Frequently Asked Questions (FAQs)

What is cloud seeding?

Cloud seeding is a technique for increasing rainfall or precipitation using naturally occurring clouds. Cloud seeding is only effective if suitable clouds are present.

Where does cloud seeding come from?

Cloud seeding was first demonstrated by Schaefer from the General Electric Laboratories on 13 November 1946. The first case of documented man-made rain occurred near Bathurst, Australia in February in 1947 by CSIRO (Commonwealth Scientific and Industrial Research Organization – Australia). Ongoing research led to the commencement in Tasmania of the first cloud seeding experiment by Hydro Tasmania and CSIRO in 1964.

What are clouds?

Clouds are composed of water droplets and sometimes ice crystals. The usual mechanism for cloud formation is for air that is rich in moisture near the Earth's surface, to be raised higher into the atmosphere either by an encroaching air mass or the heat of the sun. As the air is lifted, the pressure drops and the air is subsequently cooled. The combination of the two causes water vapour to condense into water droplets, which are visible to the naked eye.

What type of clouds are suitable for seeding?

There are two main types of clouds suitable for cloud seeding:

Cumuliform Cloud: These clouds are characterised by a dense discrete appearance with sharp outlines, which develop vertically in the form of towers or domes. The sunlit part of the cloud is often a brilliant white while the bottom is a darker grey and flat in appearance. They are convective in nature with a great deal of vertical mixing present.

Stratiform Cloud: These clouds are characterised by their layered structure. They are generally grey with a fairly flat base extending for many kilometres in all directions. Generally there is very little convective activity in the cloud.  

Picture of Cumulus cloud (lower part) and stratiform cloud (upper part) seen on the West Coast of Tasmania

Are all clouds suitable for seeding?

No, not all clouds are suitable. Significant levels of supercooled liquid water have to be present in the cloud. The cloud must be deep enough and the temperature must be within a range suitable for seeding. The wind also has to be below a fixed value. Once all the criteria are met, cloud seeding can be successfully utilised to produce rainfall.

How does cloud seeding work? The theory of cloud seeding states that the number of naturally occurring ice nuclei present in a cloud is related the amount of rain the cloud can produce. The amount of these naturally occurring ice nuclei is generally much lower than the optimum number required for effective rain formation. Cloud seeding seeks to increase the number of nuclei present by putting into the cloud artificial cloud condensation nuclei.

A simplified diagram of the seeding process

The concept is that the cloud droplets form small ice crystals on the surface of the nuclei (natural and artificial). Ice crystals falling through the cloud collide with more cloud droplets and grow. Eventually when these ice crystals fall from the cloud they melts as they pass through the melting point (0ºC), and fall as rain or snow if the temperature is low enough.

What cloud seeding agents are used?

Three types of seeding agent can be used:

Silver Iodide - is very similar in structure to naturally occurring ice. Water deposits form on the surface of the silver iodide crystal and the ice crystals continue to grow as if they were naturally occurring ice crystals.

Dry Ice (CO2) – acts to cool the water far beyond 0ºC thereby causing the cloud droplets in the air to freeze, thus growing as water freezes on the surface of the ice.

Hygroscopic salts - Salts (KCl, NaCl) attract the water vapour to themselves, growing larger and eventually forming into raindrops.

Hydro Tasmania has used both dry ice and silver iodide experimentally and has determined that silver iodide is the most effective for Tasmanian conditions.

Does cloud seeding work?

Hydro Tasmania has completed three successful experiments. Two of these have been in conjunction with CSIRO (Commonwealth Scientific and Industrial Research Organization – Australia). All of these experiments show statistically significant increases in rainfall over the Central Plateau target area.

Below is a list of experiments and some brief information about each:

Stage I : 1964-1971 This was an alternate year experiment providing randomisation on a seed/no seed 1:1 ratio in conjunction with CSIRO. The experiment was concluded in 1971 when the reservoirs were at their full supply level. The target area was the Central Plateau and silver iodide was used as the seeding agent.

Stage II : 1979 – 1983 The experiment used a ratio of suitable seeded/unseeded days at 2:1 to provide randomisation and was also conducted in conjunction with CSIRO. Silver iodide was used as the seeding agent.

Stage III: 1992 –1994 This experiment was very similar to Stage II except that dry ice was used and the work was completed by Hydro Tasmania alone.

How long does take for cloud seeding to work?

Once the cloud is seeded it takes about 30 minutes for the ice crystals formed to grow to sufficient size and fall out of the cloud under their own weight. As ice falls, it passes through the melting level (0ºC) and becomes rain.

Hydro Tasmania therefore seeds 30 minutes upwind of its chosen target area boundary. For example, if the wind speed is 40 knots, seeding will be 20 nautical miles or about 37 km upwind of the target area boundary.

