Every cloud has a silver (iodide) lining

Every cloud has a silver (iodide) lining
The haze is caused by widespread slash-and-burn land-clearing practices in Indonesia. The Indonesian government has engaged cloud-seeding crew to induce rainfall from clouds so that the haze pollution is kept under control. But cloud-seeding provides only a partial solution as most forest fires burn underground, and need heavy, continuous rainfall to be completely doused.

The Indonesian government has sent two aircraft and is readying a third for the cloud-seeding operations. Singapore has offered the use of one aircraft for cloud seeding as part of an aid package. Indonesia said it would consider the aid.

2. CLOUD-SEEDING CHEMICALS

Chemicals are released from the aircraft into the clouds below. A common substance used is silver iodide. Other substances such as dry ice, calcium oxide, propane and butane have been used too.

3. CLOUD REACTION

Clouds form when water droplets condense around ice crystals in the sky. Rain falls when the clouds hold too many water droplets. The chemical particles mimic the function of the ice crystals so that water condenses around them, increasing the likelihood and amount of rain to fall.

How effective is cloud seeding?

While cloud seeding has been shown to create water droplets in clouds, the air under the clouds is often too dry, which causes the rain that falls to evaporate before it hits the ground. So cloud seeding can work, but the results so far have been largely disappointing.

Would it be better to cloud seed over Singapore to clear the smoke, or over the fires in Indonesia?

Inducing rain over Singapore would be only a partial solution, as many smoke components are not soluble and would not be dissolved in the falling rain. As soon as the rain stops, wind would simply bring in more haze from Indonesia.

While it would be more effective to put out the source of the fires, cloud seeding over them has its own problems. The clouds moving over and away from the fires would have smoke particles embedded in them, resulting in smaller water droplets being formed and reducing the chance of rain.

Also, many of the fires are peat fires that burn underground.

Are there any health concerns?

There have been concerns raised over some of the chemicals used in cloud seeding, but there is nothing definitive. Certain chemicals used in cloud seeding (such as calcium oxide) have been shown to cause skin problems to the cloud-seeding crew in aircraft.

? Information adapted from an interview with Associate Professor Simon Watts of National University of Singapore's Environmental Research Institute

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