Temperature

Marble Bar heatwave, 1923-24

Where is the cool change?

Heatwaves in South Australia

Low level snow

Abundant snow seasons

Heat and cold

Australia is not only a dry country, but is also subject to fierce heat. As the sun tracks into the southern hemisphere in early spring, it begins to strongly heat northern and interior parts of the country. By November, average maximum temperatures have already climbed to the high 30s over wide areas of northern and central Australia - and into the low 40s in parts of northwestern Australia and western Queensland. The heat does not relent until the following autumn.

Heat waves

On occasions when the synoptic situation is favourable, the hot air extends south over the southern States. In South Australia and the southeast, this occurs when high pressure systems lie to the east, and a cold front is advancing from the west - a combination that directs a hot northerly airstream across these states. In southwestern Australia, the hottest conditions are normally associated with low pressure troughs that direct east to northeasterly winds from the hot interior. In both cases, eastward movement of the front or trough introduces cooler air from the oceans or higher latitudes - the so-called, and often eagerly awaited, “cool change”.

These “hot weather” patterns occasionally become slow-moving, and the trough or front bringing the cool change may stall, or even dissipate. On such occasions, very high temperatures - high 30s or even 40s - can persist for days, and in inland areas, for weeks on end. These are the “heatwaves” of the southern States. Such heatwaves can lead to heat exhaustion, and even death, particularly among the very young or old. Heatwaves have, in fact, accounted for more deaths in Australia than any other natural hazard: according to Emergency Management Australia the January 1939 heatwave in South Australia, Victoria and NSW killed 438 people.

“A typical hot weather situation over southeastern Australia, with hot, dry northerly winds ahead of a cold front. This situation, on 16 February 1983 - the day of the Ash Wednesday bushfires in Victoria and South Australia - produced temperatures up to 45°C”

Cold and snow

On the other hand, Australia is largely spared the extremes of cold that afflict many northern hemisphere countries in Europe, North America, and Asia. Snow is rare outside the highland areas of southeastern Australia, and is a noteworthy event when it does fall to low levels. Snowfalls at sea-level are rare, even in Tasmania, and exceptionally rare on the mainland. The path taken by a cold air mass largely determines the location of a snowfall, and occasionally a northward-moving pool will bring snow along the Great Divide as far north as southern Queensland. A typical synoptic situation producing a cold outbreak is when south to southwesterly winds rapidly transport very cold air northward from far southern latitudes.

Low overnight temperatures in Canberra during July 1994, combined with a malfunctioning automatic sprinkler, produced this display of icicles on play equipment at a Canberra pre-school centre (courtesy of Gary Schafer, Canberra Times).

At the higher elevations of southeastern Australia snow often persists for weeks or months at a time. In most years there is enough snow above about 1500 metres for a viable ski industry. However the amount of snow that falls can vary substantially from year to year: in some years, such as 1973 (when it was too warm) and 1982 (too little precipitation), the ski season fails. The fact that 1973 was a La Niña year (SOI highly positive) and 1982 an El Niño year (SOI highly negative) reflects a generally poor relationship between snowfall and the SOI. On the other hand, some years (such as 1981) have abundant snow and a “bumper” ski season. In recent years, snow- making machinery has reduced this uncertainty somewhat, although seasons of heavy snow are still welcomed - provided the depth does not become so great as to make ski tows inoperable.

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