Hot or cold?
Remembering that air flows clockwise around Low Pressure systems and anticlockwise around High Pressure systems, a fairly typical summer Australian weather map (below: Image One) shows:
North to northwesterly winds over eastern Australia on the western side of a Tasman Sea high. They carry hot, dry air from inland Australia southward over Victoria and Tasmania. With strong winds associated with an approaching trough and cold front, this represents a classic weather situation with extreme bushfire risk.
Moist, easterly flow from the Coral Sea onto the Queensland coast causes very warm, humid and sultry weather east of the Great Dividing Range. This air, often susceptible to the development of showers and thunderstorms and is described as ‘unstable’.
The cold front approaching Tasmania will replace the hot, dry north-westerlies with south-westerlies carrying cooler, often relatively humid air from waters south of the continent.
Such summer fronts are often quite shallow and may not penetrate far inland, particularly if they are distorted and slowed over the Victorian mountains.
Tropical Cyclone Freddy is moving offshore and the low heading for the Queensland coast is something to pay attention to, this time of year.
Image Two (below) shows a relatively common winter weather map.
Very cold, unstable air from well south of Tasmania flows northward over Tasmania, Victoria and southeast New South Wales, reducing normal day temperatures typically by five degrees or more. Note the cold front, the Low Pressure centres over Tasmania and the South Island of New Zealand, and the high (1036 hectopascals) over the Bight. Occasionally, rapid interaction with other weather systems can almost halt the pattern’s eastward movement, causing successive cold fronts to bring a prolonged spell of cold, showery weather to southern Australia.
Easterly winds over inland Australia. Although southern cold fronts become shallow and diffuse as they move into northern Australia they often trigger a surge in the strength of the easterlies and this, combined with their extreme dryness, creates a very high fire danger in the tropical savanna region.