The Climate and Weather of Australia/Chapter 7
VII.—SPECIAL FACTORS CONTROLLING CLIMATE.
Before considering the vital question of rainfall, which our study of temperature and barometric changes now enables us to do, it may be well to glance at two factors of considerable importance in this connexion.
Ocean Currents certainly affect the coastal climates to a great extent, while the topography practically determines the local deviations from the general simple curves of the isohyets around the heart of the continent.
Ocean Currents.
Sub-dividing the coast into six regions, as in the case of the winds, and comparing the charts issued for the U.S.A. and British pilots, we get a fair idea of the variability of the currents in some of these regions.
Northern Coasts.—A fairly constant flow from east to west characterizes the winter (April, May, June, July, and August). It will not affect the climate of Northern Australia appreciably. During the rest of the year east and west currents seem both to occur in coastal waters.
Queensland Coast.—During winter and spring (May-October), there is a fairly permanent set to the south-east. This brings warmer waters in temperate regions. During summer streams seem to show a tendency to flow north-westward along the Barrier Reefs.
New South Wales Coast.—Generally a flow to the southward, but varying especially in midwinter. This has a tendency to warm the coast lands.
South Australian Coast.—A very general westerly drift, but in the Bight the streams vary considerably, especially in Autumn (February-June). There is a southern component in this current, so that the result is to cool the southern shores of Australia.
South-west Coast.—A fairly steady current from the south-west most of the year. In winter it appears less constant, and during April-July eddies near the coast, and exhibits a movement to the south. This is a definitely cool current on the whole.
North-west Coast.—The same condition as along the south-west coast; hence a cold-water current occurs, but not so important as further south.
Topography and its Effect on Climate.
A contour map of Australia shows that the continent is strikingly devoid of strong contrasts. Three-quarters of the land mass lies between the 600 and 1,500 feet contours in the form of a huge plateau. Of the remainder, there is a low-lying area comprising the Murray and Lake Eyre Basins, partly separated by the Flinders and Barrier Ranges, and, secondly, a fringe of land with an elevation of 2,000 or 3,000 feet, culminating in 7,000 feet at Mt. Kosciusko, extending through Victoria, Eastern New South Wales, and Eastern Queensland. Isolated elevated areas such as the MacDonnell and Musgrave Ranges in Central Australia, and others in Ashburton, Kimberley, Arnhem Land reach 3,000 or 4,000 feet, but are usually of the nature of bulges on the surface of the plateau rather than true mountain ranges.
Surrounding the central dry area of Australia, the isohyets describe almost concentric curves, any modifications being almost entirely due to variations in elevation. Thus, the Darling Ranges to a great degree account for the excellent rainfall of the south-west corner of Australia. The Flinders Range (South Australia) and Australian Alps in the south-east have heavier rainfalls than the surrounding tracts owing to their cooling effect on the air currents. Along the eastern elevated margin of Australia every ridge between large river valleys accounts for a somewhat greater rainfall. Examples of the latter type are the Peak Range and Darling Downs in Queensland. Where the eastern ranges of Northern Queensland (Bellenden Ker, 5,000 feet) obstruct the South-east Trade Winds, there occurs the heaviest rainfall (165.58 inches) in Australia. In Western Tasmania there is a superfluity of rain for similar reasons, though here, the constant "stormy westerlies" play the part of water bearers.
A brief notice may be devoted to the south-east corner of Australia where, in only 15 per cent. of the total area, no less than 85 per cent. of the population of the continent dwells. Here the contours have been very approximately charted and the occurrence of alternating areas of a drier and wetter type than normal is a very interesting and marked feature. In the map shown on Fig. 56, it will be noticed that the so-called Dividing Range in the south consists really of five or six more or less disconnected "massifs." In the north is the largest of all—The New England Plateau. Then a broad low gap (near Cassilis), of much physiographic importance, and hence specially named a Geocol, separates the first massif from the Blue Mountain massif. Another geocol around Lake George is the northern boundary of two well-marked mountain areas. These latter—the Snowy and Tindery Ranges—are separated by a long north-south fault valley, or subsidence area, which has been named the Australian Rift Valley, with its summit at the Cooma Geocol.
The Snowy Mountains continue as the Bowen Mountains in Victoria, and are separated from the more eastern massif (the Barry Mountains) by the Omeo Geocol. At Kilmore the cordillera practically finishes, though a well-marked low plateau near Ballarat prolongs the main divide to the westward.
If we now consider the distribution of rainfall, we shall recognise how closely it is bound up with this alternation of a highland and geocol.
Fig. 56.
This is perhaps shown most graphically in a table:—
| Name. | Height. | Rainfall. | |
| Wet | New England Plateau | 3,000 ft. + | 30-40 in. |
| Dry | Cassilis Geocol | 1,000-2,000 ft. | 20-30 in. |
| Wet | Blue Mountain area | 3,000 ft. + | 30-40 in. |
| Dry | Lake George Geocol | 2,000 ft. | 20-25 in. |
| Wet | Tindery Range | 3,000-5,000 ft. | 25-30 in. |
| Dry | Cooma Geocol | 2,000-3,000 ft. | Under 20 in. |
| Wet | Snowy Ranges | 3,000-7,000 ft. | Over 50 in. |
| Dry | Omeo Geocol | 2,000-3,000 ft. | 20-30 in. |
| Wet | Barry Mountains | 3,000-5,000 ft. | Over 50 in. |
| Dry | Kilmore Geocol | 1,200 ft. | 25 in. |
| Wet | Ballarat Uplands | 2,000 ft. | 30 in. |
One or two other isolated areas deserve brief mention. The noticeable effect (in the far West) of the comparatively low Grampians and Cape Otway Ranges on the rainfall is very evident. The latter, though less than 2,000 feet high, lead to precipitations of over 50 and at a few stations nearly 80 inches annually. Near Geelong is one of the peculiar locally dry areas, due no doubt to its occupying a low-lying situation, sheltered from north, south, and north-west winds.
A similar occurrence of heavy rainfall (50 inches) on the insignificant Gippsland Hills (1,000-2,000 feet) seems to have robbed the atmosphere of the moisture due to the more eastern regions around Sale. The latter has 5 inches less rainfall than the rest of Eastern Victoria.
We have noticed the dry belt in the Omeo region, and the similar but, larger dry belt in the Cooma Valley (The Rift). Behind Sydney is a very interesting patch extending from Picton to the Emu Plains, with less than 30 inches. This area lies between the Blue Mountains and the highlands about Bowral and Moss Vale. The former intercept the westerly rains, the latter those coming from the south-east.
The volcanic ranges of the Warrumbungles and Nandewars also give rise to considerable increase in the rainfall in the vicinity.
Similar relations of great local importance must necessarily obtain in the other regions of the continent, but absolutely no data of the contours is available for the comparison.