Synoptic chart lines?

[DVP1960]

I understand how to see cold, warm and occluded fronts on the surface analysis charts.

However, I am confused by the simple, often short thick black lines seen on the British met office charts. I think they are fronts but what are they and what is their effect.

I guess that this is pretty basic stuff, but I cannot find the answer on the internet.

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[danP]

If it's the lines I think you are referring to, those are troughs. UKMO depicts them as solid black lines, NWS draw theirs as dashed black lines, other agencies either go dashed or solid lines, or don't show them at all.