Page:Hunt - The climate and weather of Australia - 1913.djvu/106

Rh (1).

During the month of August every station in South-Eastern Australia recorded above the normal fall, except the coastal fringe from the Gippsland Lakes northward. In South Australia, where the mean fall in the settled districts ranged up to more than double the normal, and in Victoria, where the greater part of the State had over twice the normal amount, this month will rank as one of the wettest on record.

The rains were not only abnormally heavy, but well distributed through the month, though the rain producing storm systems were actually few in number (only four). They moved slowly, developed well inland, and the path followed by their centres was further north than usual, all passing over or very little south of Tasmania. Each of these disturbances was marked by cold, wet, and stormy weather, but the storm tendencies of the month culminated on the 19th in torrential rains and fierce thunderstorms, which caused one of the most disastrous floods Victoria has ever experienced.

The graphs appended to this book include a series of the 9 am. weather charts (see Figs. 77—82) during the passage of an extensive storm system of cyclonic energy which traversed the whole of sub-tropical Australia, from Cape Leeuwin to Gabo, between the 16th and 21st of the month, and was the main factor in the development of the exceptional storm rains and floods now under review. Moving over an area already saturated by a period of almost constant rain—for scarcely a day passed during the previous fortnight without widespread and at times heavy downpours, especially in southern and South-Eastern Australia—its effect was most disastrous, and creeks and rivers, already swollen, were turned into raging torrents which overflowed their banks, and spread ruin and desolation over the surrounding country. The map of the 16th shows the advent of the disturbance, which had rapidly advanced from the southern Indian Ocean during the previous 24 hours, in the extreme west of the continent. Its centre was then off the south-west coast of Western Australia, the embracing isobars being somewhat irregular in shape and gradients moderate, and its rain area covered the whole of that State south from the tropic, with heavy falls near the coast (Perth, 213 points; and 23 other stations from 1 to 2 inches). Next morning (17th) the centre occupied much the same position, the gradients had lessened and the isobars indicated the tendencies to form a doubleheaded depression, and another general, though less heavy, rain was registered south from the latitude of Geraldton.

On the 18th the gradient had still further declined, but the low area had assumed a deﬁnite "Λ" shape, with its apex intruding well inland to much lower latitudes than on the previous day. This carried the rain, which was again general, on to the tropic, though the area of greatest precipitation was still in the south-west (Perth, 141; and nine others over 1 inch). The chart of the 19th shows that the storm area had made a distinct surge eastward to the South Australian coast, with an extension of its isobars into still lower latitudes, and a general intensification and steepening of the gradients. A new high-pressure area of considerable energy had developed over Western Australia, which stimulated the circulation in the rear of the disturbance, while the stationary anticyclone on the east coast and the Tasman Sea maintained velocities in the advancing side. Very disturbed