The following are some of the factors which have to be considered in discussing this phenomenon:—
(1.) The effect of the friction of the land mass, combined with differences of latitude, in gradually tilting the axis of the Λ-depression, so that as it passes New South Wales, it tends to lie N.W. and S.E.
(2.) The differences between the wind forces over land and water for the the same barometric gradients.
(3.) The convectional effects of the evaporation from the warmer waters off the East coast of Australia.
(4.) The effect of the ridges of the Kosciusko Ranges, which attain a height of over 5,000 feet for nearly 100 miles, in delaying the Eastward advance of the "highs" and "lows."
That "lows" coming from the West are intensified as they reach the warm coastal waters of New South Wales is a matter of common observation, as is the tendency for cyclonic storms to form there, and persist for days at a time. This is probably due to factor (3) above. If we assume that the Northern portion of a passing Antarctic depression undergoes a decided deepening from this cause, and that the axis of the depression is tilted slightly towards the N.W., as suggested in (1), the winds in rear of the trough will not only be strengthened by the fall in the barometer, but will come from a point sufficiently near Southerly to blow mostly over water. The two factors will, therefore, assist one another in increasing the force of the Southerlies.
As regards the influence of the Kosciusko Ranges, it may be premised that they have a greater retarding influence upon the "highs" than the "lows," since the increase of atmospheric density downwards near the surface is greater in "highs" than in "lows," and this may cause the change to anticyclonic high pressures along the New South Wales coast to be delayed slightly, and then to be more sudden than elsewhere, causing a temporarily steep barometric gradient on the rear side of the trough with corresponding wind force. Some influence may be traceable also to the contrast between the cooling effects of the Alpine summits, partly covered with melting snows, and the abnormal warmth of the strip of country lying between the mountains and the coast.[1]
XII.(a).—FORECASTING.
In framing official predictions of the weather the daily synoptic and isobaric chart is the chief evidence relied upon.
To forecast with any degree of accuracy requires considerable experience, a knowledge of the distribution of the normal local climatic elements, and a due appreciation of the significance of the varying features of atmospheric systems and the degree to which they are affected by latitude, longitude, the seasons of the year, and, at times, even by the prevailing conditions of the land surface over which the atmospheric systems are passing.
The methods of working in the Commonwealth Meteorological Organization are similar to those adopted in all kindred scientific institutions in other countries.
- ↑ For fuller discussion of this subject see "Essay on Southerly Bursters," by H. A. Hunt, Proceedings of the Royal Society of New South Wales, 1894.
