Results

Experiments in Victoria indicated that frontal clouds are unsuitable. The dry north-west winds from inland, ahead of a cold front (Chapter 13), already contain too many nuclei in the form of dust particles, whilst the maritime winds behind the front carry cool clouds whose ice crystals splinter, again leaving no shortage of nuclei. The only clouds there worth seeding are stratus (Section 8.5) associated with 'closed lows' (Chapter 13). Such stratiform clouds were found to occur on twenty-five days during a three-year study in the northern wheatbelt of Western Australia, enough to make the difference between a farmer's ruin and prosperity. Unfortunately, the occurrence of usable clouds is variable; there were fifty-eight days with cold clouds containing adequate water near Melbourne during May-October in 1991 and 1992, but merely fifteen in the previous two years.

It can be seen that there are several problems with cloud seeding, as follows:

1 Suitable clouds may be rare.

2 Seeding of unsuitable clouds actually reduces the rainfall (Note 9.E). If the number of active nuclei in a cloud is already high, adding more simply increases the number of drops that are formed, reducing their average size perhaps to the extent that they become too small to fall to the ground as raindrops.

3 It is hard to determine the benefit with absolute certainty; sophisticated statistics, with many repetitions of the experiment, are needed to discern any enhancement of rainfall (Note 9.F).

4 Seeding at the wrong stage of the growth of a raincloud may thwart its development. Latent heat is released suddenly when supercooled droplets are frozen by seeding, which intensifies the updraught. As a result, the cloud extends upwards to levels where the air is cold and therefore dry, causing the ice crystals to sublime away.

5 There is the complication of inadvertent cloud seeding by air pollution or the extra convection caused by urban heating; there are claims of a 40 per cent increase in summer rainfall at La Porte, downwind of Chicago, and an increase of 10-17 per cent is measured 5-25 km downwind of St Louis in Missouri (Chapter 10).

6 It appears that seeding is most effective where natural rainfall is high already, at least in Australia. There, it is carried out routinely only in Tasmania, already the wettest state.

7 A farmer downwind of cloud-seeding operations might claim damages if he lacks rain or is flooded by excess. Then the cloud-seeder has either to plead ineffectiveness or accept the blame.

8 Cloud seeding can yield only marginal results because it does not affect the factors basic to rainfall—atmospheric instability (Section 7.1), moisture content and low-level convergence.

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