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<br />I. INTRODUCTION <br /> <br />Since Bergeron's [1]1 analysis of the effect of artificial ice <br /> <br /> <br />nuclei on supercooled clouds and Ludlam's [2] classic study on <br /> <br /> <br />artificial snowfall from mountain clouds, considerable progress <br /> <br /> <br />has been made in the field of winter orographic snowpack augmen- <br /> <br /> <br />tation. Bergeron's and Ludlam's works 'were theoretical concepts <br /> <br /> <br />based on laboratory and field observations. By combining relatled <br /> <br /> <br />facts pertaining to mountain clouds and the production of arti- <br /> <br /> <br />ficial ice nuclei, they were able to suggest possible seedability <br /> <br /> <br />criteria which would allow successful augmentation of orographic <br /> <br /> <br />snowfall. <br /> <br />Since the late 1940's, experimental and operations.! cloud seeding <br /> <br /> <br />to increase precipitation in the mountai.ns has been carried out <br /> <br /> <br />to refine these theories. These efforts have been conducted under <br /> <br /> <br />widely differing topographical and meteorological conditions. <br /> <br /> <br />Additional seedability criteria have been developed which have <br /> <br /> <br />permitted application of artificial seeding to progressively more <br /> <br /> <br />suitable cloud conditions. Some of these seedability criteria are <br /> <br />simplified indices based on a single major cloud condition, while <br /> <br /> <br />others are combinations of conditions. Today, most investigators <br /> <br />1 Numbers in brackets refer to items in the Bibliography. <br /> <br />1 <br />