![]() ![]() In summer, lows travelling over warm water in the tropics can produce cyclones. In winter, these may be thick enough to give rain or snow, and explain why people living in the southern states associate low pressure systems with cold and sometimes stormy weather. As the warm humid air spirals upwards, it cools and clouds form. "When you have a low pressure system you're dragging air inwards and its being pulled upwards", explains Siems. However, during winter high pressure systems over the centre of Australia bring "dry, cooler conditions with rain along the southern portion," says Siems.Īir in high pressure systems moves in an anticlockwise direction (in the southern hemisphere), while air in low pressure systems moves in a clockwise direction due to the rotation of the Earth.Īt the surface of the Earth air flows from high pressure systems into low pressure systems. In summer, high pressure systems are associated with warm day time temperatures in mid-latitude regions. "In a high pressure system you have dense air subsiding downwards and pushing down at the surface, Because the surface air is being pushed down from the air above it spreads outwards, which is why we generally have fair weather when we have high pressure systems". There isn't a particular number of hectopascals that distinguishes a high pressure system from a low pressure one it's the relative differences between the two that count, and the interplay between them that causes weather.Ī high pressure system occurs where the air mass above the Earth is denser than in surrounding areas, and therefore exerts a higher force or pressure. "The surface pressure measures the mass or density of air above it, so even if it's warm at the surface, if there's a lot of air above it, you can get a high pressure, or if it's cold at the surface but there's not much air above it you could get a low pressure system."Īir pressure is measured in hectapascals. To understand how high and low pressure systems form and interact you have to look beyond the Earth's surface temperature to the troposphere, says Associate Professor Steve Siems from the Weather and Climate Program at Monash University. So why do lows produce cooler weather across Australia if cooler air is denser, and highs can produce hot, dry weather given that hot air rises? The monsoon finally arrived three weeks late in mid January bringing rain to the Top End, Tropical Cyclone Peta lashed the Pilbara, and Oswald drowned the east coast of Australia. The result: a column of hot air circulating around the continent drying and frying southern Australia. Meanwhile, a stationary high pressure cell in the Tasman Sea forced hot air from the north across southern Australia and prevented cooler air from further south penetrating inland. The monsoon which normally brings cooler, wetter conditions from the north failed to show up. This exceptional weather was caused by a combination of factors. At the beginning of 2013, Australia experienced a record-breaking heatwave that extended over a greater area for a longer period of time than ever experienced before. ![]()
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