Advice on the Impact of Pollution on Rainfall and the Potential Benefits of Cloud Seeding

A report to the Secretary Department of the Environment and Heritage, 5 December 2002
Neville H Fletcher
Research School of Physical Sciences and Engineering, Australian National University
Department of the Environment and Heritage, 2003

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Executive Summary

  1. Mr Gingis claims that studies made by Professor Daniel Rosenfeld of the Hebrew University of Jerusalem and by his own company have established that annual rainfall in the Snowy Mountains and in the Victorian Alps has been reduced by at least 30 percent over the past 50 years. He identifies the cause of this decrease as being industrial pollution from activities near Adelaide and near Geelong. He further claims that cloud seeding techniques developed by Professor Rosenfeld are able to reverse this effect, and proposes a program of investigation followed by operational seeding, these programs to be carried out by his company.
  2. Professor Rosenfeld is well known and respected for the techniques he has developed for deducing cloud properties by satellite observation. These observations do show clear tracks downwind of industrial sites near Adelaide and indicate that cloud properties are affected within the tracks for distances of up to 300 km downwind. Cloud droplets within the affected clouds are smaller than in clouds outside the pollution track and there is evidence that rainfall is inhibited within the track. Similar satellite data was later presented for other locations in Australia. These data are, however, the results of observations on only a few selected days when cloud properties were appropriate, and no long-term observational program has been carried out.
  3. Professor Rosenfeld subsequently found apparent evidence in official rainfall records for a decline in rainfall in some areas of the Snowy Mountains and Victorian Alps which he attributed to the effects of these pollution tracks. Investigations by the Bureau of Meteorology, however, showed that these apparent decreases could be accounted for by the closure or relocation of certain observation stations in high-rainfall areas of the districts included in the official area rainfall calculations.
  4. Long-term records of Australian rainfall show that there has, in fact, been an increase in rainfall in all areas over the past century, with the exception of a region in Southwest Western Australia where the rainfall has decreased at a statistically significant level, and in Tasmania where the decrease is small and not statistically significant. Rainfall in any given area, however, has large variability on an annual scale, and even when averaged over ten years to give decadal rainfall. Patterns of declining rainfall over periods as long as 20 years can be seen in records as long ago as 1900, but are later compensated for by periods of increasing rainfall.
  5. Rainfall records in California show that there is evidence for an inhibiting effect of pollution plumes originating in coastal cities on rainfall in mountain regions downwind from those cities, the distance involved being 50 to 100 km. The rainfall records support, though only weakly, the proposition that the rainfall deficit was remedied by Clean Air legislation in recent decades.
  6. It is concluded that there is no reliable evidence that pollution plumes have actually decreased annual rainfall in the Snowy Mountains, in the Victorian Alps, or in Western Victoria. Some such effect cannot, however, be ruled out and is not unreasonable on physical grounds, but may be minor and limited to certain cloud types and wind trajectories.
  7. Theoretical treatments, computer models, and measurements in real clouds show that seeding some clouds with large (1-2 micrometre) salt particles can change their microstructure in a direction favouring later precipitation. This is effective only for clouds with unusually small droplet sizes and can be counterproductive if the natural droplet size is large. Similarly, seeding clouds of large vertical extent with silver iodide can induce ice formation and so enhance precipitation. This is effective only for clouds with very small numbers of natural ice nuclei, and can be counterproductive if the concentration of natural nuclei is large.
  8. Irrespective of any influence of industrial pollution, experiments in Australia and overseas have shown that cloud seeding with silver iodide has probably been able to increase rainfall in some mountain regions by about 10 percent. In other regions extensive experiments have failed to produce a clear increase. Controlled experiments must generally be continued for 5 to 10 years to have any hope of giving a statistically significant result. In Australia, long-extended experiments and operations in Tasmania appear to have produced a rainfall increase of about 8% in the target area without adverse effects in either upwind or downwind areas, though evaluation of more recent seeding operations raises questions about the actual increase achieved in the target area. In the Snowy Mountains, long-continued experiments failed to produce a non-controversial result. A carefully designed experiment in Western Victoria was abandoned because suitable cloud-seeding opportunities were too few.
  9. The seeding methods used by Professor Rosenfeld and advocated by Mr Gingis are similar to those that have been used over the past fifty years, but operational convenience and efficiency have been refined. In addition, there have been great advances in observational techniques using special instrumentation in satellites and advanced radar techniques. The approach of seeding individual clouds, as is done in Texas, produces apparent striking successes, as was first demonstrated in Australia in the 1950s, but gives little information on the potential of the technique to increase rainfall over a large area, even if carried out on an "operational" basis. There is controversy about the success of both the Texas program and the related long-running program in Israel.
  10. Given that any cloud-seeding program capable of producing statistically significant results will need to be conducted over at least five years and will cost many millions of dollars, careful consideration must be given to it in advance. Before deciding on any such program, a careful analysis of cloud statistics in the experimental region should be undertaken to decide whether the proposed cloud-seeding technique is appropriate and has a chance of success. The design and evaluation of any such program should be in the hands of, and under the continuing supervision of, a broadly based scientific committee, involving both the Bureau of Meteorology and CSIRO as well as independent scientists.
  11. If a major concern is the effect of industrial pollution on rainfall, it might be much less expensive and more effective to attempt to reduce or modify the effluent at its source, particularly since in Australia these sources are identified industrial operations such as power stations or smelters. Doubtless the newer operations already have pollution control measures installed on their effluent stacks, but there is always the possibility of increasing the effectiveness of these devices. Consideration might also be given to a controlled experiment in which the effluent is seeded with large condensation nuclei, essentially by spraying sea water into the hot effluent plume, thereby providing at source the hygroscopic nuclei that cloud seeding would later introduce.