Page:Hunt - The climate and weather of Australia - 1913.djvu/90

Rh Examination of the weather charts of other drought years shows that they partake more or less of the characteristics just deﬁned. For example, the Winter months of 1899, showed a very decided tendency to the pressure distribution of 1902, though tropical cyclonic storm systems off the east coast of New South Wales were much more pronounced, with the result that the coastal rains there were well above the average for that year; 1895 resembled 1888, but the "Antarctics" were stronger; 1896 favoured the 1902 type, but the tendency to cyclonic development was much greater; 1897 also tended to resemble 1902, but with occasional departures to the other type. The years 1907 and 1911 do not show any special likeness to any drought years, and seem to have been dry from various causes; 1907 can hardly be taken as a typical drought year, inasmuch as the truly drought conditions were limited to the eastern States in area, and to the spring months in time. The great deficiency was in September and October, the previous shortage not being very serious. A study of the Weather Charts for these two months reveals two tendencies, one of which, at all events, tends to relate their pressure distribution to that of 1888, the southern circulation being of pronounced westerly type. This is shown by a decided tendency to flattened "∧" depressions, so that winds in front of a storm would be more from north-west than north-east and in rear from west than south. This was probably due to exceptional rapidity of eastward storm movement, nin [sic] higher latitudes, an idea suggested by the frequency with which monsoonal troughs in connexion with Antarctic storms were given a north-west to south-east lie. The charts of 18th and 19th October are typical of these conditions, and are distinct dry Spring types. These show a high-pressure system pressing eastward over the head of the bight. This is a very unfavorable sign for rain over southern parts, as it has the apparent effect of cutting off trough connexion with the tropical low-pressure belt, and, in addition, means the establishment of anticyclonic conditions over south coastal areas. It is characterized by persistent westerly winds over southern waters. The spring of 1909 is another good example of this tendency and was similarly dry. The type exemplified in Figures 66 and 67 may be regarded as a special one characteristic of dry weather in spring, as it most frequently occurs between August and December.

Another special type, which does not seem inconsistent with a moderately humid condition of the atmosphere and yet often causes prolonged dry Spells over the inland portions of South-eastern Australia, is that already mentioned as of frequent occurrence in 1899. Here high pressures tend to centre over Tasmania, or to form an elongated system extending from Southern New Zealand to Tasmania, and thence towards Central Australia. The apparent cause of this arrangement practically always seems to be a cyclonic storm, usually of tropical origin, located on its northern side, and often off the New South Wales coast line, along which the rainfall is generally very heavy owing to the upthrust given by seaward slopes to the strong south-easterly winds in the south-west quadrant of the cyclone. Inland areas do not benefit appreciably, and as these systems tend to persist for several days at a time, and seem not only to prevent the advance of rain-bearing storms from the west, but to destroy their energy, the occurrence of a number of them during the rainy season may have a very appreciable effect in lessening the inland rainfall. See charts for 12th August, 1899, and 28th September,