The Climate and Weather of Australia/Chapter 9
IX. CHARACTERISTICS OF DROUGHT YEARS IN AUSTRALIA.
As would naturally be expected, years drier than normal in Australia are, in general, years with air pressure above normal, and vice versâ. A comparison of the barometric and rain records for the various capital cities bears this out, though in some cases with only moderate emphasis. For example, Melbourne gives 30 cases in favour of this rule and 24 against it. Sydney, 36 for, 18 against; Brisbane, 20 for to 5 against; Adelaide, 40 for to 15 against; Perth, 28 for, 9 against; and Hobart, 22 for, and 18 against. The probability of this rule holding seems to vary with the latitude, being greater as the tropic is approached. Thus the probability that this will be so, for Hobart is .55, for Melbourne .56, for Adelaide .73, for Sydney .66, for Perth .76, and for Brisbane .80.
Confining attention to the period since 1880, the information for previous years being rather too scanty for any adequate review of their peculiarities, the principal drought years affecting the inland areas of South Australia, Victoria, and New South Wales, were 1881, 1884, 1885, 1888, 1895, 1896, 1897, 1899, 1902, 1907, and 1911. In considering drought years, attention is directed rather to the failure of the rains during the critical periods for grass and crops, which may be taken as extending from 1st April to 31st October. The small annual rain totals for all but one of these constituted them drought years in any case, the exception, and a very remarkable one, being 1911, which, if the total rainfall alone is considered, stands out as one of the wettest years on record for Victoria, owing to two tremendous rainfalls in February and March. From July onward it was extremely dry, as were also the first five months of 1912. The years 1884 and 1885 were not universally bad, the former being very dry only in the eastern States, and the latter in South Australia.
The following tables show for Melbourne and Adelaide the mean pressure and temperature departures from the normal for each month of these years. As may be seen from these, considering the year as a whole, for Melbourne, 8 out of the 11 years are above normal pressure, and a similar proportion below normal temperature. The Adelaide records agree as regards pressure, but disagree altogether as regards temperature, only 4 out of the 11 being years colder than the average.
From a seasonal forecast point of view, the Melbourne temperature figures for the first three months are very interesting, and suggest the possibility of using them to forecast the character of the following nine, no less than 25 out of 33 being below the average. The Adelaide temperatures suggest a similar possibility, though not so strongly, only 20 of them being below and 13 above normal. As regards pressure, Melbourne suggests nothing, but Adelaide shows 23 above to 10 below normal for the first three months.
Similar data for Alice Springs show that, as at Adelaide, pressure and temperature are generally above normal for the drought years as a whole.
Drought Years.
Melbourne—Mean Air Pressure Departures from Normal.
| Year. | January. | February. | March. | April. | May. | June. | July. | August. | September. | October. | November. | December. | Year. |
| 1881 | + .00 | + .05 | + .04 | + .07 | - .02 | - .06 | + .16 | + .09 | + .04 | - .07 | - .08 | - .04 | + .030 |
| 1884 | - .05 | + .02 | + .06 | + .05 | + .04 | - .02 | + .15 | - .08 | + .00 | + .01 | + .05 | - .15 | + .008 |
| 1885 | + .06 | - .04 | - .04 | + .13 | + .05 | + .05 | + .13 | - .08 | + .08 | + .17 | + .08 | + .12 | + .060 |
| 1888 | + .01 | + .03 | + .01 | + .16 | + .07 | + .00 | - .08 | + .07 | + .16 | + .16 | + .02 | + .05 | + .056 |
| 1895 | + .03 | - .00 | + .09 | - .01 | + .08 | + .00 | - .11 | - .12 | - .09 | + .03 | + .09 | - .11 | - .008 |
| 1896 | - .01 | + .05 | - .02 | - .11 | + .03 | - .02 | - .14 | + .02 | + .11 | + .08 | + .09 | + .06 | + .012 |
| 1897 | - .03 | + .01 | - .01 | - .03 | + .00 | + .15 | - .00 | - .02 | - .01 | - .10 | - .03 | + .08 | + .003 |
| 1899 | - .13 | + .03 | - .06 | - .04 | - .01 | - .03 | + .15 | + .13 | + .11 | + .07 | - .11 | + .04 | + .014 |
| 1902 | - .09 | - .03 | - .01 | + .06 | + .11 | + .07 | + .10 | + .19 | - .03 | + .04 | + .02 | - .05 | + .035 |
| 1907 | - .02 | + .06 | - .03 | - .12 | + .10 | + .11 | - .07 | - .18 | - .04 | - .04 | - .02 | - .03 | - .021 |
| 1911 | + .08 | - .06 | - .04 | - .09 | - .03 | + .01 | - .00 | + .08 | - .00 | + .09 | + .00 | - .14 | - .005 |
Drought Years.
