Warm front symbol

How to read it

NOAA’s Weather Prediction Center defines a warm front as the transition zone between warm air and the cold air it is replacing, with the red semicircles, or pips, on the side towards which it moves. NOAA’s JetStream calls them half-moons, and the Met Office compares them to half suns. WPC’s Unified Surface Analysis Manual draws the warm front as a red line with bumps facing into the cooler air, and the WMO’s manual on weather charts gives warm fronts red when a chart is in colour.

JetStream says warm fronts extend from the centre of a low, like cold fronts, but as a rule on its east side. The Met Office describes the warm air rising over the colder air as it replaces it at the surface.

Also searched as: red bumps on a weather map, red line with half circles, red semicircles on a weather map, warm front.

Look-alikes

A blue line with triangles is a Cold front symbol. Red half circles alternating with blue triangles on the other side of the line make a Stationary front symbol. A brown line with open half circles is not a front: on WPC’s charts it is a Dry line symbol.

On black-and-white charts

A Met Office teaching pack describes warm fronts as bold lines with semi-circles or “humps”, coloured red on colour charts. The shape tells the fronts apart even without colour: half circles for a warm front, triangles for a cold one.