Air masses, fronts and frontal (mid-latitude) depressions
This domain tests air-mass source regions, frontal identification and the life cycle of a mid-latitude wave depression.
Air masses and fronts
Air masses are classified by source: polar or tropical, maritime or continental, giving cP, cT, mP and mT. Fronts are boundaries between air masses. A warm front has a gentle slope, stratiform cloud, steady precipitation and a gradual wind veer. A cold front has a steeper slope, cumuliform cloud, showers, squall lines and a sharper wind veer. Frontal fog can form when rain evaporates into cool air and saturates it.
Wave depression
A frontal wave on a polar front develops through wave, open-wave, mature, occlusion and decay stages. The warm sector lies between warm and cold fronts. The cold front usually moves faster, catches the warm front and forms an occluded front. Pressure falls ahead of the system and rises behind. In the classic sequence, warm-front cloud thickens and lowers, rain begins, the warm sector gives milder air and drizzle, then the cold front brings a sharp wind shift, showers and clearing.
Worked example
A station reports pressure falling, cirrus thickening to altostratus, then continuous rain, wind veering from southeast to south and temperature rising. This is a warm-front passage. If the wind then shifts sharply from southwest to northwest, temperature falls and showers begin, the cold front has passed. The warm sector lies between the two events.
Traps
An occlusion is not a third air mass. Warm fronts are not always fast. Veering with a cold front is clockwise in the Northern Hemisphere. Frontal precipitation can occur ahead of the surface front. Tropical systems are not frontal depressions.