WEATHER AND CLIMATE OF THE BLUE MOUNTAINS

1. Weather and Climate of the Blue Mountains
   
   The climate of the Blue Mountains has been classified in various ways. It could be perhaps described as warm temperate eastern maritime with variations due to altitude. Overall summers are warm to hot and winters are mild to cold. Temperatures are modified by altitude and rainfall benefits from proximity to the tasman sea. The Blue Mountains region has a summer to autumn rainfall maximum with drier winter months.
   
   
2. The climate experienced over the whole Blue Mountains region varies according to altitude, distance from the sea and local topography. There is a change from approximately Wentworth Falls due to:
   
   1. a decline in temperature with altitude, average temperature range in the Lower Blue Mountains is 9 - 23 C and 7 - 16 C in the Upper Blue Mountains. In winter the temperature in the Upper Blue Mountains is on average 10C cooler than in coastal Sydney.
      
   2. a rise in rainfall due to orographic effect. Rainfall rises from about 802mm in Emu Plains to 1076mm at Springwood and 1410-1500 at Wentworth Falls. Rainfall peaks at Wentworth Falls and then begines to decline further west. This is basically because much moisture has already been extracted from the tropical maritime air mass that moves in from the tasman sea and the continued rise in elevation from Wentworth Falls to Mount Victoria (about 150 metres) is not enough to lower the dew point sufficently to drag significantly more moisture from the air mass.
      
      
3. On the eastern side of the Blue Mountains the rainshadow effect of sinking, warming and evaporating air is experienced. Hartly recieves about 740mm and Lithgow 865mm. In this region precipitation may be received more from orographic effects on south-westerly changes.
   
   
4. More snow in higher altitudes of the Blue Mountains. It is the Upper Blue Mountains that provides the extra orographic uplift to extract snow from cold, low pressure cells and fronts from south-west. Numbers of snow days and amounts of snow peak at Mount Victoria. both then decline towards Wentworth Falls. It is rarer for snow to fall and stay on the ground east of Bodington Hill.
   
   
5. Frost days generally incrase with altitude, though cold air drainage can cause frost to develop in valleys, on the Cumberland Plains and towards Lithgow.
   
   
6. Gound mist (fog) is more common in the Upper Blue Mountains and particularly in the Grose valley and Jamison Valley. Upslope fogs less commonly move west from the Lower Blue Mountains, forming when warm moist air from the Tasman Sea meets cooler Lower Blue Mountains air.
   
   
7. Hailstorms and thunderstorms affect the Blue Mountains region. Both often develop in the Upper Blue Mountains and move east to north-east.
   
   
8. Wind is a feature of all the mountains region as well as the Blue Mountains. Different winds brings different conditions to the region:
   
   a. Easterly air streams usually bring moisture and gentle to moderate winds
   b. South-westerly winds may bring cool changes in summer but less rain. In winter, cold temperatures and snow may occur.
   
   c. Winds from the north-west and west bring hot dry winds especially in summer and help to promote bush fires.
   d. Local winds are also important. Anabatic (upslope) winds by day and katabatic (downslope) winds by night move to and from plateaus and valleys depending on the local pressure conditions.
   
   
9. Bushfires are a response to climate, weather and other environmental conditions in the Blue Mountains. In spring and summer, conditions of strong hot winds and low humidity promoted by the trailing edge of slow moving high pressure and/or the leading edge of low pressure, both of which drag in tropical continental air, contribute to the bushfire risk. Dry thunderstorms with lightning can act as ignition, though it is more frequently a person that starts a fire either accidentally or intentionally.
   
   
10. Microclimates developed by variations of landforms, aspect and vegetation cover an important feature of the Blue Mountains region:
    
    Plateau surfaces may be very wind blown, exposed to high temperatures in summer and low in winter and have high evaporation rates. Water is limited and fire frequency grater. Heath vegetation is a common response. Sheltered valleys however are wetter, cooler, less wind affected and have a reduced fire frequency.
    
    
11. Aspects vary markedly because of the dissected plateau topography. North facing slopes are hotter, drier and more wind and fire affected. South facing slopes are shaded, can be quite cold, have more moisture and are less likely to burn intensley.
    
    
12. The distribution and growth of plants is greatly influenced by weather and climate in the Blue Mountains:
    
    Differences in the average rainfall are of some importance. It is however microclimate that is more significant. Hailstorms and strong winds associated with thunderstorms can be damaging to the local flora.
    
    Plants and animals in the Upper Blue Mountains experience colder overall temperatures than in the Lower Blue Mountains. The winds affect plant growth rates and survival. Species need to be more frost tolerant in the Upper Blue Mountains and in known frost hollows. Likewise snow is an additional stress but not to the same extent as in true alpine areas.
    
    Bushland plants, while being adapted to particular fire frequencies and intensities, are being subjected to greater pressures as people change fire regimes. Cultivated plants should be selected so that fire is not further promoted.