Representation of alternating troughs and ridges in upper-level
Westerlies for the
Northern Hemisphere, with regions of convergence and divergence labeled.
A trough is an elongated area of comparatively low atmospheric pressure, usually in association with weather fronts.
Convective cells may form in the surrounding area of troughs and give birth to a tropical cyclone. Some tropical or subtropical areas, such as the Philippines or south China are greatly affected by convection cells along a trough. In the mid-latitude westerlies, troughs and ridges often exchange with each other, especially when upper-level winds are in a high-amplitude pattern. For a trough in the westerlies, the region just west of the trough axis is typically an area of convergent winds and descending air - and hence high pressure - while the region just east of the trough axis is an area of fast, divergent winds and low pressure. Tropical waves are a type of trough in easterly currents, a cyclonic northward deflection of the trade winds.
Images for kids
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A very large trough (about 8000 km or more) crosses the North Atlantic Ocean from north east to south west. The elongated cloud is surrounded by two big areas of higher atmospheric pressure, clearly shown with no clouds at all.
