Today's Radio Solution from 4BC

31 January 2008

Kelsey asked us a great question today about cloud seeding. It produced a fairly long answer though - hope you enjoy it!

How does cloud seeding work and how do they actually know when it has worked?

First we need to know how rain starts. It starts as tiny droplets of water suspended in clouds. Then the droplets clump together into bigger drops (or freeze together into bigger crystals). Once the drops or crystals are big and heavy enough, they fall out of the sky. The frozen drops can melt on the way down, becoming rain, or they can fall to the ground as snow or hail.

Cloud seeding aims to jump-start this process by helping droplets to clump or freeze together when they otherwise wouldn't. To do this they usually sprinkle silver iodide from above by plane. Silver iodide (AgI) is a chemical compound that can be used in photography, as an antiseptic in medicine and for rainmaking and cloud seeding. Pictured is a Cessna Plane with cloud seeding equipment fixed to it.

To get the best from cloud seeding, weather forecasting techniques are used to detect suitable clouds in the location and target area as it is only effective in a limited number of weather conditions. A cloud must be deep enough and of a suitable temperature (between -10 and -12 degrees) and the wind must also be below a certain speed – mountainous areas seem to be the best. The silver iodide, which has a cyrstalline structure similar to that of ice, speeds up the freezing

In some places they use dry ice or propane as the gas expansion cools the air to such an extent that ice crystals can form spontaneously from the vapour phase and unlike silver iodide, the clouds don’t need ice crystals in them already.

It is difficult to measure just how much rain would have occurred had the cloud not been "seeded". The cloud did have rain in it and whether or not the seeding produced more rain is really not known. It may speed up the rain process though. However, there is credible scientific evidence for the effectiveness of winter cloud seeding over mountains (to produce snow) than there is for seeding warm-season cumuliform (convective) clouds.

The most successful seeding in Australia has been carried out in Tasmania around the Hydro Water scheme area. The perfect cloud which has been seeded should take 30 minutes to precipitate.

There have been concerns about the long term environmental effects of using silver iodide crystals and the rain containing this chemical can be absorbed by animals, plants and collect in catchment areas. The effects of this are still being debated. Not everyone is for cloud seeding as some scientists believe manipulating the natural pattern may impact on eco systems reliant on a certain amount of rain. Farmers downwind of cloud seeding events sometimes believe their rain is being stolen by seeded clouds.

In Beijing they fire silver iodide rockets into the sky where rain is desired. They plan to use cloud seeding before the Olympic Games to help clear air pollution.