Melbourne, Page 2
 

Melbourne sits on the southeastern coast of Australia, and its summer weather is highly variable.  A number of factors contribute to this unpredictability. One of the most interesting has to do with the orbit of the Earth around the sun. Because the Earth's orbit is eccentric, it is 7% closer to the sun during Australian summer (January) than during summer in the northern hemisphere (July). As a result, Melbourne's total solar radiation is increased by 14% in the summer, and reduced by 18% in the winter, compared to that of a city at a similar latitude in the northern hemisphere. This difference in solar heating contributes heavily to meteorological instability. The sun, that great generator of summer weather, shines down powerfully on Australia.

                  Australia-map1-B and W.gif (4661 bytes)

Melbourne's weather systems usually come from the west.  To the north of Melbourne are vast flat dry farming areas; to the north west is desert; to the west is a mix of farming and desert; the sea is to the south and east. So weather from the west has to cross the desert which makes the air dry and hot in the summer. If the system is a high pressure system it rotates anti-clockwise so that, as it moves from west to east, Melbourne gets south winds at the beginning of the high and north winds at the end of the high. The north winds can be very hot. If the high moves slowly, the north winds can continue for several days which pushes the temperature up to around 40C (104F). But a high will often be followed by a cold front which can result in the temperature dropping by 15C (27F) in 15 minutes as the wind suddenly switches to the south (off the sea).

Here are the seasonal average high temperatures and daily high temperatures for Melbourne for 1986-1991, from the work of Melbourne mathematician Rob Hyndman:

                             Melbourne-bimodal winter weather-seasonal average.gif (4197 bytes)

This chart shows that wintertime daily maximal temperatures fall within a narrow range (the middle 72% of high temperatures fall within an 8.1F range in July). Conversely, summer daily maximum temperatures cover a much wider range and the closer to mid-summer the date, the greater the variation.  (In January, the middle 72% of high temperatures fall within a 26.1F range).

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