Melbourne—Mean Temperature Departures from Normal.
| Year. | January. | February. | March. | April. | May. | June. | July. | August. | September. | October. | November. | December. | Year. |
| 1881 | + 0.2 | - 1.2 | + 0.3 | - 0.6 | + 2.1 | - 2.5 | + 0.8 | + 0.9 | + 0.2 | - 1.4 | - 0.9 | - 1.4 | - 0.1 |
| 1884 | - 3.3 | - 2.2 | - 0.2 | + 0.4 | - 0.8 | + 0.5 | - 1.2 | + 2.3 | + 1.0 | - 1.8 | - 0.6 | - 2.1 | - 0.5 |
| 1885 | - 1.9 | - 1.9 | - 2.0 | - 1.4 | + 1.1 | - 1.6 | - 1.5 | + 1.7 | + 0.2 | + 0.6 | - 0.7 | + 1.4 | - 0.6 |
| 1888 | - 2.2 | - 0.9 | - 4.1 | - 1.0 | + 0.1 | + 1.7 | - 0.1 | - 2.6 | + 0.2 | - 1.7 | + 1.9 | + 2.8 | - 0.5 |
| 1895 | - 0.6 | + 3.0 | - 0.5 | + 1.0 | - 1.4 | - 0.4 | - 1.3 | + 2.2 | + 0.1 | + 3.1 | - 0.1 | + 1.6 | + 0.6 |
| 1896 | + 1.2 | - 0.4 | - 1.8 | - 0.8 | - 0.7 | - 1.1 | - 0.4 | - 1.5 | - 1.2 | + 0.4 | + 1.8 | + 2.7 | - 0.1 |
| 1897 | - 2.3 | - 1.4 | - 4.0 | - 1.2 | - 2.1 | + 0.8 | + 2.0 | - 2.1 | + 1.1 | - 1.4 | + 2.3 | + 4.3 | - 0.3 |
| 1899 | - 3.6 | + 3.2 | + 2.9 | + 0.6 | - 1.3 | - 1.0 | - 1.5 | - 0.3 | + 1.8 | - 2.0 | - 0.1 | + 2.7 | + 0.2 |
| 1902 | - 0.8 | - 2.4 | - 2.2 | - 0.7 | + 0.9 | - 1.3 | + 0.8 | - 3.1 | - 0.4 | + 0.4 | + 4.1 | + 1.0 | - 0.3 |
| 1907 | - 0.9 | - 2.6 | - 2.6 | - 1.5 | - 1.3 | - 2.1 | - 0.5 | + 1.6 | + 1.8 | + 0.6 | + 2.9 | - 1.4 | - 0.2 |
| 1911 | + 0.3 | + 1.1 | - 0.3 | - 2.4 | + 1.2 | - 1.7 | - 1.0 | + 3.2 | + 1.6 | - 0.5 | + 2.1 | - 1.0 | + 0.3 |
Drought Years.
Adelaide—Mean Air Pressure Departures from Normal.
| Year. | January. | February. | March. | April. | May. | June. | July. | August. | September. | October. | November. | December. | Year. |
| 1881 | - .03 | + .07 | + .09 | + .08 | - .01 | + .01 | + .18 | + .09 | + .04 | + .08 | - .03 | - .015 | + .046 |
| 1884 | + .02 | + .01 | + .03 | + .04 | + .035 | - .05 | + .17 | - .06 | + .00 | + .03 | + .04 | - .10 | + .014 |
| 1885 | + .06 | - .00 | + .01 | + .09 | + .04 | + .11 | + .10 | - .07 | + .08 | + .12 | + .02 | + .065 | + .060 |
| 1888 | - .01 | + .03 | + .07 | + .11 | + .05 | + .03 | - .04 | + .05 | + .13 | + .16 | + .01 | + .025 | + .051 |
| 1895 | - .005 | + .01 | + .12 | - .02 | + .12 | + .05 | - .09 | - .095 | - .075 | + .05 | + .115 | - .06 | + .009 |
| 1896 | + .00 | + .015 | - .03 | - .09 | + .06 | + .01 | - .11 | + .06 | + .12 | + .06 | + .05 | + .06 | + .018 |
| 1897 | + .03 | + .01 | + .04 | - .00 | + .03 | + .08 | + .03 | - .01 | - .00 | - .07 | + .01 | + .03 | + .015 |
| 1899 | - .02 | + .01 | - .05 | - .05 | + .03 | - .01 | + .13 | + .11 | + .09 | + .06 | - .08 | + .06 | + .023 |
| 1902 | - .06 | + .01 | + .03 | + .06 | + .10 | + .07 | + .07 | + .175 | - .01 | - .00 | + .015 | - .05 | + .033 |
| 1907 | - .005 | + .05 | + .01 | - .06 | + .10 | + .05 | - .07 | - .17 | - .015 | - .03 | - .03 | - .01 | - .015 |
| 1911 | + .03 | - .045 | + .01 | - .06 | - .005 | + .07 | - .03 | + .01 | - .02 | + .10 | - .01 | - .08 | - .001 |
Drought Years.
