DPFS_Manual_Appendix-II-4
GRAPHICAL REPRESENTATION OF DATA, ANALYSES AND FORECASTS
1. THE SURFACE PLOTTING MODEL
The “boxes” are included in the diagram simply to fix the relative positions of the elements and are not included in the actual plot. The wind plot is not shown in the model. Ship identification letters or buoy identifiers should be plotted above the model. In the case of automatic weather stations, an equilateral triangle should be plotted round the station circle so that the apex of the triangle
points towards the position of the medium-cloud symbol.
2. GRAPHIC REPRESENTATION OF DATA ON WEATHER CHARTS
2.1 The following rules concern the symbols to be used for the plotting of various elements figuring in a surface observation:
(b) Clouds should be plotted with the same symbols as used on surface charts.
4.2 Charts of isobaric surfaces
1. THE SURFACE PLOTTING MODEL
If it is required to plot the elements shown in the model, they should be placed in the relative positions shown. Any of the elements may be omitted.
The “boxes” are included in the diagram simply to fix the relative positions of the elements and are not included in the actual plot. The wind plot is not shown in the model. Ship identification letters or buoy identifiers should be plotted above the model. In the case of automatic weather stations, an equilateral triangle should be plotted round the station circle so that the apex of the triangle
2. GRAPHIC REPRESENTATION OF DATA ON WEATHER CHARTS
2.1 The following rules concern the symbols to be used for the plotting of various elements figuring in a surface observation:
2.2 The following rules determine the symbols to be used for the plotting of the various upper-air observation elements which appear on the constant pressure charts.
(a) The wind at the level of the chart should be plotted with a solid shaft touching the station circle, the barbs and solid pennants flying to the left of the wind shaft in the northern hemisphere and to the right of the wind shaft in the southern hemisphere. The full barbs represent 5 m s-1 or 10 knots, the half-barbs represent 2.5 m s-1 or 5 knots, and the solid pennant represents 25 m s-1 or 50 knots.
Derived winds should be plotted with the shaft touching the station circle and the barbs and solid pennants flying towards the side of higher pressure. If one derived wind is plotted, the shaft should be a solid line. If two derived winds are plotted, one of them should be plotted with a broken shaft.
Colour separation between the observed and derived winds is recommended. In wind field analyses code figures may replace the barbs and pennants.
(b) Clouds should be plotted with the same symbols as used on surface charts.
3. ANALYSES AND FORECASTS ON WEATHER CHARTS
3.1 General rules
(a) The basic symbol shown in the table below is placed on the chart along the line of the phenomenon and it is repeated as necessary to indicate the extent of the phenomenon;
(b) The arrows on items 1 to 10 of the table are not part of the symbol but are entered to indicate the orientation of the symbol with respect to the direction of motion of the phenomenon.
3.2 Symbols
3.3 Representation of weather features
Weather features on charts may be shown in the manner indicated below:
4. REPRESENTATION OF THE ANALYSIS AND FORECAST ON SPECIFIC CHARTS
4.1 Surface charts
4.1.1 Fronts
Fronts will be shown using the symbols given in paragraph 3.2.
4.1.2 Isobars
It is recommended that isobars be drawn at intervals of 4 or 5 hPa. Multiples or sub-multiples of these basic intervals may be used depending on the scale and purposes of the chart but, whatever the intervals, the 1 000 hPa isobar should always be included in the series.
4.1.3 Pressure centres
(a) The location of a pressure centre may be indicated by a cross. To indicate the nature of the centre, a capital letter appropriate to the language of the country is entered above the symbol marking the centre.
(b) In the case of tropical cyclonic circulations the centre is marked by a special symbol as shown below:
| For a tropical cyclonic circulation with observed or estimated maximum winds of 17 to 63 kt (29 to 117 km/h); | |
| For a tropical cyclonic circulation with observed or estimated maximum winds of 64 kt (118 km/h) or more. |
(c) The letter or the symbol for a tropical cyclonic circulation should be aligned parallel to the adjacent meridian.
(d) Pressure centres may be given an identifying letter to assist in their tracking from chart to chart. This should be written as a suffix to the letter or symbol defining the pressure centre. A tropical cyclonic circulation may have a name assigned to it. This may be entered in block letters near to the centre.
(e) The value of the pressure at the centre should be entered in whole hectopascals immediately below the symbol marking the centre, the number being parallel to the adjacent line of latitude.
4.1.4 Tracks of pressure centres
The previous positions of a pressure centre may be entered by means of symbols in the same way as the present position. Above each symbol may be entered the corresponding time in hours (two figures) and below it the pressure of the centre at that time in hectopascals. The symbols should be joined by a thick broken line. The forecast position of a pressure centre may also be indicated by a symbol in the same way as the present position, the time and the estimated pressure being entered above and below the symbol respectively. The present position and the forecast position should be joined by a solid arrow drawn along the track the centre is forecast to take.
4.1.5 Isallobars
Isallobars of three-hour change should normally be drawn for intervals of single hectopascals. Large intervals may be used if the scale of the chart is small or if the period is longer than three hours. The “no change” line will be numbered with a zero and the numbers on the other lines will be preceded by a plus sign if the pressure has risen and a minus sign if it has fallen.
4.2 Charts of isobaric surfaces
4.2.1 Fronts
If fronts are entered, the symbols given in paragraph 3.2 should be used.
4.2.2 Isohypses of absolute topography or contour lines
It is recommended that contour lines be drawn at intervals of either 40 gpm (80, 20 and 10 when appropriate) or 60 gpm (120, 30, 15 when appropriate). The lines should be numbered in geopotential decametres, e.g. 5280 gpm should be labelled 528.
4.2.3 Height centres
The positions present, past and forecast of high and low centres in the contours may be indicated in the same way as for pressure centres on surface charts (see paragraphs 4.1.3 and 4.1.4). Above the symbol marking a centre may be entered a capital letter appropriate to the language of the country. The value of the height at the centre should be entered immediately below the symbol marking the centre to the nearest ten metres, e.g. 5280. The number should be entered parallel to the adjacent line of latitude.
4.2.4 Isotachs
Isotachs should normally be drawn at intervals of 20 kt (40, 10 and 5 when appropriate). Centres of regions of minimum and maximum speed may be marked according to national practices. On the maximum wind charts, however, the maximum should be marked by a “J” followed by the estimated maximum speed, e.g. J 120.
4.2.5 Jet streams
A jet stream should be marked by a heavy, solid line with arrow-heads placed at intervals along it pointing in the direction of the flow in the stream.
4.2.6 Isohypses of relative topography or thickness lines
If thickness lines are drawn, the following intervals are recommended: either 40 gpm (80, 20 and 10 when appropriate) or 60 gpm (120, 30, 15 when appropriate).
4.2.7 Isotherms
Isotherms will not normally be drawn on charts on which thickness lines are entered. Isotherms should be drawn at intervals of either 5°C (10°C and 2.5°C when appropriate) or 2°C (1°C when appropriate).
4.2.8 Moisture lines
If lines of equal dew point are drawn, the same intervals as for isotherms may be used.