CSIRO Marine and Atmospheric Research

Cloud seeding

Paul Holper
July 2001

Cloud seeding is a way of trying to artificially generate additional rainfall from clouds. It may involve attempting to produce rain when none would normally fall or it may be working to increase precipitation over a particular area.

Clouds and rain

Air always contains moisture. Whenever air cools, water vapour may condense into tiny droplets of liquid. Clouds are made up of millions of these water droplets.

Before these tiny droplets can form raindrops, snowflakes or hailstones, they have to join with millions of others if they are to become heavy enough to fall to the ground. They will only do this if particles are present in the atmosphere. These particles are called cloud nuclei and may be dust, salt from evaporated sea spray, sand or other material from forest fires, volcanic eruptions and pollution.

Under the cold conditions in clouds, droplets of water 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.

Cloud seeding

The particles that scientists add to clouds during seeding mimic the structure of ice and serve as additional nuclei for crystal formation. Condensation and freezing of water release a large amount of heat that makes clouds more buoyant and may double their size and height. As 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. Researchers can seed cold clouds with silver iodide particles, which have a crystal structure similar to that of ice particles. 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 with dry ice pellets, 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 way of seeding clouds is via a process known as hygroscopic seeding. This involves using flares to generate smoke full of salt. The salt particles 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 from a plane. Clouds may also be seeded from the ground using silver iodide generators.

Australian cloud seeding experiments

Cloud seeding experiments began in Australia just a year after the world’s first laboratory trials in the USA.

From 1947 to 1952, CSIRO scientists used Royal Australian Air Force aircraft to drop dry ice into the tops of cumulus clouds. The method worked reliably with clouds that were very cold, producing rain that would not have otherwise fallen.

CSIRO carried out similar trials from 1953 to 1956 in South Australia, Queensland and other States. Experiments used both ground-based and airborne silver iodide generators.

During 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 four experiments, only the one conducted in the Snowy Mountains produced statistically significant rainfall increases over the entire experiment.

In the late 1960s, the Governments of Victoria, New South Wales, Queensland, South Australia and Western Australia seeded clouds. Results were either inconclusive or controversial.

On the other hand, CSIRO’s activities in Tasmania in the 1960s were successful. Seeding over the Hydro-Electricity Commission catchment area on the Central Plateau 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 cloud seeding experiments in Emerald, Queensland (1972-1975), and in Western Victoria (1979-1980). The Western Australian Government ran a study in 1980-1982 examining the viability of seeding in the northern wheat belt. None of these activities found that seeding would be an economical, reliable way of increasing rainfall.

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.

Does cloud seeding work?

CSIRO has shown that in Australia cloud seeding is effective only in a limited number of weather conditions. Cloud seeding will never break droughts; cloudless skies will never produce rain. In fact, many types of clouds cannot be successfully seeded. Cloud seeding is most likely to be effective when used on cumulus or stratiform clouds in air forced up over mountains.

Seeding is unlikely to be effective during winter and spring over the inland plains of southern and eastern Australia. It is also likely to fail in summer over eastern and north-eastern Australia plains and immediately to the north of Perth.

In the tropics, the high rainfall variability makes proof of increased rainfall from cloud seeding extremely difficult.

Based on over 50 years’ experience with cloud seeding, CSIRO has established procedures for undertaking a cloud seeding experiment. These rigorous guidelines ensure that at the conclusion of seeding operations there will be a clear-cut answer to whether or not the activity was successful. In other words, has the seeding netted a statistically significant increase in rain over the catchment? If the answer is no, there is no point in persevering.

It may be worth again attempting rainfall enhancement experiments in areas where past efforts have failed, but proper planning needs to be done first, along with rigorous independent evaluations.

Site updated 1st October 2003

Modified: April 3, 2008

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