Adelaide—Mean Temperature Departures from Normal.
| Year. | January. | February. | March. | April. | May. | June. | July. | August. | September. | October. | November. | December. | Year. |
| 1881 | - 0.4 | - 3.0 | + 0.6 | - 0.7 | + 1.1 | - 2.4 | - 0.5 | - 0.5 | + 0.1 | - 2.4 | - 1.6 | - 0.7 | - 0.8 |
| 1884 | - 4.0 | + 0.4 | + 1.9 | + 1.2 | - 0.6 | + 0.6 | - 1.9 | + 2.2 | + 0.4 | - 2.0 | - 1.0 | - 4.0 | - 0.7 |
| 1885 | - 3.4 | - 3.2 | - 3.3 | - 1.0 | + 3.0 | - 1.9 | - 0.3 | + 1.3 | + 0.2 | + 2.5 | - 0.5 | + 2.7 | - 0.3 |
| 1888 | + 0.6 | - 3.0 | - 2.2 | + 2.2 | + 0.3 | + 1.6 | + 1.0 | - 1.7 | + 2.7 | + 0.9 | + 4.4 | + 3.7 | + 0.9 |
| 1895 | - 0.3 | + 1.8 | - 0.3 | - 0.4 | - 2.1 | - 0.4 | - 0.8 | + 1.6 | + 0.2 | + 3.7 | - 0.1 | - 0.3 | + 0.3 |
| 1896 | + 0.7 | + 1.0 | + 2.3 | - 0.3 | - 1.2 | - 2.1 | - 1.1 | - 1.1 | - 0.4 | + 3.0 | + 4.2 | + 0.3 | + 0.5 |
| 1897 | - 3.4 | + 1.4 | - 2.6 | + 0.4 | - 0.9 | + 0.6 | + 1.7 | - 1.2 | + 0.8 | - 1.5 | + 2.6 | + 6.3 | + 0.4 |
| 1899 | - 7.2 | + 4.5 | + 2.2 | + 0.8 | - 1.0 | - 0.0 | - 2.5 | 0.0 | + 0.9 | - 0.3 | 0.0 | + 0.9 | - 0.1 |
| 1902 | - 1.4 | - 3.0 | - 2.9 | + 1.9 | + 3.5 | + 0.2 | + 1.4 | - 0.8 | + 1.3 | + 2.1 | + 4.6 | - 1.6 | + 1.5 |
| 1907 | - 3.4 | + 2.0 | - 4.7 | - 2.8 | + 0.7 | - 1.2 | + 0.3 | + 1.0 | + 3.0 | - 0.1 | + 1.2 | - 3.0 | + 0.4 |
| 1911 | + 0.6 | - 3.0 | - 2.0 | - 1.7 | + 0.3 | - 0.5 | + 0.7 | + 2.7 | + 0.8 | - 1.1 | + 5.0 | - 2.8 | + 1.0 |
Drought Years.
Alice Springs—Mean Air Pressure Departures from Normal.
