Trough

An elongated area of relatively low pressure, extending from a low-pressure region. It is the opposite of a ridge. Troughs mark the locations of converging and rising air that is flowing from higher to lower pressure and are often associated with cloudiness and precipitation. A trough can be at the surface or aloft. If a trough develops at the surface on the leeward (sheltered) side of a mountain range, then it is called a lee trough.

• Unsettled weather in a trough may include low cloud ceilings, precipitation with low visibility, icing, and convective storms and their associated hazards. This is caused by air convergence and uplift, and the severity is dependent on the characteristics of the trough and the surrounding air mass.
• Sudden wind shifts and strong wind gusts are possible due to changes in the strength and direction of the pressure gradient around a trough and can cause wind shear as well as turbulence.
• Troughs often coincide with the location of cold fronts and warm fronts, at the surface and aloft, and this can intensify or add to any aviation hazards.