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Chart Guide

There's information here to help you understand:

But first, TWO WARNINGS

The page-top green warning about always checking times and dates will save you lots of grief. I can't count the number of times I've gotten excited over the weather shown on a chart or satellite image only to discover that it was a few days or months ago. Also, these files update every 30 minutes, so there's every chance you're looking at a cached copy.

The "Always check times" warning expands to include (in purple) notes on any problems that I'm aware of (but only, of course, if I'm aware of them!)

Understanding the Hourly AWS synoptics

The black lines are isobars of pressure, spaced just 1hPa apart. Normal charts are spaced 4hPa apart, so the AWN charts will look windier but give a lot more fine detail. These are pretty accurate where there is data; hopeless elsewhere

The purple lines are isohyets, or lines of equal rainfall. Use these to locate areas of rain, but don't rely on their accuracy -- the algorithm that computes them has some difficulties.

Some charts also show red lines of equal temperature, green lines of equal relative humidity, and wind streamlines (black lines with arrows in them).

Around the + that shows the location of each station, the following information is plotted:

TT       PPP
RR   +
Td  

TT is temperature in whole degrees Celcius. M = missing.
Td is dew point in whole degrees Celcius. M = missing
PPP is the tens, units and decimal pressure, so 997.6 = 976 and 1024.5 = 245
RR is rainfall in millimetres for the previous hour
There is also a wind barb, thus:


(Thanks to Joseph Bartlo for the graphic)

Understanding the 3-hourly synoptics

Again, the black lines are isobars of pressure,purple ones are rainfall, red for temperature, green for humidity and black with arrows for wind streamlines (see above).

The station plots are much more detailed:

         Ch
         Cm
   TT       PPP
RR ww +    a pp
   Td   Cl  
    L    Cn 

Wind, TT, Td, and PPP are the same as for the hourly synoptics

RR 3-hourly synoptics show rainfall for the previous 3 hours at 00, 06, 12 and 18 local (not previous 1 hour as indicated in the legend). Rain is for the previous 6 hours at 03, 15 and 21 local, and for the previous 24 hours at 09 local.

Much of the additional information in these plots is conveyed by graphics which are summarised in the two tables below (for which thanks to Tim Vasquez). Click on them to open them in new windows, then either keep them open for reference or print them out and keep them handy. You may need to use landscape formatting for the weather symbols sheet. If you're having trouble reading the fine print, try Tim's WeatherGraph from which the symbol sheets originated.

The + in the middle of the station plot is replaced by a circular graphic showing how much of the sky is covered by cloud, in eighths or octas. Use the N total sky cover column in the Miscellaneous Symbols chart at left.
If the cloud circle is blue then visibility is less than 8km and/or cloud base is less than 3,000 feet.
If the cloud circle is red
then visibility is less than 5km and/or cloud base is less than 1,000 feet.

ww is a symbol for present weather. Use the Weather Symbols chart below. A ? means no weather of note.

a is a symbol showing what the pressure trace on the barograph (recording barometer) in the past 3 hours looks like. Use the a pressure trend column in the chart at left.
pp is the tenths of hPa that the pressure has risen or fallen in the past 3 hours; e.g. 25 = 2.5hPa

Cn is the amount of low cloud in octas
L
is the level, in hundreds of feet of the lowest cloud base, so L35 = 3,500 feet, L6 = 600 feet, L0 means the cloud's on the ground.
Ch, Cm, Cl
are symbols for the types of high, middle level and low level cloud observed. Use the appropriate columns in the chart at left.

If you've read this far, you will probably be interested in downloading the free Weathergraph observation, plotting and forecasting summary chart from Tim Vasquez at Weathergraphics. This enormously useful chart is in pdf format and prints out onto two A4 pages. While it is oriented towards US and northern hemisphere circumstances, it is still a powerful reference for Australian users. It contains concise summaries of the main surface weather codes and the symbols used in plotting synoptic charts, summaries of standard hailstone sizes, the Fujita tornado scale, beaufort wind scale, meteorological conversion units, the Saffir-Simpson Hurricane Scale, weather system categories, stability indices, computer forecast models, and a range of forecasting rules-of-thumb which need some adjustment for use in Australia.

Updated 20/03/03

Understanding the 6-hourly international charts

Once you've mastered the 3-hourly synoptic charts above, these are a doddle. They are produced from the synoptic reports transmitted at the four main standard hours (00, 06, 12 and 18 utc) using World Meteorological Organisation standards and the WMO Global Telecommunications System.

