blizzards occurring in the highlands), gusty icy winds, and hail in the south-eastern parts of the country, such as in Victoria, Tasmania, the southeast coast of South Australia and the southern half of New South Wales (but only on the windward side of the Great Dividing Range, whereas the leeward side will be affected by foehn winds). Identification The bases of the two polar vortices are located in the middle and upper troposphere and extend into the stratosphere. Beneath that lies a large mass of cold, dense Arctic air. The interface between the cold dry air mass of the pole and the warm moist air mass farther south defines the location of the polar front. The polar front is centered, roughly at 60° latitude. A polar vortex strengthens in the winter and weakens in the summer because of its dependence on the temperature difference between the equator and the poles. Polar cyclones are low-pressure zones embedded within the polar air masses, and exist year-round. The stratospheric polar vortex develops at latitudes above the subtropical jet stream. Horizontally, most polar vortices have a radius of less than . Since polar vortices exist from the stratosphere downward into the mid-troposphere, a variety of heights/pressure levels are used to mark its position. The 50 hPa pressure surface is most often used to identify its stratospheric location. At the level of the tropopause, the extent of closed contours of potential temperature can be used to determine its strength. Others have used levels down to the 500 hPa pressure level (about above sea level during the winter) to identify the polar vortex. Duration and strength Polar vortices are weakest during summer and strongest during winter. Extratropical cyclones that migrate into higher latitudes when the polar vortex is weak can disrupt the single vortex creating smaller vortices (cold-core lows) within the polar air mass. Those individual vortices can persist for more than a month. Volcanic eruptions in the tropics can lead to a stronger polar vortex during winter for as long as two years afterwards. The strength and position of the polar vortex shapes the flow pattern in a broad area about it. An index which is used in the northern hemisphere to gauge its magnitude is the Arctic oscillation. When the Arctic vortex is at its strongest, there is a single vortex, but normally, the Arctic vortex is elongated in shape, with two cyclone centers,