The Climate and Weather of Australia/Chapter 5
V.—RAINFALL.
Distribution of Rainfall.
Australia and Tasmania cover 2,974,581 square miles. Of this total area, 1,045,073 square miles have less than 10 inches per annum on the average; 651,961 square miles have from 10 to 15 inches; 416,135 from 15 to 20 inches; 502,929 square miles from 20 to 30 inches; 198,608 square miles from 30 to 40 inches; and 159,875 square miles have an annual average rainfall of over 40 inches.
Chief Rain-storm Types and Seasons of Rain.
The rains of Australia fall mainly in connexion with two storm types, viz., tropical depressions and southern depressions, the latter locally known as Antarctic depressions.
The former rain-bearing factor operates over two-thirds of the continent, roughly, over that portion of Australia lying to the north of a line extending approximately from Cossack on the North-west Coast to Sydney on the south-east coast, the rainy season being from December to March inclusive, and the wettest month, January.
The remaining third of Australia's area receives its rain principally through the southern depressions which operate during the autumn, winter, and spring months, with the heaviest monthly totals in June.
Convectional rains may occur at any season in the interior, and cyclonic rains visit the east coast in northern parts during the summer months, and in the winter and spring months on the Southern or New South Wales coastal regions.
A considerable amount of anti-cyclonic rain falls over the eastern coastal districts of the continent at any time of the year, when the recurring high-pressures pass in high latitudes and are actually centred to the west of Tasmania.
Rainfall—Monthly and Seasonal Distribution.
In a map showing the annual rainfall of the continent the isohyets form approximately concentric ovals around the arid centre of the continent. This arrangement does not, however, mean that the coastal regions are always wet; although unfortunately it is true that the interior is nearly always dry. As we shall see, in each month the region with over 1 inch of rainfall forms a somewhat "crescent" shaped area, whose concavity lies toward the centre. These "crescents" slowly swing round from north, through east to south, and then back again during the year.
Let us now consider the months in some detail.
In January the sun's heat effect is greatest, and the monsoon is at its height. The cold Tibetan Plateau and warm centre of Australia act in unison to produce many on-shore northerly winds in Northern Australia, and hence an average of ½ inch of rain falls each day at Port Darwin—gradually decreasing towards the south. Alice Springs (in the centre) gets about 1 inch in the month, while south and south-west Australia receive practically no rain.
In February the rain belt is less intense at Port Darwin, the monthly fall being 10 inches instead of 15 inches; but otherwise the distribution over the continent is much the same.
In March the summer rains have retreated still further north, and the southern limb of the "crescent" is beginning to cover the south-west coast. (The S.E. Trades have given the Queensland coast near Cairns over 20 inches during each of the three months considered.)
In April the winter rains are beginning to be felt along the whole south coast, though only to the extent of an inch or so during the month.
In May the southern type of rainfall is in full swing and no rain falls in the north. The "crescent" has now tilted round to face the south and its two limbs cover the east and west coastal areas.
In June, July, and August, similar conditions obtain; the rain belt (over 1 inch) having still the shape of a crescent, and covering the coast from North-west Cape around the south and up the east coast to Cooktown.
September is, perhaps, the driest month in the year; if we consider Australia as a whole. Only the extreme south-west corner of Tasmania and the Cordillera region in the south-east have a rainfall of over 2 inches. The southern rains are now moving south as the sun retraces his path from his northern limit.
In October there can hardly be said to be a rain belt, for the rain region is divided into three isolated areas. The northward bulge of the Southern Ocean in the Bight has severed the winter rain area in two, while in Northern Territory the summer rains are again creeping southward and are giving that settlement over 2 inches. For six months (since April) the north-west coast and York Peninsula have had practically no rain.
In November the winter rains are over. The "crescent" faces toward the N.E. again, and the monsoonal rains are dominating the north of the continent. In December the summer rain has reached Alice Springs in the centre of the continent and over an inch has fallen in that region.
Many interesting points in connexion with the origin of the rainfall are brought out by these monthly charts. They will be discussed briefly in the next chapter. It will have been noticed that the "crescents" face north in summer (Figs. 50, 39, 40), east in autumn (Figs. 41, 42, 43), and south in winter (Figs. 44, 45, 46), but never occupy the west coast; while the centre also is never covered by the crescentic monthly rain areas.
Fig. 38.
MEAN MONTHLY RAINFALL.
MEAN MONTHLY RAINFALL.
Fig. 52.