| Year. | January. | February. | March. | April. | May. | June. | July. | August. | September. | October. | November. | December. | Year. |
| 1881 | - .05 | - .00 | + .03 | - .00 | - .07 | + .01 | + .08 | + .06 | + .04 | + .00 | - .05 | - .00 | + .003 |
| 1884 | + .05 | - .01 | - .03 | + .01 | + .02 | - .02 | + .06 | - .06 | - .02 | + .03 | - .00 | - .03 | - .001 |
| 1885 | + .01 | + .03 | + .04 | + .07 | - .00 | + .10 | + .02 | - .01 | + .03 | + .08 | + .08 | + .02 | + .042 |
| 1888 | - .05 | + .01 | + .08 | + .04 | + .02 | + .04 | + .02 | + .02 | + .09 | + .10 | + .02 | + .03 | + .036 |
| 1895 | - .01 | + .00 | + .08 | - .02 | + .06 | + .04 | + .00 | + .00 | - .06 | + .02 | + .07 | - .03 | + .014 |
| 1896 | - .02 | - .04 | - .07 | - .03 | + .06 | - .01 | - .08 | + .07 | + .09 | - .00 | + .02 | + .01 | - .001 |
| 1897 | + .06 | + .03 | + .05 | + .00 | + .03 | - .02 | + .04 | + .01 | + .00 | - .08 | + .00 | - .02 | + .010 |
| 1899 | + .02 | + .01 | - .02 | - .05 | + .00 | + .01 | + .02 | + .05 | + .03 | + .03 | - .03 | + .05 | + .009 |
| 1902 | - .05 | + .02 | + .02 | + .03 | + .05 | + .06 | + .03 | - .08 | - .01 | + .01 | + .03 | + .02 | + .026 |
| 1907 | - .04 | + .04 | - .01 | - .00 | + .03 | - .02 | - .06 | - .04 | + .02 | - .04 | + .03 | - .02 | - .009 |
| 1911 | - .03 | - .02 | + .01 | - .06 | - .03 | + .05 | - .06 | - .03 | - .02 | + .04 | - .02 | + .00 | + .015 |
Drought Years.
Alice Springs—Mean Temperature Departures from Normal.
| Year. | January. | February. | March. | April. | May. | June. | July. | August. | September. | October. | November. | December. | Year. |
| 1881 | + 3.7 | + 0.7 | - 0.4 | + 3.4 | + 7.4 | - 1.1 | - 0.4 | + 1.5 | - 1.0 | + 0.6 | + 1.7 | + 1.7 | + 1.5 |
| 1884 | + 2.1 | + 3.7 | + 3.3 | + 0.7 | - 2.3 | + 2.1 | - 1.8 | + 4.9 | + 2.8 | - 1.1 | + 0.1 | - 0.5 | + 1.2 |
| 1885 | + 0.4 | + 0.7 | - 6.1 | - 4.1 | + 4.9 | - 3.7 | - 0.7 | + 2.3 | + 3.2 | + 3.1 | - 0.3 | + 2.0 | + 0.1 |
| 1888 | + 5.0 | + 1.1 | - 2.3 | + 4.1 | - 0.1 | + 1.5 | - 0.3 | + 1.8 | - 1.8 | - 1.4 | + 3.8 | + 4.2 | + 1.3 |
| 1895 | - 7.0 | - 3.7 | - 1.3 | - 0.1 | - 2.1 | - 0.3 | - 2.6 | - 1.5 | - 0.4 | - 2.1 | - 2.7 | + 2.7 | - 1.7 |
| 1896 | + 1.8 | + 0.3 | + 4.9 | + 2.5 | - 1.5 | - 2.7 | + 0.4 | - 5.5 | - 3.3 | + 4.5 | - 0.2 | + 5.8 | + 0.6 |
| 1897 | + 0.6 | - 0.3 | - 0.3 | + 1.7 | - 1.0 | + 2.9 | + 1.6 | - 0.1 | + 0.0 | + 4.5 | + 1.7 | + 0.8 | + 1.0 |
| 1899 | - 6.4 | + 1.5 | + 0.9 | + 2.1 | - 1.9 | + 0.3 | - 3.7 | - 2.6 | + 2.4 | - 0.8 | + 1.9 | + 0.3 | - 0.5 |
| 1902 | + 3.6 | + 3.0 | + 0.3 | - 0.7 | - 0.1 | - 0.6 | + 2.2 | - 2.3 | + 2.8 | + 1.9 | - 0.4 | - 2.5 | + 0.5 |
| 1907 | - 1.6 | - 1.9 | - 0.5 | - 1.9 | - 1.4 | - 0.5 | + 3.0 | + 0.5 | - 1.8 | + 2.5 | - 4.5 | - 1.0 | - 0.7 |
| 1911 | - 0.5 | - 1.9 | - 2.1 | + 1.1 | + 2.1 | - 1.0 | + 2.3 | + 1.7 | - 0.6 | + 1.0 | + 0.8 | + 0.1 | + 0.3 |
As the years 1888 and 1902 stand out beyond all others as drought years, a comparison between the two may be instructive. It is worth noting, to begin with, that the former was sandwiched between years of excessive rainfall, but that the latter was the culmination of a long stretch of years with low average rainfall which began in 1895. The year 1888 was characterized by extreme simplicity in weather types, there being scarcely any departure from the normal type. Pressures were higher than normal throughout the year in Melbourne and Adelaide, except in July, and expecially so in the Spring months. The low pressure systems were almost entirely of "Antarctic" type, there being few inland rains of tropical origin, even in summer, and during the seven winter months—April to October—practically no rains due to tropical incursions of low pressure systems. Cyclonic developments from "Antarctic" low pressures were absolutely wanting during the whole year, the tendency being towards a succession of high-pressure systems on the mainland, and low-pressure systems with centres far to southward passing along the south coast line. For the year as a whole, the departures from the average mean pressure were very large throughout Australia, averaging four-hundredths of an inch in excess at the six capital cities.
