Four decades of snowfall experience from snowpack augmentation studies in the Australian Alps and the Sierra Nevada of southwestern United States

The high mountainous regions of the Western United States as well as the principal Alpine region of Southeastern Australia have provided snow-forming conditions which have been studied by many atmospheric scientists for more than four decades. The mesoscale condition under which snow is formed and is precipitated to the surface, are now well understood, as are the specific cold cloud conditions within which the ice-phase precipitation is principally produced. These preferred regions occur at elevations consistent with the temperature regimes in which ice crystals undergo most rapid growth. Experience from as many as twenty five independent snowpack augmentation programs in the Sierra Nevada since the early 1950s, has shown that the ranges of elevations of the freezing level and the -5C isotherm have remained substantially steady over this 40-year period. The same was true for the Snowy Mountains region of Australia where in the period 1955-59 the mean elevation of the freezing level was observed by aircraft during the passage of winter storms in the months of June through August to be approximately 1500 metres. During the Snowy Mountains Atmospheric Research Program (SMARP) in 1988 and 1989, rawinsonde release showed the mean elevation of the freezing level during cold front passages at 1300 metres; during low pressure troughs, 1400 metres; during synoptic low pressure systems, 1300 metres. These are the conditions under which snow occurs over these mountains in winter. It is well known that the number of snow-producing storms varies greatly from year to year. This is true in the Sierra Nevada and probably in the Snowy Mountains, although the number of such events in the Snowy during a 1977-81 period of analysis by SIROMATH for the Snowy Mountains Authority, was almost identical to that experienced in the 1988-89 SMARP study period. Recent studies in the Sierra Nevada have shown that it was not possible to detect any silver or silver iodide in waters from two lakes located within a seeding target area, after 42 years of winter snowpack augmentation activities.