Cold front symbol

How to read it

NOAA’s Weather Prediction Center defines a cold front as a zone between two air masses in which the cooler, denser one is advancing and replacing the warmer, and puts the blue triangles, which it calls pips, on the side the front is moving towards. NOAA’s JetStream school and the Met Office describe the same blue line with triangles, and the Met Office compares the triangles to icicles. The WMO’s manual on weather charts lists the cold front first among its frontal symbols and gives it blue when a chart is drawn in colour.

“Cold” and “warm” are relative, JetStream notes: the air behind a summer cold front can still be hot, only cooler than the air it replaces. JetStream also says cold fronts, as a rule, run south and west from the centre of a low, and not from a high.

Also searched as: blue line with triangles, blue triangles on a weather map, cold front, triangles on a weather map.

Look-alikes

A red line with half circles is a Warm front symbol. A purple line with triangles and half circles on the same side is an Occluded front symbol, and a line of alternating red and blue, with triangles and half circles on opposite sides, is a Stationary front symbol.

On black-and-white charts

The colour is not essential to the symbol. A Met Office teaching pack describes cold fronts as bold lines with triangles that are blue “when displayed on colour charts”, and the WMO manual sets out a monochrome and a coloured form of every front.

Forming and weakening fronts

A front that is forming or dying is drawn differently, and the conventions differ. The WMO lists separate symbols for frontogenesis (a front forming) and frontolysis (a front weakening). WPC draws a weakening front as a dashed line with its triangles on every other segment. On the Met Office’s charts a front whose line is broken by crosses is weakening.