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Chart
Guide
|
There's information here to help you understand:
The hourly synoptic charts produced from automatic weather station observations
The 3-hourly synoptic charts made from the full synoptic manual and automatic network
The large 6-hourly international synoptic charts covering the whole globe
The upper air charts produced from balloon ascents made at airports around the nation twice daily (temporarily suspended)
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.
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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. 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. Cn
is the amount of low cloud in octas 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 |
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