The year 1902 was also marked by pressures much above normal, but in other respects differed very strikingly from 1888. In 1888, the depressions followed one another over the southern waters with but little hindrance and without making many inroads upon the high-pressure systems upon their equatorial sides; in 1902, at all events during the five critical months, April to August, high-pressure systems were continually being built up over Tasmania and South-eastern Australia, apparently through the agency of large cyclonic depressions operating over Tasman Sea or New Zealand, many of which were of tropical origin. The result upon storm systems approaching from the west was generally disastrous; some were deflected too far south to affect Eastern Australia; some seemed to die out before arrival, and some were converted into feeble cyclonic circulations, which had little rain-producing effect. The last two months in both years were, however, a good deal alike, in that depressions of tropical origin became frequent; possibly owing to the approach of improved conditions.
The pressure excesses at the different capitals for the years 1902 were almost as large as in 1888, and bear out to some extent the tendency for high-pressure systems to form over Tasmania.
This would suggest two main dry weather types:—
- (a) High and low-pressure belts maintaining their respective latitudes, with little tendency to departure from normal position—probably the result of a general lack of humidity in the atmosphere. Of this, 1888 is most typical.
- (b) Unfavorable location of high-pressure systems over South-Eastern Australia and Tasmania, co-incident with the operation of great storm systems over Tasman Sea and New Zealand, and a tendency to the formation of slow-moving feeble cyclonic circulations advancing from the westward. This was most characteristic of 1902.
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, n 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, ISOBARIC CHARTS CHARACTERISTIC OF DROUGHT YEARS.
Note.—Shading shows where rain fell during previous 24 hours.
1909. Figures 68 and 69. The tendency to this distribution was very pronounced during the seven winter months of 1899, and in particular during the months of July and August, when the inland rains were very scanty.
In the following table the years are arranged in two sections—(a) and (b)—representing the two most distinct drought types, and the barometric departures from the normal for the whole year at the various capital cities are also given to show how far these support the classification. As will be seen, they do this fairly well:—
Departure from Normal Annual Air Pressure.
| Year. | Hobart. | Melbourne. | Sydney. | Brisbane. | Adelaide. |
| Type (a). | |||||
| 1888 | +.029 | +.060 | +.053 | +.039 | +.051 |
| 1895 | .. | -.004 | +.010 | -.001 | +.009 |
| 1907 | -.035 | -.017 | +.007 | -.013 | -.015 |
| Type (b). | |||||
| 1902 | +.058 | +.029 | +.032 | +.020 | +.033 |
| 1896 | +.021 | +.016 | +.017 | +.009 | +.018 |
| 1897 | +.032 | +.007 | +.015 | +.007 | +.015 |
| 1899 | +.041 | +.020 | +.020 | +.010 | +.023 |
| 1911 | +.022 | -.009 | -.008 | -.012 | -.001 |
The weather charts, Figures 60-62, illustrate the 1888 type, and Figures 63-65 show some typical of 1902 pressure distribution.
Variation of the Winter Rainfall over Northern Victoria with Mean Pressure and Pressure Ranges (1887—1911).
A detailed study of the relation between the inland rains over Victoria, the mean air pressure at Melbourne and the extreme pressure ranges as shown during the passage of storm systems over Victoria during each month, gave the following results for the seven months, April—October inclusive. "Agreement" in the case of mean pressure requires departures from normal opposite in sign to those from rainfall, and in the case of pressure ranges the same sign with those for rainfall:—
| — | April. | May. | June. | July. | August. | September. | October. | |
| Mean Pressure | Agreement | 12 | 16 | 15 | 16 | 17 | 13 | 12 |
| Disagreement | 10 | 9 | 7 | 9 | 8 | 11 | 11 | |
| Pressure Ranges | Agreement | 12 | 19 | 11 | 18 | 17 | 12 | 11 |
| Disagreement | 13 | 6 | 14 | 7 | 8 | 13 | 14 |
This shows a certain tendency for the rainfall to vary inversely with the mean monthly pressure, but it is not a very pronounced one for the first and last two months of the series, and seems to be more especially a winter relationship.
