Page:Hunt - The climate and weather of Australia - 1913.djvu/163

Rh only low-pressure system being an Antarctic passing eastward over and south from Tasmania, and a large monsoonal depression covering the western and northern interior of the continent. The two were separated by a large area of "flat" barometric pressures, never varying much from 30.0 inches. This is, of course, known to be rather favorable to the development of local squalls and thunderstorms inland, but these do not seem to have occurred, at all events in any isolated fashion, as no rain fell in New South Wales and very little in Victoria. On the morning of the 12th a "low," apparently cyclonic, was centred off Robe, where the barometer reading had fallen during the 24 hours from 29.99 to 29.52, nearly half-an-inch. The formation of this storm centre seems to be intimately related to the monsoonal depression of the preceding day, as a well-marked trough along the front of which northerly winds blow runs well beyond Alice Springs. These winds would necessarily be hot—at 9 a.m., 97° at Alice Springs; 95° at Farina; 90° at Broken Hill, &c.—and the rapid fall of pressure to southward producing steep gradients made them strong. Next day's chart shows the intensification still in progress. Gradients were very steep, especially in rear of the trough, the fall in pressure from Adelaide to Southern Tasmania being exactly 1 inch. Hence wind strength did not lessen at all with change of direction as the trough passed. All the conditions necessary to maximum dust production were, therefore, present—(1) preceding drought; (2) great heat; (3) strong and variable winds; (4) no rain with the storm itself. Three months later, on the 14th February, 1903, a very similar storm occurred, dust being carried upwards to such an extent that the clouds of alto-cumulus level became of a curious copper colour, and the rain which afterwards fell was at first loaded with dirt. In many parts of Victoria remote from the inland plains the surface of the ground took on a different tint owing to the foreign matter deposited upon it. The amount of surface soil transported in this way during this summer was undoubtedly enormous. Another effect of these wind storms was a redistribution of plant life. In the Mallee the number of new varieties of grasses and small flowering plants springing up after the next winter's rains was a matter of frequent comment.

. 151, 152, 153. Fogs.—Although fog is not usually regarded as one of the most important meteorological phenomena, its occurrence often has a very practical bearing upon the affairs of life. In the country, and especially among the hills, it may provide no inconsiderable portion of the moisture supply, while at other times its formation when destructive frosts are threatened is very welcome. In large cities its value is not so apparently positive, and from some aspects becomes very decidedly negative. This is seen in its effect upon the speed of traffic and consequent hindrance to business. What its value may be in providing excuses for tardy arrivals in public offices is, of course, another matter. Fogs are very local phenomena, the general causes in the way of pressure, distribution, &c., which favour their development in one locality not being at all effective in others. One factor must, of course, be present in every case; that is the presence of water vapour in the lower stratum of air in