Page:Hunt - The climate and weather of Australia - 1913.djvu/12

Rh Finally, it may be mentioned that the two chief gaps in the oval outline the Continent—the Gulf of Carpentaria and the Great Australian Bight—would appear to control the movement of the cyclonic disturbances to some extent, as will be noticed later.

 

The sun reaches the southern limit of his annual movement on 22nd December or thereabouts. In consequence of a slight lag in the heating effect, January is, in general, the hottest month in Australia. The year, therefore, may be divided as follows:—

Summer.—December, January, and February. Autumn.—March, April, and May. Winter.—June, July, and August. Spring.—September, October, and November.

Since Australia extends over so many degrees of latitude, its northern area obviously comes under the influence of equatorial conditions, where the four seasons are not so well marked as, for instance, in Europe. Here the major divisions are the wet and dry seasons. But there is a difference of 10° between the mean temperatures of January and July, and there is a remarkable dissimilarity between the muggy conditions in January (when the heaviest rain for the year falls) and the dry heat of July.

In the south the division of the year into four seasons is well marked, though over the greater portion of the year definite wet and dry periods are still noticeable.

A study of the mean monthly temperature charts shows a marked control of temperature by latitude, modified, however, by well-defined variations. These latter may be classed as—

Alpine Cool Loops. Hinterland Hot Loops.

Let us start in August, when the sun is moving south and approaching the equinox. The isotherms run almost due east and west, the hottest region being the north-west coast. A cold loop covering the south-east corner of Australia is due to the presence of the Australian Alps and Tablelands of Eastern New South Wales. This factor brings the 50° isotherm nearly to Dubbo, New South Wales. In September, this region of elevation is warming and the loop has disappeared, but two hinterland loops have appeared in the north—one in the north-west, behind the Pearling Coast (Cossack and Condon), and the other along the N.E. Queensland Coast—whereby the 70° isotherm is carried far south to Rockhampton. As the months advance, the north-west loop becomes a closed heated area over the Pilbara Goldfield (Nullagine, &c.), where all the summer (December, January, February, and March) a mean temperature of 90° is experienced.

