Weather and Gardening – When Will It Rain?  

On 28th September Julie Evans, a retired meteorologist, gave us an excellent explanation of the factors affecting weather in Australia, and how to view weather information available from the Bureau of Meteorology. Australia’s weather is the result of many factors. Looking at a world map it is obvious that, while the Northern Hemisphere contains huge land masses, the Southern Hemisphere is largely ocean. The existing land masses in the south are mostly a very long way from Antarctica. Prevailing winds are not interrupted by land masses, and ocean temperatures with their associated winds have huge effects. In equatorial areas, warm humid air ensures significant rain. Nearer the poles lies the turbulent frontal zone ensuring regular rain. Between these broad weather zones lies the subtropical high-pressure belt where the land masses are much drier, particularly around 30degree latitude. In the northern hemisphere this latitude crosses the dry areas of Northern Africa and the Middle East. In the south this latitude is right across Australia, a little south of the NSW-Queensland border. Australia is a dry continent.

Ocean temperatures with the associated winds in both the Pacific and Indian Oceans greatly affect Australia’s weather. The Pacific Ocean changes affect Eastern Australia, particularly NSW, causing the systems known as El Nino and La Nina. In El Nino changes in Pacific Ocean temperatures cause rain-bearing winds to miss Australia, leading to higher land temperatures and less rain, especially inland. In La Nina the rain-bearing winds reach Australia resulting in good rain. Sometimes there is too much rain, causing flooding. Australia’s rainfall has always been very variable, with extremes of wet and dry.

Looking more locally, our land masses affect weather. Australia does not have the high mountain ranges found in the northern hemisphere. For rain to fall, moist air currents need to rise. Mountains force prevailing winds to rise suddenly. In NSW we have the Great Divide running roughly parallel to the coast and not far inland. Humid winds from the Pacific Ocean to our east, particularly in summer, tend to get forced upward on the Great Divide and trigger rain. On average, coastal NSW has a much higher rainfall than west of the Divide. Even the North Shore ridge catches humid air, and Turramurra has the highest annual rainfall in Sydney. On the other hand, the Warragamba Dam catchment lies in a relatively dryer region, although its large area usually manages to maintain dam levels. Inland NSW needs humid air crossing from Western and South Australia to get rain. These winds are largely during winter and spring. In recent years these humid winds have remained too far south, missing most of the Australian land mass. This is worsening the current severe drought in Eastern Australia. It is adding to the effects of the current El Nino.

Julie then diverted our attention to local weather and particularly to the vast amount of information available on the Bureau of Meteorology website, if you know where to find it. Looking at a weather map, our best chance of rain in Sydney is when there is a low pressure area around central NSW and the high pressure area is way down south. Currently there are too many highs too close to us. On the Bureau’s website there are very detailed weather records and much information predicting weather outcomes in the near future. Obviously the accuracy of prediction varies with how far ahead it is. Weather can change unpredictably despite the best predictive science. For today or tomorrow, predictions are remarkably accurate, as in MetEye which gives localised information including the likelihood of rain every few hours. A great deal of predictive information is provided for specific assistance to agriculture and shipping especially. Julie has given us a list of particularly helpful links for this website.

Our members found this talk extremely enlightening and helpful, and thanked Julie enthusiastically.