Page:The climate of Western Australia, from meteorological observations made during the years 1876-1899.djvu/22

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backs slowly towards the N. Within a few hours, rarely more than a day, it veers to the N.W. again, with a renewal of the stormy conditions, and this may be repeated for many days before the end of the "low" area passes Eastward and a well-defined "high" appears.

From October to December the weather is in a transitional state. The summer type is endeavouring to assert itself, but winter "lows" not infrequently pass along, giving a very mixed set of weather maps, and causing rather uncertain weather with scattered thunderstorms and capricious showers.


THE SUMMER TYPE (Overland "Lows" AND "Willy-Willies").

In normal summer weather there is usually a moderate "high" extending along the South coast, or over the ocean still farther South, with falling gradients thence Northwards. This condition is interrupted by two kinds of disturbance. A "low" of the winter type may pass along well to the South-ward, or a "low" may come down from the tropics. After what has been already said, the former kind may be dismissed in a few words. It is usually preceded by great heat and followed by a gradual cool change, with probably a few coastal showers.

The latter kind of disturbance is the more distinctively summer one. It is generally an amorphous sort of affair, with shallow gradients and badly defined outlines, which seem to push generally downwards from the North coast against the Southern "high." Frequently the "high" refuses to give way, when it will back off to the coast line again and lie in wait for a better opportunity, or perhaps endeavour to cross into the Eastern colonies through central Australia. Sometimes the "high" appears to split and move Eastward and Westward, leaving room for the "low" to pass through and so on to the Southern Ocean, where it behaves like one of the ordinary winter type. But whichever course it follows it proves to be distinctly objectionable. It is generally preceded by a most undesirable increase of temperature, and accompanied by terrific heat and scattered thunderstorms. These characteristics are especially noticeable inland, and help to cause the disagreeable summer features in the climate, of which more presently. When the "low" succeeds in travelling right across, it is usually followed by a refreshing cool change, but when it is blocked and retreats, the weather remains in a sultry and unsatisfactory condition.

A series of maps (1899, January 16-20) illustrates the passage overland of an unusually vigorous "low." The high temperatures were absent in this case, because this series represents the second attempt. A few days previously it had apparently opened up an avenue right down to the South coast, but was unable to travel any further and forced to retreat. On this former occasion temperatures had ranged up to 110° in the shade. It is just possible that it succeeded in getting past and round the Southern side of the "high," and is represented by the "low" off the South-East corner of Australia on January 16th. Anyhow, the interior was left in a very unsettled condition, and the passage of the disturbance from the N.W. coast, on the 16th, to Tasmania, on the 20th, is distinctly marked, and was accompanied by heavy rain storms throughout. The general weather remarks at the end of the series give the weather in Western Australia only, but on the morning of the 18th Sir Charles Todd, Government Astronomer for South Australia, reports: "As anticipated, we have had general rains, extending as far North as Powell's Creek (lat. 18°), etc."; and on the 19th Mr. P. Baracchi, of Victoria, reports: "Light to moderate rain recorded generally, and heavy over the central, N.W., W., and N."

This series is an exaggerated instance of the type we have been discussing, but it was necessary to choose an unusually severe storm area for graphical representation, as our observing stations in the interior are so very few and far between that it would be difficult or impossible to trace an average one.

Occasionally the tropical "low" assumes the character of a tornado, and is then known locally as a "willy-willy." This bursts with great fury upon the N.W. coast, and is often felt for some little distance inland. Sometimes it can be traced in a mild form across the continent, but is frequently lost to sight after its principal outburst. It is by no means an unusual thing to find traces of its progressive path down the N.W. coast; the centre keeping out to sea. In about latitude 20° it apparently recurves and moves direct upon the coast line, causing great destruction if it happens to strike a township. The barometer does not as a rule fall below about 29·4 or 29·5, except close to the centre, where it is sometimes below 28 inches. A very severe "willy willy" visited the N.W. coast at the end of March and commencement of April, 1898, the progress of which can be traced on the accompanying series of weather maps (March 28th–April 2nd). After 9 a.m. on the last day the barometer commenced to fall very rapidly at Cossack, but a full description appears in the "introduction" to the annual report for 1898, which is here reproduced.