Hydro Tasmania Cloud Seeding Operations Date: 2nd May 2007 Av Wind: 40 knots at 320º Flight Start: 05:03 Flight Duration: 4 hr 11 min Seeding Start: 06:21 Seeding Duration: 1 hr 30 min  Target Area(s): Upper Pieman, Mersey Forth, Great Lake & Upper Derwent In the example shown, the average wind direction is 320º (or North Westerly) with a speed of 40 knots. The seeding track (shown in red) is approx 20 nautical miles (nm) (or 37 km) upwind from the target area boundary.  Two tracks have been seeded; one seeds the Upper Pieman and Mersey-Forth areas and the second seeds the Upper Derwent and Great Lake regions.

When does Hydro Tasmania undertake operations?

Currently, the cloud seeding season is from the 28th of April until the 31st October. Seeding takes place when it is safe to fly and suitable weather is present, 24 hours a day 7 days a week.

Why does Hydro Tasmania cloud seed?

Tasmania produces the majority of its power from hydro-electricity. This power is produced from rain falling on the hydro catchments, where it is stored in dams until the energy is required). Water is run through power stations to produce electricity. The more water in storage, the greater the capacity of Hydro Tasmania to meet the energy demands of Tasmania in particular through the drier periods such as summer. Cloud seeding is a cost effective means of increasing rainfall in selected catchment areas.

How does Hydro Tasmania seed the clouds?

Hydro Tasmania uses a twin engine aircraft, a Cessna Conquest which is under contract for the period of cloud seeding each year. This plane has two silver iodide generators or burners, containing the silver iodide solution under pressure, one mounted under each wing. Every cloud seeding flight is conducted by one pilot and one cloud seeding officer.

When airborne the cloud seeding officer advises the pilot of a seeding track upwind of the target area. Once suitable cloud is encountered on the seeding track, the cloud seeding officer ignites the burner using switches mounted inside the aircraft.

Hydro Tasmania’s cloud seeding aircraft with burners seen under each wing.

Where does Hydro Tasmania cloud seed?

Hydro Tasmania seeds over six catchment areas. 

These are the:
Upper Pieman
Mersey Forth
Great Lake
Upper Derwent
King
Gordon

Hydro Tasmania’s Cloud Seeding Target Areas

How long has Hydro Tasmania been cloud seeding?

Hydro Tasmania commenced seeding in Tasmania in 1964. Below is a list of seeding work, and associated target areas completed:

1964-1971: Seeding was conducted in alternate years over the Central Plateau area, and concluded in 1971 when the reservoirs were at the full supply level.

1979 – 1983: Seeding was conducted on the Central Plateau target area.

1988-1991: Seeding was conducted on the Central Plateau, Upper Pieman/ Mersey Forth target area and occasionally the Gordon catchment area.

1992 –1994: Seeding was conducted over the Central Plateau target area.

1994-1995: Seeding of agricultural areas in the Tasmanian Midlands and East Coast.

2000: Seeding of agricultural areas in Tasmania’s Central Highlands and South East.

1998: Seeding operationally, September and October only, in the Central Plateau, Upper Pieman/Mersey Forth, King and Gordon catchment areas.

1999-2008: Seeding operationally; every year from 1st April to 30th November in the Central Plateau, Upper Pieman/Mersey Forth, King and Gordon catchment areas. Some seeding of agricultural areas in 2008.

from 2009: Seeding operationally; every year from 28th April to 31st November in the Central Plateau, Upper Pieman/Mersey Forth, King and Gordon catchment areas.

How does Hydro Tasmania know when to fly?

The decision to fly is made by the cloud seeding duty officer. All cloud seeding officers are trained in weather forecasting. They base the decision to fly on available weather information (including satellite, radar etc.) provided by the Australian Bureau of Meteorology and their own experience.

Are the seeding agents used harmful to the environment?

Hydro Tasmania has completed an Environmental Impact Assessment of cloud seeding work. The results indicate that there are no adverse impacts on the environment from the silver iodide released in cloud seeding activities.

Does cloud seeding cause an increase in rainfall outside the target areas, for example, the west coast of Tasmania?

There have been numerous studies undertaken by CSIRO and more recently by the Antarctic Cooperative Research Centre that have shown cloud seeding does not cause any detectable increase in rainfall outside the identified target area.

Does cloud seeding cause a decrease in rainfall downstream of the catchment areas, for example the east coast of Tasmania?

Studies undertaken by CSIRO and more recently by the Antarctic Cooperative Research Centre show that cloud seeding does not cause any measurable decrease in rainfall on the East Coast and its associated farming areas.

Who decides when and where Hydro Tasmania will carry out cloud seeding operations?

Hydro Tasmania has always used a referee who is independent of cloud seeding operations and an expert in the operational requirements of Hydro Tasmania. The referee identifies target areas to be cloud seeded based on the storage situation, and continues to monitor the level of storages through the cloud seeding season. The referee will halt cloud seeding in areas where storages are approaching full capacity.

Is Hydro Tasmania available to undertake cloud seeding outside of Tasmania?

Hydro Tasmania with its 38 years of experience in cloud seeding is available for both international and national consultancies. Hydro Tasmania has completed two interstate projects and has provided extensive advice to overseas operations.