Page:Hunt - The climate and weather of Australia - 1913.djvu/153

Rh. 131 136. Hot Spells in Southern Australia.—In the southern parts of the continent or, say, within 200 or 300 miles of the south coast-line, spells of heat unpleasantly great are usually of short duration. The immediate cause is a low-pressure trough connected with an Antarctic disturbance, and as the average rate of summer Antarctics is about 800 miles per day the northerly winds in advance of the trough rarely last more than 24 hours. This is more expecially [sic] the case on or near the coast-line; the further inland we go the longer the preliminary period during which light variable or easterly winds preceding the true northerly induced by the trough are allowing an accumulation of heat. But occasionally even coastal towns are subjected to hot spells lasting several days. The prime factor for this is always a stationary anticyclone centred over Tasman Sea, but with its western slopes overlying eastern and south-eastern Australia. This usually goes with some slow-moving monsoonal depression over the western interior of the continent, or an Antarctic disturbance which fails in its advance eastwards. The real problem here is the cause of the stationary condition of the anticyclone. This set of maps provides reason for thinking that it may be the same as has already been noted in connexion with the dry winter of 1899, when cyclones off the east coast held back or built up the following anticyclones so as to form a semicircle of high pressures on their south-eastern and south-western to north-western boundaries. In this case there is evidence that a tropical cyclone for some days was following the usual parabolic course not near the east coast, but away to the north-east in the neighbourhood of New Caledonia. Such a course would certainly tend to maintain high pressure over Tasman Sea, and it may be that this is the usual cause of the phenomenon under review. The maximum temperatures in Melbourne for the five days 30th January—3rd February, 1912, were 96.4°, 102.6°, 106.5°, 105.9°, and 102.5° respectively. Much the same operating factors appear to have been at work during the still more severe and, in Melbourne, record spell of heat from 15th to 20th January, 1908, the successive maxima at the Weather Bureau being 102.0°, 106.7°, 109.3°, 104.1°, 105.7°, and 107.5°. The charts for this period also show evidence of the existence of a very persistent tropical low-pressure system operating for some days away to the north-east beyond the high over Tasman Sea. This is most marked from the 16th to the 19th, when barometers were falling over Norfolk Island and the extreme north of New Zealand, and rising over eastern New South Wales and Victoria. On the 20th, when the tropical low was moving off, as shown by pressures rising again at Norfolk Island, the centre of the "high" moved northwards, allowing the trough of a slight Antarctic to move eastwards along the south coast of Victoria, thus bringing the much desired cool change. The charts for the 16th, 18th, and 20th illustrate the changes noticed on this occasion.