A trough is an elongated region of relatively low pressure or low geopotential height, drawn on a map as a southward dip in the height contours and often associated with colder, denser air in the column below. Troughs appear at all scales, from a small surface trough along a boundary to a deep longwave trough spanning the western United States.
The physics of a trough
A trough on an upper-air map is a region where the height of a given pressure surface, most commonly 500 mb, dips lower than the surrounding air. NOAA's upper-air primer notes that this happens where the column below is colder and denser, so a given pressure level sits closer to the ground.
Air within the trough flows cyclonically, curving counterclockwise through the base in the Northern Hemisphere. That curvature and the wind speed changes through the trough set up regions of vorticity that drive vertical motion in the layers below.
How forecasters read it
Forecasters look at troughs on the 500 mb chart first because that level tracks the middle of the troposphere and steers most weather systems. NOAA JetStream summarizes the pattern rule this way: unsettled weather generally sits between the trough and the downwind ridge, while fair weather sits between the ridge and the downwind trough.
In a severe weather context, the base and downstream side of a mid-latitude trough is where the polar jet dives south, where cyclogenesis is favored at the surface, and where warm moist air ahead of the system is forced to rise. The Plains outbreak setup is often a deep western trough digging into California and the Great Basin with a surface low forming on its downstream side.
What it does not tell you
A deep trough on the map is not the same as active weather. Cutoff lows can sit for days without producing much because they lose their connection to the jet and to fresh moisture return. Very sharp, negatively tilted troughs, in contrast, can force strong ascent even when they look modest.
The trough is a piece of the setup, not the setup itself. Its interaction with the surface pattern, moisture axis, and low-level shear is what determines the story on any given day.