What’s that about?Cloud Seeding 2012

We thought the best way give you information on cloud seeding is to answer some common questions we are asked. If you have any other questions about our cloud seeding program please contact us: contactus@hydro.com.au.

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.

Who came up with the idea?Cloud seeding was first demonstrated by Vincent Schaefer from the General Electric Laboratories on 13 November 1946, in the United States. The first case of documented man-made rain occurred near Bathurst, Australia in February 1947 and was carried out by CSIRO (Commonwealth Scientific and Industrial Research Organisation – Australia). Ongoing research led to the first cloud seeding experiment in Tasmania 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. This causes water vapour to condense into water droplets, which are visible to the naked eye.

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 used to produce rainfall.

Which clouds are suitable for seeding?There are two main types of clouds that are 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.eeding

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 to 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 aims to increase the number of nuclei by putting artificial cloud condensation nuclei into the cloud.

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 melt as they pass through the melting point (0ºC) and fall as rain, or as snow if the temperature is low enough.

Silver iodide crystal

Small snowflake

Large snowflake

Melting level

Raindrop

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What’s that about?Cloud Seeding 2012

What is used to seed the clouds?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 causing the cloud droplets in the air to freeze, additional water condenses on the outisde of the frozen droplet, making the droplet grow.

Hygroscopic salts (KCl, NaCl) - attract water vapour to themselves, growing larger and eventually forming into raindrops. Hygroscopic salts work in warmer, typically tropical, types of cloud.

Hydro Tasmania has used both dry ice and silver iodide experimentally and has determined that silver iodide is the most effective for Tasmanian conditions. Seeding with hygroscopic salts has not been trialled in Tasmania.

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 amounts of silver iodide released in cloud seeding activities.

Does cloud seeding work?Yes. Hydro Tasmania has completed three successful experiments. Two of these have been in conjunction with CSIRO. All of the 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 in conjunction with CSIRO providing randomisation on a seed/no seed 1:1 ratio. The experiment was concluded in 1971 when the reservoirs were at their full supply level. The target area was the Central Plateau (Great Lake) 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.--------

Cloud seeding makes an important contribution to Hydro Tasmania’s annual generating capacity by increasing rainfall over hydro storages. A Monash University study found a consistent increase of at least five per cent in monthly rainfall over seeded catchment areas. Read the report here > -----

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Flight Date:Start Time:Flight Duration:Seeding Start:Seeding Duration:Seeding Altitude:Catchments Targeted:

Legend

Flight PathBurners OffBurners On

Target AreasGordonMersey ForthGreat LakeUpper DerwentUpper PiemanTargeted Catchment(s)

19 May 201114:56 EST3 hrs 1 min16:24 EST1 hr 2 min12,000ft Gordon

47 knots at 260o

Average Wind Speed & Direction:

¿

0 50 100 150 20025

Kilometres

Cloud Seeding

What’s that about?Cloud Seeding 2012

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 cloud seeding season 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. The aircraft is fitted with a number of scientific instruments to measure the atmospheric conditions encountered and to determine the suitability of clouds for seeding.

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.

How long does it 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 to fall out of the cloud under their own weight. As ice falls, it passes through the melting level (0ºC) and becomes rain.

Hydro Tasmania 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.

Example: Hydro Tasmania cloud seeding operations Date: 19 May 2011

In the example shown, the average wind direction is 260º (or westerly) with an average wind speed of 47 knots. The seeding track (shown in red) is approx 25 nautical miles (nm) (or 46 km) upwind from the target area boundary.

One track has been seeded for the Lake Gordon catchment; conditions for the other catchments were checked out (track shown in blue), but no suitable cloud was found.

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Mersey ForthUpper

Pieman

Great Lake

Upper Derwent Gordon

What’s that about?Cloud Seeding 2012

When does Hydro Tasmania undertake operations?The cloud seeding season runs from April to October. Seeding takes place when it is safe to fly and suitable weather is present, 24 hours a day 7 days a week.

Where does Hydro Tasmania cloud seed?Hydro Tasmania seeds over six catchment areas.

These are the: Upper Pieman (Lake Mackintosh), Mersey Forth (Lake Rowallan), Great Lake, Upper Derwent (Lakes St Clair, King William and Echo) and Gordon.

How long has Hydro Tasmania been cloud seeding?Hydro Tasmania started 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 (Great Lake), 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 areas 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 (Lake Burbury) and Gordon catchment areas.

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What’s that about?Cloud Seeding 2012

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 April to October in the Central Plateau, Upper Pieman, Mersey Forth 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 on their own experience.

Does cloud seeding increase rainfall outside the target areas?There have been numerous studies undertaken by CSIRO and 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 decreases in rainfall downwind of the target catchment areas?Studies undertaken by CSIRO and by the Antarctic Cooperative Research Centre show that cloud seeding does not cause any measurable decrease in rainfall downwind of the target 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 more than 40 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.

Would you like to know more?If you would like to know anything else about Hydro Tasmania’s cloud seeding program please email us at contactus@hydro.com.au.

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