Definition
Elongated areas of relatively low atmospheric pressure, shown on weather depiction and surface analysis charts as a dashed line running through the axis of lowest pressure. Troughs are not closed low-pressure systems but extensions of low pressure into surrounding higher pressure, often associated with wind shifts, cloudiness, precipitation, and unsettled weather along or near the trough line.
Plain English
A long, narrow zone of lower air pressure that stretches out from a low-pressure area, marked on weather charts as a dashed line. Weather along this line is often cloudy, rainy, or bumpy.
Context Anchor
Seen on weather depiction charts and other preflight weather charts when checking the overall weather pattern along a route.
Derivation
From Old English 'trog,' meaning a long, narrow container or hollow. The shape of a feeding trough — long and dipped in the middle — matches the shape of a pressure trough on a weather map: an elongated dip in pressure compared to the air around it.
Why Pilots Care
Troughs often signal areas of clouds, precipitation, turbulence, and reduced visibility that affect route planning and safety decisions.
Analogy
Think of a trough as a long shallow valley in the pressure pattern. Weather can collect or organize along that low area, much like water tends to collect in a low place.
Grounding Statement
Picture a long valley running through a landscape of higher ground — a trough is the same shape, but made of lower air pressure stretching out across the chart.
Intuition Check
Do not think of a trough as a cloud type, a storm, or a physical dip in the sky. In aviation weather, it means a stretched-out area of lower air pressure.
Example Sentence 1
The briefer pointed out a trough moving across the route and warned of building clouds and showers along its path.
Example Sentence 2
Surface winds ahead of the trough were southerly and moist, bringing widespread low ceilings along the planned route.