Page:Hunt - The climate and weather of Australia - 1913.djvu/94

Rh The tendency for the rainfall to vary directly with the range of pressure is also evident, but the balance of probability that this will be so is even less than in the preceding case. In May, July, and August large pressure ranges favour increased rainfall; the other months are almost indifferent.

The winter rainfall of Australia, or, at all events, that of its southern portion, occurs in connexion with barometric depressions of various types which may be classified as follows:—(1) Antarctic V-depressions, i.e., storms the centres of which pass too far south to be indicated by closed isobars and which, in many cases, seem to be of such vast extent that they may almost be regarded as portions of the great southern low-pressure belt. If rain-producing inland their northern isobars take the form of inverted "Vs," though some may open out to northward so as to define a barometric trough running well into the interior of the continent, or even connecting with the tropical low-pressure belt. (2) Antarctic cyclones, circulatory systems originating over southern waters and passing from the Bight over South-eastern Australia, or through Bass Straits. (3) Tropical depressions shown isobarically by (a) dips southwards from the tropical low-pressure belt into a high-pressure system south of or over southern parts of the continent; (b) troughs connecting with and originating in the tropical low-pressure belt; (c) cyclonic systems of tropical origin.

Tropical depressions when well developed are much the most productive of good inland rains, and judging by their cloud circulation are caused by southward flows of the atmosphere of wide extent and considerable depth. The first form of depression, however, "Antarctic disturbance," is much the most frequent in winter, and when it is supplemented by trough-development extending well into the northern interior brings much rain to the inland areas of South Australia, Victoria, New South Wales, and even Queensland. But, in general, the absence of cyclonic development indicates a dry season even though the "Antarctics" themselves are of considerable intensity.

In the following table is shown for the several winter months the average rain per storm received over Northern Victoria when (a) all the storms are of simple Antarctic type; (b) when some of the storms experienced were cyclonic in form or tropical in origin; also the monthly rainfall when (A) all the storms were Antarctic; and (B) when some were cyclonic or of tropical origin. This method of grouping is adopted owing to the impossibility at present of determining the rainfall for each storm separately:—