The only differences from the Australian 3-hourly charts are in the data I've selected to put in the station plot. Some cloud information has been omitted to make room for maximum and minimum temperatures for land stations and water temperature and swell height for ships. So the station plot now looks like this:

          Tx
    TT  Tn  PPP
RR ww  + a pp
   Td    Cl   
Sea temp   Swell height

RR is reported for different periods on different continents -- check the legend in the bottom left corner of the map for the period for each map. Note that in South America standards vary from country to country, and some countries don't report rain at all.

Maximum (Tx) and minimum (Tn) temperatures are also a mixed bag, sometimes reported, sometimes not, and varying in the time of reporting from continent to continent.

Sea temperature and swell height are reported from some coastal stations and also from some ships and drifting buoys, which make their first appearance on these charts. Unlike Australian waters, which are pretty sparsely populated by shipping, some sea areas such as the North Atlantic can boast more ship reports than parts of Australia, Africa and South America. Sea temp is in °C, and swell height is in metres.

Understanding the upper air charts (temporarily suspended)

The charts show the pressure, temperature, humidity and wind at 7 standard levels in the atmosphere. They are drawn from the measurements made by weather balloons released from major airports around Australia twice daily, at 9am and 9pm. The balloons measure barometric pressure, temperature and humidity as they rise, and silver reflectors on them are tracked by radar so as to determine wind direction and speed from the way the balloons drift.

The levels are stated in pressure (hPa or hectopascals) rather than in physical height, as determining your precise height above the ground in a balloon is not so easy. However, the pressure levels (or surfaces) roughly correspond to the followiong altitudes, give or take a bit depending on the temperature of the air:

925hPa = about 900m
850hPa = about 1.5km
700hPa = about 3km
500hPa = about 5.75km
400hPa = about 7.5km
300hPa = about 9.5km
250hPa = about 10.5km

Once above the ground, temperature, humidity, pressure and wind don't vary as much over small distances as they do in the bottom kilometre or so of the atmosphere. That is why quite reliable upper air charts can be drawn with only a few dozen observations. The actual observations at each level are plotted in a similar way to all the other charts:

TT   HHH
   +
TdD  

But there are some differences:

TT is temperature in °C as usual

TdD is the dew point depression. So if the plot showed a temperature of 10 and a TdD of 15, the dew point would be -5. This makes it easier to see dry and moist areas.

HHH is the geopotential height of the pressure surface above sea level at this point. This is usually close to the geometric height, or altitude. At the 925hPa level, the figures in this position are the number of (geopotential) metres above sea level. At 850 and 700hPa, the thousands digits are dropped off, and you need to add 1000 to the 850hPa level and 3000 to the 700hPa level; so, for example, if the plotted height on a 700hPa chart is given as "154", you can work out that it means 3.154km. At 500hPa the units digit is dropped off, so 565 = 5,650km.

Wind is shown by the usual wind arrows, but watch out for the 50 knot barbs at higher levels, where wind speeds often exceed 150 knots.

The upper charts put the essential third dimension into understanding the weather. I find the best way to build an understanding in my head of what's going on is to open a number of charts in different windows, make sure they are aligned, then click between them. Be sure you have a surface map handy or in your head, too. The temperature and pressure surface height charts only go up to 500hPa, as these measures are of little importance in understanding weather developments at higher levels. The humidity and wind charts go right up to 250hPa, as it is in the upper levels that jet streams make their appearance and the presence of humidity at high levels is an indication of deep moisture through the atmosphere.

Contours are given for:

Geopotential height: Although these black contours are lines of equal height above sea level, they still produce the familiar high and low pressure systems, and winds flow roughly parallel with the contours. The closer together the lines, the higher the wind is likely to be. Look for upper closed low pressure systems, upper troughs, and check whether they align vertically. The height labels on the contours follow the same procedures described in HHH above.

Temperature: The red isotherms are one of the most vital signs of what is going on broadscale. Watch for tongues of cold air pushing up from the south and warm air pushing down from the north, especially on the 850, 700 and 500 charts.

Wind: Use the wind barbs at stations to get an idea of wind directions. Then use the isotachs, or lines of equal wind speed, to see where the strongest winds are. The jet stream usually lurks above 500hPa, and a check of the 400, 300 and 250hPa charts will usually find it. Or them, because there are usually a number of jets snaking across the Australian chart. Look for areas where the wind speed is accelerating into a jet, as these are commonly areas where bad weather develops.

Relative humidity: The green dotted contours show relative humidity. Although the humidity is analysed at 10% intervals, they are only important in giving a broad idea of where the main areas of moist and dry air are. Watch the areas of >70% humidity, and in particular >90% at the 500 and 700hPa levels, and see which way the wind is advecting them.