METEOROLOGISTS over the years have learned a great deal about the workings of the atmosphere, and have become highly skilled at predicting its capricious whims. They, have had little success, however, in their attempts to control the elements.
In a way this is not surprising. The amount of energy involved in even the most localised of atmospheric phenomena is on a scale almost unknown in human activities. A summer thunderstorm, for example, lasting only half an hour, releases as much energy as the burning of 7,000 tonnes of coal. Since it is impossible to apply energy to the atmosphere on this scale in any controlled way - even if it were desirable to do so and the results could be forseen all we can hope to achieve by way of changing our weather is to give nature a slight push in the appropriate direction, by adding that vital extra ingredient to initiate a process just on the point of happening anyway, but which might not otherwise have occurred.
"Cloud seeding" is a case in point. The technique originated in 1946 when an American chemist called Vincent Schaefer found that if powdered carbon dioxide, or "dry ice", were dropped into a bank of cloud from an aeroplane, it sometimes facilitated a fall of rain. The reason is that the particles of dry ice, or the silver iodide that is sometimes used nowadays, act as a catalyst, encouraging the water droplets of the cloud to change into natural ice crystals, which in turn enhance the cloud's ability to produce rain. But in practice it is difficult to assess the efficacy of such a process rain may well, occur after the operation but who is to say that it might not have rained in any case - even without this human intervention?
During the early 1980s, detailed studies were carried out in Australia, Israel and the US, to test the effectiveness of cloud seeding, and the results were analysed by the British cloud physicist Sir John Mason. He concluded that "in the case of the Tasmania and Florida experiments the evidence does not provide strong support for a positive seeding effect. Statistical evaluation of the Israeli experiment, however, provides much more convincing evidence, and suggests an average increase in rainfall due to seeding of about 15 per cent. But why the convective clouds in Israel should be more responsive to seeding than rather similar clouds in other parts of the world is not immediately fear." And thus the situation stands, even to this very day.