CAN WE MAKE RAIN.

Dunstan Times, Issue 3133, 4 September 1922, Page 8

CAN WE MAKE RAIN.

(By Donald Campbell Macfie, M.A

M.8., etc.) Mr C. H. Hatfield, during the drought of hist vjwmmer. excited much interest by his claim that he had discovered ways and means of producing rain in times of drought, and in South. California, where he exhibited his powers as rain-maker, he was actually paid for his services in bringing down rain. Such a wonderful discovery Ava.s difficult to credit, and Mr Carlo Salter, the superintendent of the British Rainfall Organisation of tlie Meteorological Office, threw doubt on the rain-maker's achievements by pointing out that, m nature, rainfall is produced by the elevation, with consequent expansion, of great masses of air, and that, in order to produce a rainfall one inch in excesw of normal over square ten miles, Mr Hatfield would require to' devise a means of raising 65,000,000 tons of water-vapor.

But the superintendent's argument was not quite fatal to Mr Hatfield's claim. No dOubt when air currents rise they expand, and when they expand they cool, and when they coof they are inclined to drop the water collected at lower heights and at higher temperatures, and no doubt elevation. expansion, and cooling are the chief cause of rain.

Yet they are not the sole cause or the chief cause of rain. Another very common cause of rain is the meeting of cold and warm currents of air, and the cooling of the warm current by the colder one. And another very common cause of rain is the drifting of a warm moisture-laden wind against cold mountain-tope. On Table Mountain at Cape Town, for instance, the warm wind from the sea condenses as it meets Table Mountain, even as warm breath condenses on a cold windowpane, and then there is almost perpetual mist and rain. So the western mountainous regions of Scotland and England have a, heavy ramfaiTT, because the moist warm winds from the Atlantic, and from the Gulf Stream, con.dense on the cold mountain-tops, the condensation no doubt being assisted by the cooling of the clouds as they risv upwards to cross the mountains. These are two supplementary causes of rainfall; but since Mr Hatfield can neither produce cold clouds nor cold mountains at will, it is evident these are not the means he uses to bring down rain. There is a third cause of rain, which, however, it is possible he mav use.

Air may be almost saturated with water-vapor, or even completely saturated with water-vapor, and yet it docs not condense and fall in rain, because the water-vapor lacks centres of condensation. Most of the rain which falls upon earth and sea consists of water-vapor condensed on little particles of atmospheric; dust. The particles, being very small, are practically all surface, and therefore cold, and they offer millions and millions of little cold condensation surfaces for the watervapor.

Even during the recent drought the grass in many meadows dripped with dew (which is a form of rain) because the thin blades of grass offered cold! condensation surfaces for the moisture in the atmosphere. Where condensing particles are absent, ram does not readily fall, even if the air be saturated with water-vapour. Where condensing particles are plentiful rain tends to fall, even when the air is not quite completely saturated. ft is not impossible that Mr Hatfield's secret, if secret there be. may be based! on ths last principle of condensation particles. The writer was on the South African Karroo during a season of complete drought, and 1 frequently noticed great bodies of clouds passing across the sky. sometimes almost low enough to touch tlie hot kopjes. They were, of course, currents of air laden with water-vapour, and the water-vapour was so near the rain-point that more than once a \'vw drops of rain fell. ft seemed to the writer that if the clouds could be slightly cooled, and if the water-vapour could be given extra points of condensation, a heavy fall of rain might be evoked, and he wrote to the Colonial Secretary, and urged that experiments, such as spraying the clouds with liquid carbon dioxide and with fine particles of sand or salt, should be undertaken. The liquid car-bon-dioxide would cool the air, and the sand or salt would offer condensationpoints. Sail would be particularly promising, since it is hygroscopic, and would, therefore attract water as well as condense water-vapour. Possibly, it is by sonic such method that Mr Hatfield produces rain, if the production rain wero reallv a fact.

There would probably bo no necessity to spray large areas of cloud, for, once condensation started, if. would cause cooling and further condensation. A U'w aeroplanes with a supply of dust and liquid carbon-dioxido might suffice to turn on the taps of several colossal' clouds, and to wafer a considerable area.