Extending water resources
Extending water resources

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Extending water resources

5 Rain-making

'Everybody complains about the weather but nobody does anything about it.'

(Mark Twain)

These days, Mark Twain would be wrong; many people are trying to change the weather, mostly to influence precipitation. Attempting to induce an increase in precipitation by artificial means is called rain-making. Clouds consist of minute droplets of water, but not all clouds produce rain, and when it rains, it doesn't always pour; only a small fraction of the water droplets in each cloud reaches the ground as precipitation. So the idea that human intervention — a rain-dance, perhaps — might encourage a cloud to give up a little extra water has been around since ancient times. More recently, would-be rain-makers have attempted direct intervention, by delivering various chemicals from aeroplanes in an effort to wring more rain from the clouds, a practice known as 'cloud seeding'.

There is no possibility of rain-making in cloudless arid areas; the main condition for rain-making is to have water in the atmosphere as clouds. For rain to fall, the water droplets in clouds must condense around small particles of solid material, until it forms drops heavy enough to fall as rain. If there are no solid particles to act as nuclei for condensation, there will be no rainfall. Cloud-seeding supplies nuclei around which condensation can begin. This will only work for clouds where the water content is high enough for the air to be supersaturated — and the warmer the air, the more water droplets it can contain before the conditions for precipitation are reached. The substances used to seed clouds are commonly silver iodide, common salt or dry ice (solid carbon dioxide). The substance is released into the cloud from the ground (Figure 8), aircraft (a more expensive method) or rockets.

Figure 8 Ground generator of silver iodide for cloud seeding at Bridgeport, California, USA.

Rain-making was first developed in the 1940s. One of the first times it was used successfully was in Ontario, Canada in 1948. A large fire was raging in Northern Ontario. When dry ice was seeded into clouds above the fire a rainstorm developed that helped put out the blaze.

This method was also used to make snow. In Saint Moritz, Switzerland, in 1949, a hotel owner needed snow for his skiing guests and hired a 'cloudbuster' to make it. Dry ice was thrown out of an aeroplane above clouds over Saint Moritz. Snow did fall, but unfortunately for Saint Moritz it came down to the north instead, over the ski runs of Davos, a rival town.

The effectiveness of rain-making is difficult to evaluate. It is hard to tell, for example, how much rain would have fallen anyway; rainfall often has a high natural variation from year to year. In order to be certain that cloud seeding is really increasing rainfall, researchers have to compare seeded clouds with similar, unseeded ones, which is very difficult to do. In 2003 the National Research Council of the USA concluded that there was still no conclusive proof that cloud seeding works. The greatest scepticism involves summer rainfall, where rain-making seems least likely to be successful. But the Research Council noted a strong suggestion of a positive effect on winter precipitation in mountain areas. So ski resorts regularly seed clouds to boost snowfall. Power companies, farmers and cities do it and bank water for later in the spring thaw. The Snowy Mountains Scheme in Australia also uses it in winter to increase reservoir levels.

So despite no conclusive proof that it actually works, anecdotal evidence of the effectiveness of rain-making has led to its adoption in more than 40 countries around the world. Part of the reason for its use must be that the extra water it produces is very cheap in comparison to other methods mentioned in this chapter, of the order of US$0.001 m-3, about a thousand times less expensive than desalination, for example. This is mainly because rain-making does not need a large capital investment.

So why isn't rain-making used more often? The main reason is that if the clouds are not there, you cannot seed them, so this cuts out most of the arid areas of the world where it would be of most use. Another reason is that it may not work for summer rainfall. There is also a problem of identifying whether it has worked; in 2000 in Utah, USA, for example, cloud seeding projects were estimated to have increased snowfall by between 7 and 20% — a lot of valuable water — but within the annual variability of precipitation.

Another concern is that artificially removing water from the atmosphere in one area may reduce the precipitation elsewhere; rain-making may simply redistribute the precipitation. But this is to regard the atmospheric system as a fixed pool of water, which it is not; the atmosphere is a dynamic system and does not behave as simply. There seems to be no evidence of a decrease in precipitation downwind of rain-making projects.

Yet another concern is that if a cloud is moving when it is seeded, the water may not be precipitated at the intended place, as we have seen for Saint Moritz. However, this is not a problem if seeding is done to supply a large area.

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