Page:Hunt - The climate and weather of Australia - 1913.djvu/168

Rh The conditions most favorable for tornadoes inland and in the summer are (1) high temperatures; (2) considerable humidity; (3) very small barometric gradient, to which may be added a very probable fourth factor, unusually steep vertical temperature gradient. These seem to be most frequently provided by extensive but comparatively shallow monsoonal depressions, which favour a wide gentle air flow southerly from the tropical interior. The weather charts for 25th to 29th September, 1911, show conditions typically favorable and abundantly justified by results, inasmuch as two tornadoes resulted. The first was in Victoria at Marong, near Bendigo, on the afternoon of the 27th September, 1911. This supplied all the characteristic features of a tornado, the long inverted cone depending from the dense blue-black thunder cloud, the narrow track, 5 to 12 chains wide and 12 miles in length, along which indescribable damage was wrought, and the accompanying violent thunder and hail storms. Fortunately a photograph of the storm cloud with its pendent funnel was secured by a gentleman 3 miles distant, and this has been reproduced, together with a full description of the storm, &c., in the 1911 September number of the Australian Monthly Weather Report. Two days later the same atmospheric conditions resulted in a similar storm in New South Wales at Cudal, between Forbes and Orange. The results in the latter case were not so serious, as the storm occurred in a sparsely populated area.

The charts given will repay inspection. That of the 25th shows a great valley-like depression extending from the Bight to the Northern Territory. while the axis of a high-pressure system lies north and south over Eastern New South Wales and Queensland. A wide direct southward drift of air in front of the trough is thus secured. The chart of the 27th shows the same features, the very slow eastward movement allowing of steady accumulations of heat and increasing humidity. By the 29th the depression was well over South-eastern New South Wales. In connection with this disturbance no favorable feature appears to have been wanting. Slight though the barometric gradients were, they had complete control of the atmospheric circulation, the air in front of the trough from cumulus base to cirrus level flowing steadily from points near north. This is always favorable to thunderstorm development, and visual observation was sufficient to show that this case was no exception, the writer, who was in Northern Victoria at the time, making a special note of the abundance day after day of "towering" cumulus and cumulo-nimbus clouds.

From the 1st to the 5th November, 1910, a cyclonic depression with very slight gradients, but influencing enormous areas, was slowly passing from the north-west coast across the continent. During the whole of this period barometric conditions, ideal from a thunderstorm point of view, prevailed over the whole of Eastern Australia. They were, if anything, even more favorable than in the preceding example, and of much wider scope. In addition to heavy thunderstorm rains over the greater part of the interior the following exceptional phenomena were reported. At Hergott Springs (S.A.), in the driest part of the continent, a "cyclone" accompanied by a violent thunderstorm passed over the township at 5 p.m. on the 4th instant. This wrecked one house, unroofed others, and threatened loss of life. The