Page:Hunt - The climate and weather of Australia - 1913.djvu/131

Rh These winds have been described as perhaps the most remarkable of the "squall" winds which characterize various parts of the earth.

They are cold winds, succeeding a period of hot weather. They blow from the south, usually arriving as a sudden squall after calms or northerly winds. Although not unknown along the southern shores of Australia, it is the coast of New South Wales, from Port Macquarie to Cape Howe, where they are especially noteworthy. Here the topographic conditions are peculiar, and are undoubtedly contributing causes to the local intensity of the burster. Some 50 or 80 miles from the coast extends a belt of highlands from 3,000 to 7,000 feet high. They are parallel to the coast and therefore at right angles to the general easterly atmospheric drift. Between this cooler belt of highlands and the ocean is a "hinterland" which is considerably warmer than the ocean or the mountains during the summer months when these winds chiefly occur. The monthly averages for the bursters are as follows:—

So that the spring and summer half-year accounts for 86 per cent. of the bursters.

The average of their maximum velocities is 42.7 miles per hour, though 153 miles was recorded in 1877. (The maximum velocity occurs usually about twelve hours after the burst.)

The weather preceding the burster may be summarized as follows:—For a period varying from three hours to three days before high temperatures prevail, northerly winds, with a tendency to east in summer, and to west in early and late summer, are the rule.

The clouds are characteristic. Usually cirro-cumulus and thunder clouds appear in the south-west, then a long dark cumulus roll approaches with a front of 30 miles or more. As this approaches the wind drops completely, and then a whirl of dust ushers in the "burst." The windvane flies round to the south, and the southerly in a few minutes may reach gale force. Rain may accompany the burst, but it is generally due to the electrical disturbances. The warm antecedent north winds are, however, occasionally accompanied by some rain.

The southern swing in wind direction is accompanied by a very rapid fall in temperature on occasions amounting to as much as 37° Fah.; 18° is about the average total fall, but on the 30th December, 1891, the thermometer dropped this amount in the first five minutes.

There is no doubt that the burster is merely an intensification of the normal southerly wind ushering in an anti-cyclone in New South Wales. This intensification is at least partly due to the relation of topographic and temperature conditions noted previously.