The tendency for the rainfall to vary directly with the range of pressure is also evident, but the balance of probability that this will be so is even less than in the preceding case. In May, July, and August large pressure ranges favour increased rainfall; the other months are almost indifferent.
Variations in Rainfall with Storm Types.
The winter rainfall of Australia, or, at all events, that of its southern portion, occurs in connexion with barometric depressions of various types which may be classified as follows:—(1) Antarctic V-depressions, i.e., storms the centres of which pass too far south to be indicated by closed isobars and which, in many cases, seem to be of such vast extent that they may almost be regarded as portions of the great southern low-pressure belt. If rain-producing inland their northern isobars take the form of inverted "Vs," though some may open out to northward so as to define a barometric trough running well into the interior of the continent, or even connecting with the tropical low-pressure belt. (2) Antarctic cyclones, circulatory systems originating over southern waters and passing from the Bight over South-eastern Australia, or through Bass Straits. (3) Tropical depressions shown isobarically by (a) dips southwards from the tropical low-pressure belt into a high-pressure system south of or over southern parts of the continent; (b) troughs connecting with and originating in the tropical low-pressure belt; (c) cyclonic systems of tropical origin.
Tropical depressions when well developed are much the most productive of good inland rains, and judging by their cloud circulation are caused by southward flows of the atmosphere of wide extent and considerable depth. The first form of depression, however, "Antarctic disturbance," is much the most frequent in winter, and when it is supplemented by trough-development extending well into the northern interior brings much rain to the inland areas of South Australia, Victoria, New South Wales, and even Queensland. But, in general, the absence of cyclonic development indicates a dry season even though the "Antarctics" themselves are of considerable intensity.
In the following table is shown for the several winter months the average rain per storm received over Northern Victoria when (a) all the storms are of simple Antarctic type; (b) when some of the storms experienced were cyclonic in form or tropical in origin; also the monthly rainfall when (A) all the storms were Antarctic; and (B) when some were cyclonic or of tropical origin. This method of grouping is adopted owing to the impossibility at present of determining the rainfall for each storm separately:—
| April. | May. | June. | July. | August. | September. | |||||||
| — | Number of Storms. |
Average Rain. |
Number of Storms. |
Average Rain. |
Number of Storms. |
Average Rain. |
Number of Storms. |
Average Rain. |
Number of Storms. |
Average Rain. |
Number of Storms. |
Average Rain. |
| Pts. | Pts. | Pts. | Pts. | Pts. | Pts. | |||||||
| (a) | 56 | 19 | 29 | 31 | 35 | 57 | 57 | 38 | 12 | 51 | 31 | 20 |
| (b) | 62 | 43 | 72 | 52 | 71 | 60 | 86 | 34 | 130 | 31 | 132 | 26 |
| Average Monthly Rain. | ||||||||||||
| (A) | .. | 83 | .. | 125 | .. | 222 | .. | 156 | .. | 205 | .. | 126 |
| (B) | .. | 204 | .. | 218 | .. | 264 | .. | 180 | .. | 182 | .. | 174 |
TENDENCY TO ISOBARIC DISTRIBUTION NOTICEABLE IN DRY SPRINGS.
EAST COAST CYCLONES
favouring heavy East Coast Rains, byt Dry Spells Inland.
CONTRASTED TYPES
showing increased rain production inland when "Antarctic"
is supplemented by Trough connection with Tropical Low Pressures.
(a) Wet Chart—Tropical influence pronounced. (b) Dry chart—Tropical influence absent.
Note.—Shading shows where rain fell during previous 24 hours.
TYPICAL RAIN-PRODUCING STORM SYSTEMS.
Note.—Shading shows where rain fell during previous 24 hours.
As will be seen from the table, those months in which air circulation departs from the simple Antarctic type are in general the most rainy. August certainly seems to be an exception, but this may be partly explained by the fact that only three months of August, in which all storm systems were "Antarctic," occurred during the 25 years, and also that amongst cyclonic developments are included "tropical troughs" and "dips," which may not always have extended far enough south to appreciably affect Victoria as storm systems. Figures 72-74 show typical specimens of some of the systems above referred to:—
| Ordinary Antarctic depression | 2nd June, 1911. |
| Antarctic cyclone | 2nd September, 1911. |
| Tropical dip | 22nd June, 1912. |
Effect of Air Flow from Tropical Belt.
