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WEATHER Winter Alpine weather can be extremely variable. Weather is caused by large atmospheric motions organised into bands of high and low pressure. During winter, southern parts of Australia come under the influence of lows and their associated cold fronts, exposing it to cold maritime air. This leads to frequent rain which falls as snow on higher ranges. Insert an image for High & Low pressure systems on the right. In high pressure systems, ____________________________________ ____________________________________ In low pressure systems, ____________________________________ ____________________________________ When a low and a high pressure system replace each other a mixing takes place known as a front which brings with it a change in temperature, pressure, wind strength and wind direction. A cold front occurs when cold air replaces warmer air; the cold air undercuts the warm air pushing it upwards and causing large clouds to develop. Cold fronts are associated with lows moving from the west coast to the east coast of Australia. Diagram A illustrates a typical snow producing weather pattern. A cold front is preceded by thin wispy cirrus clouds at high altitude. These thicken and are replaced by lower cloud as the front begins to move in. The front brings with it rain or snow which will be wet snow if there is warmer air ahead of the front and this turns to colder drier snow as the front passes. With high pressure systems over central Australia, mountains experience almost continuous westerly winds with fronts passing every few days. Southerly Outbreaks occur when strong high pressure systems sit in the great Australian Bight with deep low pressure systems south of Tasmania. Cold air is drawn up from the south very quickly. Southerlies occur only a couple of times during the winter but brings bitterly cold conditions and huge dumps of snow. What do isolated fluffy clouds suggest to a skier? ________________________________ What does the saying “Red Sky At Night” imply? _______________________________________________________________________ SNOW AND WIND Different types of snow conditions in different areas require different ski techniques to be successful. Icy conditions require: _____________________________________________________ Where as powder snow lends itself to: _______________________________________ In spring, the night air temperature falls below freezing therefore the snow freezes at night into little ice granules. By the afternoon in 22 degree temperatures the snow tends to get heavy and slush. Moguls result from: Wind chill factor A severe wind in –20 degree temperatures can lead to a skier being prone to extreme hypothermia. Wind chill factor needs to be considered at all times by touring skiers. The chill factor is particularly pronounced at lower temperatures. Determine what the chill effect would be if : a) thermometer reading of 0 degrees and a 60km/h wind _________________________ b) thermometer reading of –20 degrees and a 30km/h wind ________________________ c) thermometer reading of –20 degrees and a 60km/h wind _______________________