The Climate and Weather of Australia/Chapter 2
II.—THE SEASONS AND THE MARCH OF THE TEMPERATURE.
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.
The March of Temperature.
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.
MEAN MONTHLY TEMPERATURE.
MEAN MONTHLY TEMPERATURE.
The north-east loop moves gradually westward and southward during spring and summer, forming finally a broad warm loop running down from the Gulf to South-west Queensland. This trend of the loop is no doubt largely due to the monsoon winds from the north, which characterize the front of the cyclonic system usually dominating North-west Australia in summer.
In April and later, the Queensland loop shrinks as the land areas cool, while the Kosciusko Alpine loop (in the S.E.) advances to its maximum in July.
Across Central and South-western Australia the isotherms move equator-wards in winter, and retreat in summer without losing their east-west direction. The 75° isotherm moves about 1,200 miles north (as winter approaches) from January to July, the cooling being quickest in April and May. (It must be noticed that our meteorological knowledge of the west centre of the continent is very meagre, and more complete data may indicate looped isotherms in the remaining quadrant of the continent, though the topography would not seem to favour such a condition.)
Details of the distribution of temperature, especially with regard to the State capitals, will be of interest.
January is the hottest and July the coldest month.
The highest temperatures are recorded over the north-western portion of Western Australia (see Fig. 14), where the maximum shade temperatures have exceeded 100° on 64 consecutive days and 90° on 150 consecutive days, the mean temperature of the hottest month being 90° and the mean temperature of the coldest 65°.
Fig. 14.
Chart showing maximum number of consecutive days with temperature reaching 90° for any one hot spell.
The coldest portion of Australia is the Australian Alps situated in North-eastern Victoria and South-eastern New South Wales, where the mean shade temperatures range from 65°, in January, to 40° Fah., in July. During exceptionally dry summers the temperatures in the interior reach, and occasionally exceed, 120°, and the same areas during the winter months are subject to ground frosts.
Taking Australia as a whole, the extremes of temperature annually, seasonally, and daily are less than those experienced in any of the other continents, and the mean temperatures prevailing are generally lower than for corresponding latitudes in the other continental land areas of the Globe. These features are due mainly to insularity and the comparative absence of physiographical extremes.
The following table gives the monthly, seasonal, and annual means and extremes of temperature for the Australian capitals:—