A very common feature of good winters is the formation of trough-like depressions lying north and south and connecting with the tropical low-pressure belt. These cause extensive rains, falling mostly in front of the trough and extending sometimes right across the continent. They occasionally end by producing a strongly cyclonic circulation over South Australia, New South Wales, or Victoria, or all three, and when this happens the tropical influence seems to come to an end with cessation of trough rains and the production only of those due to the cyclonic system itself. A tongue of high pressure usually now presses from the westward along the northern boundary of the cyclone destroying the tropical connexion, and this is followed by the lessening intensity of the cyclone itself as if its main supply of energy had been cut off. This cyclonic formation and later peculiarities of development are most frequently seen when tropical influences only are operative.
To show the closeness of the relations between the winter rainfalls over the southern and south-eastern interior of Australia and the formation of these troughs over Central Australia, the daily weather charts for all the years available have been consulted, and counts made of the number of days during the six winter months (April-September), when the barometer readings at Alice Springs reduced to sea-level have been below 30.0 inches. In some of the cases the depressions would be due to purely tropical intrusions of low pressure, but it was not thought worth while to attempt to eliminate these, more especially as they would favour the production of the trough-like depression above referred to. The table shows that 19 times out of 25 the rule holds that winter rains are above or below normal according as the number of days occupied by trough depressions giving readings below 30.0 inches and passing over Alice Springs is above or below normal. Then the eight selected dry years for which charts are available give, for the number of days thus occupied, a mean only two-thirds as large as that given by the twelve wet years. A typical wet chart is that of 31st May, 1909, in which the tropical trough connexion is a prominent feature, and dry chart that of 6th September, 1907, in which the trough is wanting. See Figures 70 and 71.
It is perhaps necessary to explain here that the term "trough" is used in a special sense. It is customary to use the word "col" when referring to the space separating the isobars defining two "highs." Analogy is drawn from land contour. Two mountain ridges may be joined by a saddle or col, or separated by a valley, along which perhaps a river flows. So in considering the connexion or separation of two barometric high pressure systems we may have the "highs" contoured by isobars in such a way that the "col" is either distinctly anticyclonic, the wind circulation of the preceding high almost merging into that of the following one, or distinctly convectional and rather forming connexion between the circulations of two low pressure systems than between those of two "highs." It is for the latter type that the word "trough" is here used. In the case of a col, the isobars of the two "highs" are decidedly convex to one another along their common axis; in the case of a trough the rear isobars of the preceding "high" or the front isobars of the following "high", or both, tend to straighten and the convexity is more apparent in the isobars of the adjacent low pressure systems, producing often an hour glass appearance. The chart of 31st May, 1909, shows the straightening of the rear isobars of the preceding high very well. When a marked north and south lie of the isobars is thus produced it almost invariably happens that much cloud is formed over the trough area and rain falls to a considerable extent over inland districts, a result probably partly due to the cooling of the southward flowing mass of the air in the trough front.
The following table gives the results in detail. The rainfall is a mean derived from twenty typical stations, ten in the northern wheat-growing area of South Australia, and ten in Northern Victoria, and refers only to the seven months' period, April-October:—
| Year. | Mean of Rainfalls of North Victoria and Upper and Lower North of South Australia. |
Number of Times Alice Springs Barometer below 30.0 inches Gravity not applied. | |
| April-October. | April-September. | ||
| Inches. | Periods. | Days. | |
| All years | 11.79 | .. | .. |
| 1881 | 9.12 | .. | .. |
| 1882 | 11.86 | .. | .. |
| 1883 | 13.20 | .. | .. |
| 1884 | 10.53 | .. | .. |
| 1885 | 9.08 | .. | .. |
| 1886 | 10.22 | ||
| 1887 | 13.46 | 7 | 19 |
| 1888 | 8.23 | 4 | 7 |
| 1889 | 20.14 | 9 | 17 |
| 1890 | 15.59 | 9 | 18 |
| 1891 | 11.72 | 7 | 12 |
| 1892 | 14.25 | 12 | 15 |
| 1893 | 16.07 | 16 | 34 |
| 1894 | 15.03 | 8 | 8 |
| 1895 | 9.79 | 10 | 16 |
| 1896 | 7.75 | 8 | 14 |
| 1897 | 9.64 | 8 | 15 |
| 1898 | 12.07 | 9 | 19 |
| 1909 | 9.30 | 8 | 11 |
| 1900 | 11.22 | 10 | 14 |
| 1901 | 11.01 | 8 | 14 |
| 1902 | 5.86 | 7 | 10 |
| 1903 | 13.46 | 10 | 25 |
| 1904 | 10.42 | 11 | 17 |
| 1905 | 13.65 | 7 | 20 |
| 1906 | 14.29 | 10 | 28 |
| 1907 | 11.23 | 10 | 22 |
| 1908 | 13.86 | 9 | 13 |
| 1909 | 15.67 | 13 | 24 |
| 1910 | 14.51 | 11 | 17 |
| 1911 | 9.14 | 9 | 13 |
As the evaporation statistics might be expected to throw some light on conditions in general during drought years, the following table is appended. That the light is not particularly bright is evident, the result being confused by the fact that the rate of evaporation is dependent upon a number of factors:—
Rainfall and Evaporation during Drought Years.
