It has been a long standing debated issue as to whether cloud seeding is reliable and a cost effective form of water management or just simply unworthy of any scientific attention.
Cloud seeding is a form of weather modification trying to artificially generate additional rainfall from clouds, attempting to produce rain when none would normally fall or to increase precipitation (rain or snow) over a particular area.
Air contains moisture and when air cools, water vapour may condense into tiny droplets of liquid. Clouds are made up of millions of these water droplets.
Paul Holper from CSIRO Marine and Atmospheric Research (Australia) highlighted that before these tiny droplets can form raindrops, snowflakes or hailstones, they have to join with millions of others to become heavy enough to fall to the ground. However, they will only do this if particles are present in the atmosphere.
Such particles are cloud nuclei, which may be dust, salt from evaporated sea spray, sand or other various materials from volcanic eruptions, fires and pollution. Under cold conditions, droplets of water with the clouds can form small ice crystals on the surfaces of the cloud nuclei. Water vapour in the cloud then freezes directly onto the surface of these crystals, which become heavier and eventually fall.
The particles that scientists add to the clouds during cloud seeding imitate the structure of ice and serve as additional nuclei for crystal formation.
Clouds can be seeded in a number of ways. Researchers can seed cold clouds using silver iodide particles, which have a similar structure to that of ice particles, encouraging the growth of new ice particles.
As Paul Holper stated, “Water can deposit on the silver iodide particles, coat them with ice and keep growing as if the entire particle were a natural ice particle”.
Cold clouds may also be seeded using pellets of frozen carbon dioxide, more commonly referred to as ‘dry ice’, which cool the nearby air far below 0°C. Cloud droplets in the cooled air freeze and form ice particles that can grow as more water freezes on their surface.
Another form of cloud seeding - hygroscopic seeding, involves using flares to generate smoke full of salt. The salt particles within the smoke act as nuclei that generate large water drops that can readily develop into raindrops.
Seeding using silver iodide burners, dry ice pellets and hygroscopic flares is done via a plane, yet clouds may also be seeded from the ground using silver iodide generators.
Professor Gary Jones, senior water scientist and Chief Executive of eWater Cooperative Research Centre, Canberra recently stated, “Cloud-seeding has a mixed reputation, both in Australia and overseas. Some see it as a reliable and cost effective way to boost precipitation and water supplies. Others…unworthy of attention by any proper scientist or water professional.”
However, there are strong scientific grounds for why some types of cloud seeding should in fact work. Techniques using silver iodide or dry ice to induce ice-formation and precipitation can’t make clouds and rainfall appear out of nowhere, alternatively Jones highlights that they “work by catalysing a natural ‘chain-reaction’ that might or might not happen in a cloud ultimately leading to snow or rain”.
Cloud seeding has a long research and operational history in Australia with initial experiments beginning in 1947 with CSIRO scientists using Royal Australian Air Force aircraft to drop ice into the tops of cumulus clouds.
The Australian national science organisation CSIRO has been involved in many of these research projects, working with various state governments and the hydro-electric power companies in Tasmania and NSW.
Trials in Tasmania between 1964 and 1994 were considered successful, and as a result cloud seeding has been adopted there as an operational technique, boosting hydro-electricity water supplies.
A key challenge is in discerning how much precipitation would have occurred had clouds not been seeded. The amount of precipitation due to seeding is difficult to quantify. Jones stresses the problems for practitioners associated with the natural variability of rain or snowfall, “creates huge problems for practitioners trying to untangle the effects of cloud seeding from the natural variability in precipitation”, in addition to dust and other background aerosols stimulating precipitation and obscuring the interpretation of what might otherwise appear positive results.
Conversely, there is now scientifically strong and justifiable evidence that cloud seeding has increased precipitation, in an area of the Snowy Mountains in Australia with increases of snowfall from 9% to 14% with no negative environmental effects detected.
In NSW, The Snowy Hydro has recently started trialling cloud seeding in attempt to increase winter snowfalls over the mountains, and furthermore spring and summer runoff levels, again to improve the reliability of flows for electricity generation and the wider area.
The ‘Snowy Precipitation Enhancement Research Project’ trial has been conducted since 2004 by the government-owned Snowy Hydro Corporation with an independent review being provided by the NSW Natural Resources Commission, which oversees the trial and ensures it does no environmental damage, including protecting alpine habitats.
The trial uses ground-based burners on mountain tops to introduce silver iodide and a tracer into suitable clouds. The ‘suitable’ clouds are identified using modern Doppler radar identification techniques. Snowfall is then sampled carefully and chemical ratios in the snow are analysed to confirm the source (natural or seeded). The trial is to continue until 2014 when final results are to be announced.
There are no known credible studies that have shown detrimental health or environmental effects from using cloud seeding technology. There is a high level of confidence that there will be no negative effects caused by the seeding agents, due to the fact that the amount of silver iodide and other chemicals being added to the environment would be in minute quantities over the very large target area.
Conversely, under the guidelines of the United States Environmental Protection Agency Clean Water Act, silver iodide is considered a “hazardous substance, a priority pollutant, and a toxic pollutant”.
Additionally, a key manufacture of silver iodide, Deepwater Chemicals, warns of potential health effects of silver iodide in their Material Safety Data sheet, where they suggest chronic poisoning by iodides may be manifested by skin rash, running nose, headache, irritation of the mucous membranes, weakness, anaemia, loss of weight and general depression. However, as any toxicologist will suggest, it’s the dosage that does the damage.
In the great dry continent of Australia it is apparent that the CSIRO has shown cloud seeding to be effective only in a limited number of weather conditions.
Cloud seeding cannot be expected to break droughts and cloudless skies will never produce rain. Indeed many types of clouds cannot be seeded and the technique is most likely to be effective when used on cumulus clouds in air forced up over mountains – But, it’s a genuine option for the rainmakers.
Some additional sources:
http://www.abc.net.au/catalyst/stories/2714955.htm
http://www.cmar.csiro.au/e-print/open/holper_2001c.htm
http://insidewater.ewater.com.au/2011/05/04/cloud-seeding-in-australia/
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