MORE INFORMATION- Water
Although this is not used on the Central Coast cloud seeding is a way of artificially generating additional rainfall from clouds. It may involve attempting to produce rain when none would normally fall or it may be working to increase rain fall over a particular area.
When air cools, water vapour condenses into tiny droplets of liquid. Clouds are made up of millions and millions of these water droplets. Before the droplets can form raindrops, snowflakes or hailstones, they have to become heavy enough to fall to the ground. They do this by joining together.
When cloud seeding is occurs the particles that scientists add to clouds mimic the structure of ice. Condensation and freezing of water release a large amount of heat through the chemical reaction that takes place which makes clouds more buoyant and they may double their size and height. As the clouds grow taller, they draw in more moist air that can add to the rain formed.
Clouds can be seeded in a variety of ways. This includes:
Cold clouds can be seeded with silver iodide particles, which have a crystal structure similar to that of ice particles. Water may deposit on the silver iodide particles and coat them with ice and keep growing as if the entire particle were a natural ice particle.
Cold clouds can also be seeded with dry ice pellets (dry ice is frozen carbon dioxide CO2), which will cool the air nearby 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 way of seeding clouds is through a process known as hygroscopic seeding which involves using flares to generate smoke full of salt. The salt particles generate large water drops that can readily develop into raindrops.
Figure 1: The Cloud Seeding Process
Figure 2: How Cloud Seeding Works
In Australia cloud seeding experiments began a year after the world’s first laboratory trials in the USA in 1946. From 1947 to 1952, CSIRO scientists using Royal Australian Air Force aircraft dropped dry ice into the tops of cumulus clouds. The method worked well with clouds that were very cold and produced rain that would not have otherwise fallen. The CSIRO carried out similar trials from 1953 to 1956 in South Australia, Queensland and other States.
In the late 1950s and early 1960s, CSIRO performed cloud seeding in the Snowy Mountains, on the York Peninsular in South Australia, in the New England district of New South Wales, and in the Warragamba catchment area west of Sydney. Of these experiments the only one to produce statistically significant rainfall increases over the entire experiment was the one conducted in the Snowy Mountains.
In the 1960s CSIRO’s activities in Tasmania were also successful. Seeding over the Hydro-Electricity Commission catchment area on the Central Plateau of Tasmania achieved rainfall increases as high as 30% in autumn. The Tasmanian experiments were so successful that the Commission has regularly undertaken seeding ever since in mountainous parts of the State.
CSIRO also conducted the following cloud seeding experiments following the above successes:
In Emerald, Queensland (1972-1975)
Western Victoria (1979-1980)
During the late 1980s, CSIRO Atmospheric Research acted as scientific advisors to Melbourne Water in a cloud seeding assessment conducted over the Baw Baw plateau. This is a major water catchment area east of Melbourne. The experiment generated no statistical increase in rainfall.
Cloud seeding in Australia has been found to be effective only in a limited number of weather conditions. It is concluded that, within Australia, cloud seeding will never break droughts as cloudless skies will never produce rain and cannot be forced to. Cloud seeding is most likely to be effective when used on cumulus (often low in the air and look like cotton wool or like cauliflower on top with a flat base, clusters of small white cumulus clouds are usually a sign of fine weather) or stratiform (appear as light grey clouds that look like even sheets and cover all or part of the sky) clouds in air forced up over mountains.
Figure 3: Cumulus clouds
Figure 4: Stratus Cloud
(Source: http://www.hydro.com.au/ )
Cloud seeding has no known detrimental health or environmental effects anywhere in the world. The main reason that it is not used on the Central Coast is weather conditions are not suitable to make the expense of the seeding process worthwhile in the long run.
For further information on cloud seeding in Australia see:
BOM Australian Cloud Seeding Symposium