Start of Winter: Water begins to freeze

May 10th, 2012
by Jo Law

The first pentad of winter started much drier than the preceeding months. There have been days when the relative humidity dropped to around 50% with the lowest of 35% recorded on May 10th at Bellambi (AWS). The crisp air unmistakably heralds the onset of winter. Statistically speaking the Illawarra winter are the dry months when temperatures, rainfall and relative humidty are at their lowest all year. It is unlikely, however, that water will begin to freeze.

Ice or snow rarely occurs at sea level on mainland Australia. The Australian winter tends to be milder compared to land of similar latitudes in the northern hemisphereas as the Southern Ocean shelters our continent from the cold polar region. Across that vast ocean lies the extreme world of Antartica with the lowest recorded temperature of −82.8 °C. It is also the driest continent on Earth with an average annual rainfall of less tan 200mm. At the Australian coastal base, Casey, the monthly highest maximum in this May is 1.9ºC and the current monthly lowest minimum is -17.8ºC. Further inland at Davis, May 9th recorded its monthly highest maximum to-date of -10.7ºC and lowest minimum of -22.8ºC.

When Australia and Antartica formed part of Gondwana in the Mesozoic, the Earth was warm and humid. Ice caps were absent from polar regions and lush forests spread from equator to the south pole, dinosauers and later marsupials roamed. The supercontinent began to break up some 55 million years ago. When Australia finally separated and drifted northwards, ocean currents flowed between the two continents, Antartica began to cool, ice began to form, and drying began in both lands.

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