Page:Types of Australian weather.djvu/18

14 and rains were not so general as on the thunderstorm type, but curiously, were recorded in an almost parallel area of country, nearly three hundred miles wide by twelve hundred long. Though great barometric changes have taken place, the pressures remain about the same as on Chart No. 13.

TYPE VII. A RAIN STORM WITH VERTICAL AND NEARLY STRAIGHT ISOBARS.

This type is one of the best defined and reliable of the series for forecasting purposes, because with them, good general rains almost invariably come. They are the rear isobars of a departing anticyclone, and the wind circulation from north and north-east brings into the interior winds laden with tropical moisture to meet in the west southerly winds laden with antarctic cold, and therefore precipitating power. The sequence of rain is rendered even more certain if rain be recorded in the north-east before the isobars straighten, or in the depression to the south.

The actual height of the barometers is not material, but the greater the number of isobars in a given area, the more extensive will be the rainfall; the rain usually lasts three days. The rain begins to fall north-west of New South Wales, spreads southwards, then eastward, and finally northwards, crossing the mountains near the Queensland boundary. A fine example of this type occurred on 15th, 16th, and 17th October, 1894. (Charts 16, 17.)

On the 15th, Chart 16, a departing anticyclone rests over Tasman Sea, and its rear isobars are shown running north and south over central Australia; another anticyclone is shown over Western Australia and a depression east of the Australian Bight; a trough of low pressure rests over Central Australia from north to south. On this day the only indications of the pending rain were found in the cloudy skies generally over South Australia, western parts of New South Wales and Queensland, and a small area of rain in South Australia.

In Chart 16, the straight isobars of Chart 15 have entirely disappeared, but the rain has come over Central and South