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.
Snowy Hydro’s cloud seeding operations are undertaken throughout the winter months, nominally from May to September. Over these months, Snowy Hydro’s atmospheric scientists continually monitor meteorological forecasts for cold, westerly fronts. When an opportunity is identified, weather balloons (radiosondes) are released from Khancoban, upwind of the target area. These provide atmospheric data over the mountains ranges, including wind speed, wind direction, temperature and relative humidity. Real-time meteorological conditions, e.g. temperature, precipitation and liquid water measurements, are also monitored in and around the target area from a comprehensive network of weather stations and remote sensing facilities. Before cloud seeding may commence, there are many operational criteria which have to be satisfied. In addition, cloud seeding may not be initiated under certain environmental criteria, for example, if there are relevant flood warnings in place.