Troughs and Ridges
Troughs
A trough is an area of low pressure, bounded by areas of high pressure of each side.
Since the air in the trough cannot move outward due to high pressure at its boundaries, nor can it descend into the ground, it is forced upwards. This upward moving air is conducive to cloudiness and precipitation.
Ridges
A ridge, or a ridge line, is an elongated area of high pressure. These are areas of descending air, which dissipate cloudiness.