Climate - Melbourne

• see also:
  • Climate - Introduction 1 - atmosphere & temperature
  • Climate - Introduction 2 - water & rain
  • Climate - Introduction 3 - air & wind
  • Climate - Introduction 4 - Climates
  • Climate - Australia
  • Bureau of Meteorology - summary Victorian climate and weather in 2003

   

Understanding Melbourne's weather:

• see also: comparison of Australian capital cities
• although Melbourne's weather seems to be quite random having 4 seasons in a day, much of it can easily be understood and predicted on a day to day basis with knowledge of the positions of nearby highs, lows, cold fronts and upper level troughs
• February is the hottest month, while July is the coldest.
• unlike many regions, Melbourne's average monthly rainfall tends to be much the same for each month of the year, as a result of the frequent cold fronts and low pressure systems passing over and the spring/summer convective thunderstorm activity, although May and October are the wettest with 65-67mm and March and June are the driest with 43mm. The western suburbs have the least rain and the most sunshine and usually suffer less severe storm damage than the eastern or southern suburbs.
  • see Bureau of Meteorology - summary Victorian climate and weather in 2003
• first one must understand that winds tend to blow parallel to isobar lines on a weather map, with strength dictated by closeness of the isobars and the direction being anti-clockwise around a high and clockwise around a low:
  • it then becomes obvious as a high moves across Melbourne:
    • there will initially be cool, moist south-westerlies associated with showers while the high is to the west
    • as the high becomes centred over Melbourne, the weather becomes more settled with clear skies, light winds and thus mild-warm weather:
      • there is often morning cloud associated with local weather phenomenon of land breezes overnight converging on Port Phillip Bay causing uplift and cloudiness over the bay which moves over Melbourne and then retreats back over the Bay by 11am -noon
      • there is usually a southerly afternoon seabreeze often creating isolated cloud build up, which settles by ~9-10pm, when a mild land & urban breeze take over after midnight creating an offshore breeze
      • on particularly hot days, especially if there is some humidity, fair-weather convection cumulus clouds may develop in the afternoon creating thunderstorms
      • NB. in winter, the high pressure systems generally pass Melbourne well to the north resulting in cool westerly winds
    • as the centre of the high passes to the east of Melbourne, the winds change to a hot, dry north-north-westerly which gradually builds in strength and usually heralds an incoming cold front which is preceded by increasing cloudiness, humidity and usually thunderstorms as the warm is is forced upwards by the advancing front
    • as the cold front crosses Melbourne, the temperature suddenly drops within minutes and the wind changes direction to a south-westerly which is associated with cumulus clouds and showers
    • the cycle begins again, often cycles such as these recur on almost weekly cycles with the 1 or 2 days of clear skies occurring on same days each week
  • sometimes the high after passing east, becomes stationary in the Tasman Sea forming a "blocking high" where it may stay for 1-2weeks causing a heat wave in Melbourne with hot, dry northerly winds from inland Australia
  • sometimes the effect of a high over Melbourne is masked by the presence of an upper level trough which causes warm, humid, unstable weather with possible showers & thunderstorms
  • when a low pressure passes over, the wind directions are opposite to a high but also as there is convergence of winds causing upward lift of air, this results in humid, cloudy conditions with showers & possible thunderstorms
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