| Year. | Adelaide. | Alice Springs. | Melbourne. | Sydney. | ||||
| Rain. | Evaporation. | Rain. | Evaporation. | Rain. | Evaporation. | Rain. | Evaporation. | |
| 1881 | 18.02 | 55.97 | 6.42 | .. | 24.08 | 38.66 | 41.09 | 30.69 |
| 1884 | 18.74 | 54.70 | 5.39 | .. | 25.85 | 31.59 | 44.04 | 34.45 |
| 1885 | 15.88 | 54.63 | 17.20 | .. | 26.94 | 32.37 | 29.91 | 36.51 |
| 1888 | 14.55 | 58.06 | 10.06 | .. | 19.42 | 37.92 | 23.01 | 36.33 |
| 1895 | 21.28 | 52.09 | 14.18 | 84.33 | 17.04 | 41.97 | 31.86 | 35.54 |
| 1896 | 15.17 | 56.65 | 10.42 | 90.21 | 25.16 | 38.13 | 42.40 | 35.81 |
| 1897 | 15.42 | 54.55 | 5.69 | 102.54 | 25.85 | 36.66 | 42.52 | 42.56 |
| 1899 | 18.84 | 55.55 | 6.53 | 100.02 | 28.87 | 38.63 | 55.90 | 38.56 |
| 1902 | 16.02 | 58.23 | 5.44 | 108.49 | 23.08 | 38.61 | 43.07 | 36.12 |
| 1907 | 17.78 | 53.33 | 9.81 | 100.66 | 22.26 | 40.61 | 31.32 | 38.95 |
| 1911 | 15.99 | 48.14 | 7.09 | .. | 36.61 | 38.87 | 50.03 | 37.55 |
| Mean all years | 20.62 | 54.44 | 11.09 | 97.10 | 25.60 | 38.38 | 47.99 | 36.92 |
| Drought Mean | 17.06 | 54.35 | 8.93 | 97.71 | 25.01 | 37.64 | 40.47 | 36.64 |
Evaporation means for the drought years are thus seen to be, in general, slightly below those for all years of record for the coastal stations, and only slightly above for Alice Springs, the one inland station given. The very low evaporation totals for Alice Springs for 1895 and 1896 were coincident with exceptionally heavy summer rains, and were what should be expected when humid conditions intervene at the time when evaporation is normally most rapid. Otherwise the results show the effect of the drier atmosphere of the drought years. At the coastal stations the lower evaporation, as well as the lower summer temperatures already noted, may be explained by assuming that during drought years the greater insolation over the interior produces greater and more frequent indraughts of moist, cool, sea air.
In the foregoing the rainfall statistics most frequently referred to were those obtained from a mean of the rainfalls at ten typical stations in the northern wheat-areas of Victoria, and a similar number from the northern areas of Southern Australia. These are—for Victoria, Swan Hill, Echuca, Yarrawonga, Warracknabeal, Charlton, Bendigo, Shepparton, Dookie, Horsham, and St. Arnaud; and for South Australia, Hawker, Quorn, Port Augusta, Wilmington, Ororoo, Petersburg, Appila, Crystal Brook, Burra, and Auburn.
The mean annual rainfall for the Victorian stations is 17.82 inches, of which 12.33 fall in the seven months April to October. The wettest year since 1880 was the year 1889, when 26 inches fell, and the driest 1902, when the total was only 10.94 inches, of which only 5.11 fell during the critical seven months. For the South Australian stations the mean annual fall is 15.0 inches, and for the seven winter months, 11.25. The wettest year in this area was also 1889, with 25.35 inches, and the driest 1888, with 9.21 inches, of which 7.93 fell during the growing period. In 1902, the year's total was 10.70 inches, and that for April—October 6.60 inches.
It will thus be seen that even in the worst years the rainfall over the inland areas considered is sufficient when aided by modern agricultural methods to insure at least moderate yields of cereals from extensive inland areas. That an occasional drought is not an wholly unmixed evil is seen in the unusually abundant crops harvested in the first good season, owing probably to an improved condition of the soil. The most serious effects of prolonged drought are the losses of stock, due to an almost entire dependence upon natural grasses. That these are easily preventable will, no doubt, be proved by future experience.