Le Fox and I, listening to BBC's wonderful Radio 4 this morning were rather struck by the assertion by someone or other that China had used various technology to make sure that there woud be no rain during the Olympics. Slightly bemused, I decided to check up what this was all about.
It turns out that indeed, the authorities in charge made sure of good weather artificially using the technique known as 'cloud seeding'. The basic idea seems to be to introduce various chemicals into clouds which contain supercooled water to form ice crystals which then cause rain. One such checmical is silver iodide (AgI) which apparently has a structure similar to ice crystals and thereby induces so called heterogeneous nucleation. For your viewing pleasure, here's a demonstration of spontaneous freezing of supercooled (colder than freezing point) water:
It turns out that actually a great numer of countries use this technique to induce precipitation from clouds, either using aeroplanes fitted out with special tanks of silver iodide (or dry ice) or using anti-aircraft guns to fire canisters into the clouds. China spends $90 million per year on cloud seeding, citing an instance in which clouds in Tibet were seeded giving an inch of snow as a major success. Such techniques, as far as I understand are used merely to induce rain to fall from existing clouds rather than to form clouds themselves. In the week preceeding the olympics huge numbers of canons and rockets were used to fire large quantities of silver iodide into clouds abover Beijing. This had the dual consequences that heavy rainfall beforehand meant that the olympics would be rain free and that the ostensible pollution for visitors would be greatly reduced.
It's a rather natural question to wonder about the environmental consequences of such meteorological manipulation. Looking around on the internet, it seems that high levels of silver are relatively harmless to humans. The only symptoms experienced by people with a high exposure to silver in factories is a series of respiratory problems, mostly caused by powdered form of the silver rather than any chemical reaction. One slightly bizarre symptom of long-term exposure to silver is so called argyria, a condition whereby silver is deposited in the skin of the sufferer, giving them a bluish-grey tint. This seems to mostly affect ingesters of so called colloidal silver, which was widely used as a cure-all around the world. Prolonged exposure leads to such cases as Paul Karason featured below. Indeed, although there is no particular danger to his health, his skin will remain blue for the rest of his life. Apologies for the slightly cheezy reporting style.
Rather more significantly, silver plays an important role in the potassium/sodium cycle in fish by inhibiting the action of a regulatory enzyme. Interference with this cycle causes fish to take large amounts of water into their tissues very quickly leading to cardiovascular collapse.
Though the idea of cloud seeding seems fairly cool in many respects, especially in countries where agriculture is heavily dependent on consistent and predictable rainfall, it seems that the environmental cost to aquatic life at least indicates that some tempering of the process ought to be necessary.
For some more info on the whole process check out these links:
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