Some countries have precipitation problems that are the opposite of ours. They are exemplified by an Australian epic, which runs in part as follows:
"If rain don't come this month," said Dan,
And cleared his throat to speak,
"We'll all be rooned," said Hanrahan,
"If rain don't come this week."
During the last half-century considerable effort has been expended on techniques to enhance natural rainfall in areas susceptible to drought. It is a daunting task: the energy involved in even very small-scale atmospheric processes is so vast that any human input can only act as a catalyst, intended to trigger something which was, perhaps, about to happen anyway.
A case in point is the technique of "cloud-seeding". It 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 resulted in a fall of rain.
The reason is that the particles of dry ice, or the silver iodide that is sometimes used nowadays, acts as a catalyst to encourage water droplets in the cloud to change into natural ice crystals, which in turn enhances the cloud's ability to rain.
A difficulty arises, however, when trying 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 happened anyway, even without this human intervention?
During the early 1980s, detailed studies were carried out in Australia, Israel and the US. The Israeli results were encouraging, suggesting an average local increase in rainfall due to seeding of about 15 per cent; the analyses elsewhere proved inconclusive.
Cloud-seeding has recently been resurrected by a team led by Dr Roelof Bruintjes of the US National Atmospheric Research Centre, involving experiments carried out in Mexico. Traditional techniques suffered from the difficulty of deploying the seeded particles throughout a sufficiently large volume of a cloud for them to be effective.
Bruintjes's method is to allow the clouds themselves to do the work, by spraying the particles from an aircraft flying underneath potential shower clouds, and letting the updraughts in the clouds effect dispersed delivery. Since injection is at low level where temperatures are not too far from zero, hygroscopic particles of natural salts, instead of ice, are used as catalysts; they rapidly absorb moisture from the cloud to form nuclei on which larger drops can grow.
The results, they say, are promising - but there is a potential downside:
And every creek a banker ran,
And dams filled overtop;
"We'll all be rooned," said Hanrahan,
"If this rain doesn't stop."