Station Weather Plot
A station weather plot is a graphic representation of the weather conditions at a specific time at an observing station. It enables a large amount of data to be displayed in a small space on a synoptic weather chart.
Synoptic weather charts are produced at regular intervals each day by national meteorological services, and may comprise the station weather plots from national and international fixed land stations, mobile land stations, ships and a variety of marine platforms all having the same UTC observation time. The charts are analysed by meteorologists, assisted by satellite imagery to locate severe and other significant features, to determine the pressure patterns and look for trends or unusual features in the plotted data as an aid in preparing weather forecasts and warnings.
The data to be presented on synoptic weather charts are provided in coded reports. After the report has been received and decoded, the data are plotted as either raw values, encoded values, code figures or weather symbols. The coded report of a surface weather observation from a fixed land station uses the SYNOP code form.
Raw & Encoded Values
The raw values of air temperature, dew point, maximum temperature and minimum temperature are plotted to one decimal place in line with standard BoM practice.
The mean sea level pressure is coded in tenths of a hPa and plotted as three digits after omitting the thousands digit(s):
- e.g. 996.4 hPa is 9964 tenths and coded as 964 after omitting the thousands digit (9), and
- e.g. 1026.7 hPa is 10267 tenths and coded as 267 after omitting the thousands digits (10).
The amount of pressure change is coded in tenths of a hPa. An amount of less than 10.0 is plotted as two digits including a leading zero if necessary. An amount of 10.0 or more is plotted as three digits. This differs from the synop code, which is always reported as three digits that includes a leading zero if necessary:
- e.g. 0.8 hPa is 8 tenths and coded as 08 after adding a leading zero, and
- e.g. 10.3 hPa is 103 tenths and is coded as 103.
The rainfall is coded as three digits for plotting. An amount of less than 1.0 is coded in tenths of a mm and prefixed by 99. An amount of 1.0 or more is coded as the nearest whole number and prefixed by leading zeroes if necessary. Nil rainfall is not coded:
- e.g. 0.8 mm is 8 tenths and coded as 998 after adding 99 as a prefix, and
- e.g. 4.2 mm is rounded to 4 and coded as 004 after adding leading zeroes.
The wind gust is coded as the nearest whole number then prefixed by G for plotting:
- e.g. a gust of 34.5 knots is rounded to 35 and coded as G35 after adding G as a prefix.
Layout & Decode
The station weather plot shown in the layout guide above includes a non-standard wind gust, otherwise the layout conforms to the standard plotting model defined by WMO. The wind direction symbol extends from the centre of the station circle towards the direction from which the wind is coming. The wind speed symbol affixed to the end of the wind direction symbol is drawn to the right side in the Southern Hemisphere and to the left side in the Northern Hemisphere.
Elements are omitted from the station weather plot in the following circumstances:
- The rainfall is excluded in accordance with standard WMO practice if the amount is 0,
- The amount of lowest cloud is excluded in accordance with standard WMO practice if the code figure is 0,
- Elements are excluded in accordance with standard WMO practice if they are not reported, and
- Only the first instance of past weather is plotted in accordance with standard WMO practice if both instances are the same.
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