FAX charts can be found here: Southweather Charts

Hi all, just thought I'd share with you the wonders of the Met Office FAX charts. They are issued a least once a day, and up to 4 times a day for the 24-hour forecasts.
Despite being a simple chart, with no animation, they can reveal a great deal about the current and forthcoming weather.

The basics - air pressure. The charts show isobars, lines of equal pressure at sea level. This is a very quick and easy way to tell whether the UK will be under High or Low Pressure, and as a rule of thumb under sunny and dry, or wet and windy weather.

You've probably heard the phrase on TV weather forecasts "the closer together the isobars, the stronger the wind". Well the reason behind this is quite simple.

The closer together the isobars, the greater the pressure gradient (for example air is being forced very quickly upwards) and hence the stronger the winds. Tight isobars are often found around LOW pressure systems, where there is a large difference in pressure between the inside and outside of these systems, which creates the large gradients.



If air is rising very rapidly in the centre of the LOW pressure from sea level to the cloud tops, then air is needed to replace the ‘missing air’. This creates wind at ground level, as the air is forced to fill the place. The stronger the rising air, the faster the wind will be at ground level as it is needed to fill the empty space much quicker, and hence the tighter the isobars, the stronger the rising air and the faster the winds.

Next we can plot weather fronts. There are three main types of these: cold front, warm front and occluded front (or occlusion).
Without getting too complicated, cold fronts tend to bring heavy rain, whilst warm fronts are mostly associated with drizzle/light rain. This is mainly due to the angle at which these fronts meet the ground, warm fronts are normally aligned at 30° to the horizontal ground whilst cold fronts are normally 70° to the horizontal ground.
This simplified diagram shows quickly why. Cold fronts can reach higher into the atmosphere, and as a result can create ‘bigger clouds’ from convection, capable of producing heavy rain.



Occluded fronts are brought about by the joining of a warm and cold front. Usually on a basic LOW pressure system, the warm front will be placed ahead of a cold front. However, due to the angle again (and other factors) the warm front usually moves at a slower speed than the cold front, and so effectively the cold front ‘catches up’ with the warm front, and the two merge into an occlusion.



Troughs are simple a line of showers or thunderstorms. These are much harder to predict compared to standard weather fronts, and so they aren’t always added on the BBC maps.

Using knowledge from pressure systems, you can work out the wind direction from a FAX chart. The wind travels anticlockwise around a LOW pressure system, and clockwise around a HIGH pressure system.
Without any predicted temperatures given, you can work out that a wind from the northeast/east is going to be a cold (winter) or cool (summer) wind, whilst one from the south/southwest is going to be a mild (winter) or warm (summer) wind.

Given HIGH pressure you can also highlight the risk of haar or sea fog. If there is a slack HIGH pressure over the UK, with little wind, but the predominant wind is an easterly, then you can predict in advance the risk of sea fog drifting in from the North Sea.

The last bit I will talk about is the dams. ‘Dam’ is a measure of thickness of the air (I’m an amateur so I don’t know much about this) but what I have learnt is that the lower the figure (and thickness), the colder the air will be whilst the higher the figure/thickness, the warmer the air will be. As a rule of thumb, if a weather front is placed well within a dam of 528 or less, then there is a high probability it will fall as snow. This is often highlighted on FAC charts during northerly blasts.

Conversely, if the 564 dam is nearing the UK, then you can expect some pretty high temperatures (often in the 20’s).



Using all of the information I have highlighted above, I will now create a simple forecast for this coming Saturday, based on no other data than the FAX chart provided by the Met Office from this evening.

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Thanks DW for a really informative thread.....I learnt something to night !!

Thanks Dolly, I can't get enough of synoptic charts. Just one question It's a long time since I've seen a "col" on a chart, do the met office not bother anymore or is it there just has'nt been one.

Am i right in thinking a 'col' is placed on a FAX chart in an area between either two Low's or two High's where there is little or no predominant wind (ie a slack area)? If so, nope have never seen one of them on a chart before, but not sure how long ago they stopped doing it?

They also used to plot convergence zones on their charts, but I haven't seen one of those symbols for a while

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Thanks Dolly, very informative and easy to understand. It only seems like half a forecast when they don't show an Atlantic chart on TV and now I understand them a bit more.

Finally they put one on at 1pm today!!

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Is that the thing that looks like a tree going down the spine of the country.

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yep they are very rare these days

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Some things I cant ingest there.

I intend to use your post DW, as a discussion stimulus.

You obviously arent a newby at this, so i am reasonably sure that you will be able to cope with this without offence or trauma.

I will be civil and gentlemanly in my criticism.

Is that okay with you DW?

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haha yeah that's fine I learnt how to read charts since i joined the forum - i had very little knowledge beforehand

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