Can I have a guided tour of Met Office 'FAX' charts?
Prior to May 2003, there was a legend, 'ASXX' or 'FSXX', which identified the product as either an analysis (A), or a forecast (F) relating to the surface (S) level - strictly mean sea level - for an undefined region (XX). The words 'Analysis' or 'Forecast' are now used where required, and/or "xx hr", where xx=the number of hours from analysis time of the forecast chart, so "84hr" is the expected situation 84 hours from the analysis time.
[ The use of ASXX and FSXX dates from the time when the bulletins were coded using the International Analysis Code when ASXX/FSXX were the headers in much the same way as SMUK is the header for a bulletin of synoptic reports made at a main hour from the UK. ]
In addition, the letters EGRR used to appear on UKMO charts, which are the ICAO identifying letters which are assigned to the Exeter Operations Centre [EXO] (before late August 2003 to the Bracknell Telecommunications Centre) - these again have been dropped from 2003.
Other abbreviations include MSLP (mean sea level pressure) and UTC - universal co-ordinated time (an acronym chosen to satisfy both the English and French speaking communities since it does not have a direct equivalent in either language - in French UTC is read as temps universel coordonne). UTC is based on an atomic standard, but for all practical purposes is equivalent to GMT.
For international transfer of pictorial information the bulletins/files containing that information carry headings such as PPVA89, PPVI89 and so on. The letter P (or Q) indicates pictorial information, the second letter P indicates the information refers to pressure, the V defines the area for which the information is provided and the fourth letter indicates whether an analysis (A), or a forecast where E,G,I,J,K,M and O are for 24/36/48/60/72/96/120-hour forecasts respectively). The figure 89 refers to any parameter at sea level.
In the top-lefthand corner, there is a geostrophic scale - used to find the (theoretical) friction-less wind speed from the spacing of the isobars;(use with caution - see "How do I use a geostrophic wind scale?".
There is no longer a distance scale on the charts, but a tip to find distances ... remember that one degree of LATITUDE is equivalent to 60 nautical miles (n miles), (thus 1.5 degrees is equivalent to 90 n miles and so on) Therefore, if you want to measure off how far a depression has travelled over the period of six hours between analyses, step off the distance with a ruler or pair of dividers, then lay this distance along a line of LONGITUDE in the same area of the chart, and count the number of degrees latitude that this represents. For example, if the distance measured off is five degrees of latitude then this is equivalent to 300 n miles (i.e. 5 times 60, which is 300 n miles in that 6 hours; The overall speed of movement of the feature is even simpler to define for that 6 hours: one has only to remember that 1 degree latitude in 6 hours (i.e. 60 n miles/6hr)=10 knots; therefore if a feature 'steps-off' 3 degrees of latitude in that 6 hours, it must be moving at 30 knots.
These simple calculations can also be used to forecast the expected movement of fronts using the simple methods described in text books (for example, active cold fronts can be advected (moved) at a speed of four-fifths the measured geostrophic wind measured just ahead of the front, the vector being in the direction of the warm sector isobars. A factor of two-thirds can be applied to the measured geostrophic wind to give the expected movement of the warm front.
Isobars are drawn every 4 millibars on charts originating in the UK, the USA and Canada but note that a 5-millibar spacing is more common on charts originating in countries in continental Europe. Isobars are labelled with values in whole millibars (or hecto-Pascals/hPa), starting at 1000 hPa.
Fronts are drawn with heavy solid lines, distinguished by solid 'triangles' for cold fronts, solid 'bobbles' for warm fronts, and a mixture of the two for an occlusion. The triangles/bobbles point in the direction that the front is heading/thought to be heading and placed on alternate sides of the line when the front is quasi-stationary. Heavy lines with no such additions indicate troughs (the word 'TROUGH' may or may not be indicated beside the line). There are occasionally variations in the graphical representation of fronts. If the front is significantly weakening (frontolysis) then a cross hatch is placed across the frontal line (itself broken) between the triangles/bobbles, and if a front is considered to be forming (frontogenesis) then the solid line is replaced by dots between the 'bobbles / spikes' [ However, in my experience, its mighty difficult to tell the difference between the two! ].
An example of many of the fronts, pressure features etc., can be seen here.
On the 'medium-range' charts (e.g. T+48, 72 etc.), there are additional long-dash lines. These are the 500-1000 hPa total thickness lines at 18 dekametre intervals.
(The medium-range charts are currently listed as Additional Products within the context of the WMO Resolution 40 (WMO Twelfth Congress 1995) and may not always be available on the Web.)
The output is now produced using on-screen analysis and field modification tools. The days of hand-drawing charts for both actual and forecast purposes (at least in the UK service) has come to an end.