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Blue for cold, red for warm, purple for occluded — where colour is available.
Also known as: Blue and red lines on a weather map, Coloured fronts
On a colour chart the front symbols are coloured as well as shaped, and the two encode the same thing twice. That redundancy is deliberate: the shapes survive a monochrome fax and the colours are faster to read.
Front lines are conventionally coloured blue for a cold front, red for a warm front, purple for an occluded front and alternating red and blue for a stationary one.
Contexts: Maps
Deliberately redundant with the symbols. The triangles and semicircles carry the same information, so a chart transmitted in black and white — as they were for most of the twentieth century — loses nothing.
The colour choice is a convention rather than a description: a cold front is not blue in any sense beyond the chart, and the association is with the temperature of the air behind it.
Curved lines across a map in blue, red or purple, each carrying attached triangles, semicircles or both.