There are two types of synoptic charts on the website. There is one analysis chart and six forecast charts.Analysis Chart
The analysis chart represents the current state of the atmosphere over Australia at any given time. The time stamp is located in the bottom right corner of the chart. This chart is updated every hour.
The main purpose of this chart is to reflect the current pressure pattern over Australia. The white lines overlayed onto the map are called isobars, which connect points of equal pressure. Wind flows roughly parallel to these isobars. Air flows in a clockwise direction around a low pressure system (represented by the symbol L) and anticlockwise around a high pressure system (represented by an H). The number below these symbols indicates the central pressure of these systems, measured in hectopascals (hPa). As a general rule, the spacing of the isobars in relation to one another reflects the strength of the surface winds, i.e. the closer isobars are together the stronger the winds will be and vice versa. Tropical cyclones can also be identified on an analysis chart.
Arrows placed onto the charts point in the direction of wind flow, while their colour indicates the temperature of the air mass. A red arrow represents a hot wind that generally causes temperatures to be greater than 5 degrees above average for that time of year. An orange arrow reflects a warm wind with temperatures up to 5 degrees above average. A green arrow specifies a cool wind with temperatures generally between 0 and 5 degrees below average. A blue arrow is a cold wind causing temperatures greater than 5 degrees below average.
The yellow lines on the chart indicate the position of significant fronts or troughs. A front or trough represents the boundary between two air masses of different characteristics. When a front or trough passes over a region there is normally a marked change in air temperature, wind direction, humidity and precipitation pattern. Cold fronts always move in an easterly direction. Troughs are slower moving and normally travel eastwards, but can remain stationary or move west during summer.
The monsoon trough which affects northern Australia from November to April tracks north and south over the tropics, accounting for the characteristic wet and dry seasons of the region. Warm fronts are rarely analysed on Australian synoptic charts, but also travel east.
The blue jet stream shaded areas on an analysis chart indicate the location of the strongest upper level winds in the atmosphere. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength and orientation of the jet streams.Forecast Charts
The rain forecast charts show the likely amount of rainfall occurring in any given area on the day of the chart. These charts are based heavily on computer model output with little meteorologist input.
The lightest shading of green indicates 0.2-2mm of rainfall. The medium shades indicate 2-40mm. The darkest shading indicates greater than 40mm.
Note that these shadings refer to amount of rainfall, NOT chance of rain.
16:31 EDT Locals on the Cocos Islands were expected to spend most of Christmas Day bunkered down in their houses, as Tropical Cyclone Kate bears down on the tiny community more than 2,000 kilometres off the West Australian coast.