When a warm air mass is moving in to replace a cold air mass, the boundary is called a warm front. The warm, less dense air rises over the top of the colder, more dense air.
The cold & dense air resists motion, causing the front to move across the ground slowly and the slope formed along the warm front to be shallow and spread out over a large area. This creates a stable atmosphere with widespread stratiform cloud and drizzle that can persist for long periods.
The warm air at altitude can be some 600nm ahead of the warm front line drawn on a chart – potentially leading to rain falling up to 200nm ahead of the front.
The passing of a warm front brings almost every type of stratiform cloud we’ve discussed so far.
In the passing of a warm front you can expect:
The main hazards of a warm front to a pilot include:
A warm front is shown on synoptic charts as red a line with semicircles. These semicircles can be thought of as half suns, with the edges of the ‘suns’ indicating the direction of movement of the warm front.