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\ Winter Weather Modific ation Research at Utah State University G. E. Hill At Utah State University research on increasing precipitation from winter orographic clouds has been focused in recent years on three main aspects of the problem. Up until about four years ago, the pr imary aspect of the research was directed at what is often called opportunity recognition, which means identification of the periods when the addition of artificial ice nuclei will initiate precipitation that would not otherwise occur. For the past five or six years the problem of deliver- ing ice nuc lei to sui table clouds has received increasing attention. This effort has led to a compl etely ne,,,, delivery system based upon utilizing small balloons. The third aspect has been the recent develop- ment of a specialized communication system for either controlling remote devices or for receiving data from remotl:! locations. In this article each of the three aspects will be reviewed and the immediate plans for the future will be described. QEP-ortunity Recognition tunity recognition is based ciency, E. That is, The conceptual model for defining oppor- upon a definition of precipitation effi- E = pic = p/(p + E) I/O + E/P) where C, P and E are the rates of condensat ion (and sublimation), pre- cipitation reaching the ground, and evaporation, respectively. For an orographic cloud the condensation takes place on the upwind side of a mountain barrier, precipitation occurs in the general region where the cloud exists and evaporation occurs on the downwind side of the barrier. The source of water for augmented precipitation by cloud seeding is supercooled I iquid water that would othen,ise evaporate if seeding were not done. The quotient E/P, which contains the essential information on precipitation efficiency, is approximately directly related to the quotient of LWC measured where the verticall motion approaches zero near the barrier crest and the prec ipitation rate over the barrier (LWC/P). Two independent types of measurements made over several years have led to the finding that updrafts and supercooled liquid water are con- fined generally to layers often less than 1 km in thickness. The typical altitude of the center of the layer is only a few hundred meters higher than the barrier crest. Often the supercooled liquid water layers are very shallow with a thickness of only 200 or 300 m. Radiometric measurements of LWC reveal a duration of high LWC from a few minutes to a few hours. High concentrations of LWC () 0.25 nun) typically occur during low precipitation rates. Also, with enhanced LWC amounts, the cloud top temperature (CTT) is generally around -220C or warmer. Furthermore, the LWC tends to increase with increasing cross- barrier wind speed (U700)' 45