Definition
An elongated area of relatively low atmospheric pressure, shown on surface weather maps as a stretched-out region extending from a low-pressure center. A trough has no closed isobars around it, but the pressure within it is lower than the surrounding air on either side. Weather along a trough is often unsettled, with clouds, precipitation, wind shifts, and sometimes thunderstorms.
Plain English
A long, narrow zone of lower pressure stretching across a weather map. It is not a closed low, but a corridor of low pressure where unsettled weather often forms.
Context Anchor
Seen on surface weather maps when reading wind and pressure patterns before a flight.
Derivation
From Old English 'trog,' meaning a long, hollow container. Meteorologists borrowed the word because a trough of low pressure looks like a long dip or valley in the pressure pattern, the opposite of a ridge.
Why Pilots Care
Troughs often signal shifting winds, clouds, and precipitation that affect route planning, turbulence, and weather avoidance.
Analogy
Think of pressure across a region like a landscape of hills and valleys. A trough is a long, narrow valley running through that landscape, lower than the ground on either side but not a closed pit.
Grounding Statement
A pilot crossing a trough line will typically notice the winds beginning to change direction and speed as the low-pressure zone draws air toward it.
Intuition Check
Do not read trough as just a farm or water container here. In weather, it means a long area of lower air pressure on the map.
Example Sentence 1
The briefer pointed out a trough extending southwest from the low over the Great Lakes, warning of scattered showers along the route.
Example Sentence 2
While reviewing the morning briefing, the instructor pointed out the trough line and noted the expected wind shift along the afternoon cross-country route.