Page:Hunt - The climate and weather of Australia - 1913.djvu/149

Rh. 125 126. Easterly Gales through Bass Strait.—These also occur occasionally though only very rarely, and, of course, when the barometric gradient is reversed, the most common case being an intense monsoonal cyclone or dip over New South Wales opposed by a strong anticyclone over Tasmania. 12th and 13th January, 1911, provided a case of this kind, the weather in Bass Strait and even in Port Phillip being exceptionally severe. Very rough weather also prevailed on the New South Wales coast.

. 127 128. Anticyclone forced Southwards over Tasmania by Cyclone off New South Wales Coast.—The warm waters off the east coast of Australia are very favorable to vigorous cyclonic development, and many depressions reaching these waters after a journey across the continent, during which their barometric effect was very feeble, give rise to great displays of energy. But whatever their origin, whether it be directly from the tropical seas to northward, or monsoonal depressions inland, or even from the heads of Antarctic disturbances, they show a marked tendency to cling for days at a time to the coastal waters. This appears to have a twofold effect upon the following anticyclone—(a) it is partly forced southwards; (b) it is merged into a semi-circular high-pressure ridge built up on its southern sides by the cyclone and extending from southern New Zealand across the ocean to Tasmania, and then north-west or north towards the Northern Territory or the Gulf of Carpentaria. The cyclone shown on the chart of 15th July, 1912, apparently originated near Alice Springs on the 12th, where thunderstorm rains were then falling and evidence of the beginning of a separate low centre being given, became cyclonic off Port Macquarie on the 14th, and held the "High" in the position shown for four days (14th to 17th inclusive).

. 129 130.

Cold Snap (South-eastern Australia).—The cold weather of 9th and 10th October, 1910, was phenomenal. It began almost simultaneously in South Australia and Victoria on the 9th, when heavy snowfalls during the afternoon were almost general from Spencer Gulf to Gabo Island.

The chart of the 10th, with its cyclonic centre off Gabo Island and steep gradients, indicates very unsettled and rough weather conditions, which were sufficiently illustrated in Melbourne during the early morning by heavy hail storms, vivid lightning, and sharp peals of thunder. Moreover, the great length of north and south isobars which overlay Eastern Australia and extended through at least 30° latitude favoured great transference of cold air from Antarctic seas, even into tropical Queensland. It is doubtful, however, if it can be taken as sufficient demonstration of the cause of the heavy snowfalls and exceptional cold, the snowfall and greatest drop in temperature in Victoria taking place in the afternoon of the 9th, when winds were still westerly. The minimum at Adelaide fell to 36.8 on the night of