Cloud seeding should not – as some would have us believe – be put in the same category as a ‘rain dance’. Many countries are having varying degrees of success with the technique to encourage the skies to open up.

The science of cloud seeding is not new. People been investigating ways of getting rain clouds to part with their moisture bounty for more than 50 years.

Alex Nazarov is principal scientific officer for Hydro Tasmania (Australia) and has been working mostly in the field of hydrology since 1979 – the past three years full-time with Hydro Tasmania.

“Successful cloud seeding needs a special ingredient – clouds,” he says. “More specifically, clouds with a large water vapor content and a low temperature.”

Hydro Tasmania has a 100-year history in power generation engineering and dam construction and has 29 hydropower stations and more than 50 large dams. Hence, good rainfall is paramount for its business. Its total generating capacity is 2568MW and its assets are valued at more than US$2 billion.

In the natural rain-making process warm, moist air rises into the atmosphere, begins to cool and forms droplets of water. Cloud droplets form around cloud nuclei – tiny particles of dust, salt, or soil that are constantly present in the atmosphere.

Cloud droplets then group together in to clouds (it takes millions of cloud droplets to form one raindrop), which can precipitate in one of two ways.

In higher temperatures, droplets in the clouds merge with countless other droplets and become heavy enough to fall as rain. In lower temperatures, the droplets form ice crystals. Other droplets freeze onto these ice crystals, which grow larger and heavier until gravity takes charge and they fall to the ground as rain, snow, or hail, depending on how much they have warmed in the process.

Cloud seeding is not as basic as it sounds – in fact, it’s a complex process. In the simplest terms, it means introducing other particles into a cloud to serve as cloud condensation nuclei and help in the formation of precipitation.

When considering the science of cloud seeding, many people visualise small aircraft flying through clouds dropping magic rain-making ‘bombs’. The process can involve aircraft, but these planes often have special burning flares attached to their wingtips that douse the cloud with hydroscopic particles of silver iodide or dry-ice pellets. Or the aircraft can spray the chemical from beneath their wings. This is the most expensive method of making rain but it can cover a wide area.

Another method is to fire large shells containing silver iodide from old anti-aircraft guns or rocket launchers – the current practice in many parts of China. Another alternative is to burn silver iodide flares on hilltops where convection currents in the air can carry the payload upwards. Moisture in clouds collects around the chemical particles until it is heavy enough to fall. Of course, using the best method to suit local weather patterns is crucial to success.

Nazarov says cloud seeding has several applications, including precipitation enhancement, reduction in hailstorm damage, fog dispersal and even firefighting. More than 30 countries including the United States, Australia, Morocco, Burkina Faso (West Africa), Thailand, Indonesia and China are using or trialing cloud seeding for a variety of reasons.

Most countries are investigating cloud seeding to help fill storages for irrigation, potable water supply and electricity generation, but there are other purposes.