The usual cycle of the passing of a depression or Low Pressure System is defined by the approach of a warm front as the warm air of a low pressure system of air slides over a colder high pressure air mass.
As the air rises the warm, moist air condenses, creating cloud. As the cloud thickens and the cloud base lowers, the barometer starts to fall as the warm front approaches. Cirrus cloud will be followed by Altostratus and Nimbostratus cloud and rain.
The air mass following behind the passing warm front is known as the warm sector. It is generally warmer than the air before the warm front or after the cold front that follows. Behind the warm front the rain becomes lighter, turns to drizzle or ceases, but it usually remains cloudy. Temperatures rise behind the warm front and the wind turns clockwise. This is known as veering.
The warm sector might be accompanied by a period of clear, dry weather. The barometer will be at it’s lowest. The air mass after the warm front and before the impending cold front is therefore known as the warm sector.