Page:Hunt - The climate and weather of Australia - 1913.djvu/160

Rh. 146 147.

Development of a Monsoonal Trough.—24th and 25th March, 1913. The way in which what appears to be an ordinary monsoonal dip in the isobars may under convectional action develop a barometric trough cutting the high to southward in two and joining the tropical and Antarctic low pressure belts is well shown here. Such an occurrence is practically always associated with rain, frequently heavy inland, and this, contrary to the usual experience inland, may be accompanied by southerly winds. The most reliable indications of rain in such cases are widespread cloud formation in the "dip" and a flow of upper air, as shown by the movements of cirrus or other high-level clouds over Victoria from some northerly point.

. 148, 149, 150.

Great Dust Storms.—11th-13th November, 1902. The raising of dust in Australia in summer time can hardly be said to require any special type of weather. That it be windy is usually sufficient. There are, however, seasons in which the production of dust becomes specially easy. Such was the spring of 1902, when the rainfall inland over large areas was so scanty that the surface soil once broken never became sufficiently compacted again by moisture to prevent strong winds, especially those of variable direction, from carrying it away. Many carefully worked fallowed lands in the northern parts of Victoria were practically swept bare of the loose soil mulch which had been so laboriously prepared by cultivation to conserve moisture and provide plant food for the next year's crop. Many a boundary fence was obliterated and lane filled to a depth of several feet by wind borne sand and loam during this year. In New South Wales the same occurred over large areas of purely pastoral country.

The charts for 11th, 12th, and 13th November, 1902, show all that barometer readings can tell us of the genesis and development of the worst of these dust raising storms. The morning of the 12th in Melbourne was beautifully fine, calm, and pleasant, the sky only partly covered by thin cirrus clouds moving from W.N.W. About 1 p.m. the calm was broken by a violent burst of northerly winds, which an hour or two later gave place to almost equally violent westerlies, and these to fierce north-westerly squalls a little later. The dust was at times suffocatingly dense, and the upper air was so loaded with it that the sun was rarely visible. In the country the effects were much more marked. At many inland towns the darkness produced almost equalled that of the blackest night, and in the houses nothing could be done without lamps or other means of lighting. Added to this were some phenomena of an even more terrifying character. At Boort and in some parts of the Riverina the storm was accompanied by a sort of globular lightning, "fireballs" were seen falling on the fields and roads, and scattering the earth. As these electrical phenomena were produced in a dry atmosphere the assumption is that they were due in some way to friction between dust particles.

The weather charts themselves are most interesting, and show that atmospheric changes and movements of a most unusual character were in progress. The chart of the 10th shows unusually quiet weather over the continent, the