerence between the two periods was that during the dry years there was a general lack <br />of low level tropical moisture moving down into South Africa from the equator while the <br />wet years were characterized by an abundance of tropical moisture during the late summer <br />months ("monsoon" period). It is thus clear that not only can difFerences in clouds and <br />rainfall occur from the early part of a summer season to the latter but also from year to <br />year. Intuitively one would also expect difFerences in seeding responses depending on the <br />type of clouds present. It is therefore necessary to understand these differences and again. <br />use covariates that describe these differences to take them into account in the evaluation <br />of cloud seeding results. <br /> <br />As mentioned in the previous section precipitation formation as discussed in the <br />literature is considered to occur through one of two processes, the 'ice process' or the 'warm. <br />rain' process. Extending this tradition clouds are further categorized as either 'continental' <br />or 'maritime', terms which describe the degree of colloidal instability. However, research <br />in recent years has made it clear that precipitation formation in many convective clouds <br />is not a simple matter of one or the other of the two classical mechanisms, but rather <br />a combination of the ice and warm rain process. In some 'continental' regions natural <br />clouds occupy the complete spectrum in microstructure between the both extremes, as <br />discussed in the previous paragraph. Clouds in which the coalescence process is active <br />in combination with an ice multiplication process are usually considered more efticient <br />precipitation producers than when only the ice process is active. In addition, precipitation <br /> <br />14 <br />