When the height contours bend strongly to the south, (as in the diagram below), it is called a TROUGH. Strong troughs are typically preceded by stormy weather and colder air at the surface. Below is an example of a trough in an upper-level height field (red contours). The trough axis is denoted by the purple line.

The image below depicts geopotential height (solid white contours) and temperatures (colored regions) at 500 mb. Temperatures decrease with color from light blue to purple. A trough is located over the eastern United States and is indicated by the dip in the geopotential height field. This is the upper level extension of a surface low pressure center, which is why troughs are also called upper level lows.

Notice the relatively cold temperatures associated with the trough. This is caused by the southward transport of colder air in the lower troposphere. The trough will intensify (deepen further southward) if cold air continues to move southward at low levels in the troposphere.