The Atmosphere
Movement in the Atmosphere
Atmospheric Stability
Localised Winds
Visibility and Fog
Aircraft Icing
Weather Reports and Forecasts
Practice Exam

Warm Fronts

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.

A warm front typically has a slope of about 1:150

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:

Rising air temperature
High cirrus (Ci) and cirrostratus (Cs) to be observed first
Followed slowly by lowering altostratus (As) and altocumulus (Ac)
Then extensive nimbostratus (Ns) forms near the surface position of the front
The wind will veer
Some rain or virga from altostratus is possible
Potential for the lowering cloud and evaporating rain to produce fog
Rain that evaporates before it reaches the ground is known as virga

Warm Front Hazards

The main hazards of a warm front to a pilot include:

A lowering cloud base and potential for fog
Continuous, widespread rain
Rain can fall from the front into the cold air below, where it will freeze onto the airframe in temperatures below 0°C (known as freezing rain)
Cumulonimbus may be hidden behind or inside lower layers of stratiform cloud

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.