A pressure trough is an axis of lower pressure due to an increased amount of lifting. Pressure troughs are common along frontal boundaries since a frontal boundary helps initiate vertical motion. When air rises it reduces the pressure underneath it. This is analogous to someone lifting up on you while on a scale. Although you stay on the scale, the upward force being exerted on you reduces your weight on the scale. The diagram below shows a pressure trough pattern associated with a mid-latitude cyclone both with fronts and without fronts. On the diagram without fronts it can be seen where the fronts should go due to the kinking of the pressure contours both south and east of the low pressure system. Look at the pattern of the isobars on a surface chart when there is a mid-latitude cyclone. Knowing this information can help you locate the position of the cold and warm front.