Trough symbol

How to read it

NOAA’s Weather Prediction Center defines a trough as an elongated area of relatively low atmospheric pressure, the opposite of a ridge, and its Unified Surface Analysis Manual says it is drawn as a wide orange, dashed line. NOAA’s JetStream school adds that a trough is not a boundary between air masses: the wind changes direction across it, but the air mass does not change. WPC also uses the trough symbol for outflow boundaries, the edge of air spread out by thunderstorms, labelled “OUTFLOW BNDRY”.

Also searched as: dashed line on a weather map, orange dashed line, surface trough, TROF, trough line.

Differences between weather services

The line looks different from one chart to the next. WPC draws a wide orange dashed line. The Met Office draws troughs as black lines without semi-circles or triangles. Washington State University’s weather column says troughs are “typically portrayed as dashed lines”, and Iowa State University’s surface map guide shows them as a yellow dashed line. The WMO’s manual lists the “axis of trough” among its chart symbols.

Look-alikes

A trough has no triangles or half circles, which is what sets it apart from the Cold front symbol and Warm front symbol. The Met Office notes that troughs often bring showers but are not boundaries between air masses “like fronts”. A brown line with half circles on WPC’s charts is a Dry line symbol.