How does it work?
Cloud seeding is a weather modification technique which involves the introduction of a seeding agent into suitable clouds to encourage the formation and growth of ice crystals or raindrops, in turn enhancing the amount of precipitation falling from the cloud.
The terrain of the Snowy Mountains region of New South Wales and the prevailing meteorology during the winter months offers significant potential for cloud seeding.
Winter precipitation over the Snowy Mountains is largely associated with moist westerly weather systems. As these systems approach the mountain ranges, the air mass is lifted and condenses further to form orographically enhanced clouds composed of tiny water droplets. Under certain conditions these droplets remain in liquid form, even at temperatures below 0°C. Water in this form is known as super cooled liquid water (SLW).
To fall out of the clouds as snow, these SLW droplets need to form ice crystals. This normally occurs through interaction with tiny airborne particles (like dust or other ice crystals), or when cloud temperatures are very cold. If there are not enough of these particles, or the temperatures are not cold enough, then not all the SLW droplets are converted into ice crystals and the clouds are considered ‘naturally inefficient’.
Under normal conditions, these clouds evaporate as they descend on the lee side of the mountains resulting in the well understood and naturally occurring phenomenon known as a rain shadow. This is why areas downwind of mountain ranges, such as the Monaro Plains, tend to be much drier than on the upwind side.
To improve the snow making efficiency of these clouds, additional particles can be introduced into the clouds. The excess SLW droplets freeze onto these particles forming ice crystals which grow and fall to the ground as snow. This process is known as glaciogenic cloud seeding.
What cloud seeding agent is used and how is it dispersed?
Trace quantities of the ice nucleating or seeding agent, silver iodide, are dispersed into the atmosphere by ground based generators. There are 23 generator locations along the western perimeter of the target area. A typical cloud seeding generator is shown below.
The silver iodide is mixed with a solvent (acetone) and sprayed as an aerosol into an LPG burner. The solvent combusts, particles of silver iodide rise above the generators from the heat generated by the flame and are transported by the wind into clouds over the mountains.
Silver iodide is insoluble in water and non-bioavailable. It is used as the glaciogenic (ice nucleating) seeding material because it has similar physical properties to natural ice crystals. The particles are invisible to the naked eye (approximately 0.06 microns in diameter), and are so small that over 300 million of them would fit onto the head of a pin.
Most generators are located on previously disturbed sites, generally in very remote locations where grid power is unavailable. The sophisticated electronics, controls and protection measures are powered by deep cycle batteries, and the burner is fuelled by LPG. A wind turbine and a solar panel are used to recharge the batteries.
Generators and gas cylinders are usually deployed in March and April and recovered around October each year. Once deployed and tested, the generators are operated remotely from the Snowy Mountains Control Centre located within Snowy Hydro’s main office in Cooma. They are equipped with many alarms and protection measures to minimise any risk of environmental harm.
Each generator when operating burns 1.25 litres of solution per hour, releasing around 20 grams of silver iodide. Only very small amounts are used: Between 2010 and 2014 an annual average of 28 kg was dispersed over a target area greater than 2100 km².
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