Occluded front

An occluded front occurs when a cold front overtakes a warm front, lifting the warm air aloft, resulting in a boundary at the surface where a colder air mass meets a less cold air mass beneath a layer of elevated warm air. This process typically develops in the mature stage of a mid-latitude low-pressure system and marks the transition toward the system’s weakening phase.

Occlusions form because cold fronts generally move faster than warm fronts. As the cold air pushes forward, it eventually catches up to the warm front, undercutting the warm air mass and forcing it aloft. The result is a complex vertical structure with cold air behind the front, cooler air ahead, and warm, moist air rising above both, completely lifted from the surface.

Trowal and weather dynamics

A key feature associated with an occluded front is the trowal, short for trough of warm air aloft. This refers to the wedge of lifted warm air that wraps around the center of the low-pressure system. The trowal is typically the most active part of the occlusion in terms of cloud formation and precipitation. It often appears in satellite imagery and weather models as a band of moisture extending from the center of the cyclone, usually in a comma-shaped cloud pattern.

Types of occlusions

There are two main types of occluded fronts, depending on the relative temperature of the air masses involved:

• Cold-type occlusion: Occurs when the cold air behind the front is colder than the cooler air ahead. The colder air lifts both the warm air aloft and the cooler air in front of it. This is more common in continental interiors.
• Warm-type occlusion: Occurs when the cooler air ahead of the original warm front is colder than the cold air behind the original cold front. In this case, the cold front boundary rides up over the colder air ahead. Warm-type occlusions are more typical in maritime climates.

Weather associated with occluded fronts

Occluded fronts are commonly associated with widespread, persistent weather, often affecting large areas for extended periods. Typical conditions include:

• Overcast skies with layered clouds (stratus, nimbostratus, altostratus), often with embedded cumuliform clouds.
• Continuous or heavy precipitation (rain, snow, or mixed).
• Significant surface temperature changes, the nature and magnitude of which depend on the specific type of occlusion (cold-type vs. warm-type), often less dramatic right at the surface boundary than with warm or cold fronts.
• Shifting winds, usually turning clockwise (in the northern hemisphere).
• Possible embedded thunderstorms in areas with higher instability, particularly within the trowal.

Because the main warm air mass is lifted away from the surface boundary, occluded fronts generally produce less abrupt temperature contrasts right at the surface than primary cold or warm fronts. However, the moisture and uplift within the trowal zone can still lead to significant rainfall or snowfall, particularly in colder seasons.

Identifying occluded fronts

On a weather map, an occluded front is typically shown as a purple line with alternating semicircles and triangles pointing in the direction of movement. These symbols indicate that both warm and cold front characteristics are present in the overall system. The cloud and precipitation pattern often takes on a comma shape, with the trowal forming the head of the comma. This area is commonly targeted in forecasts for precipitation and storm development.

What makes occluded fronts different from other fronts?

Unlike cold or warm fronts that separate two primary air masses at the surface (cold vs. warm), an occluded front involves three air masses. The warm air is lifted above the surface, creating a more complex vertical structure and a broader area of cloudiness and precipitation where a colder and a less cold air mass meet at the surface.

Are occluded fronts dangerous?

While they usually produce less severe weather (like strong thunderstorms) than strong cold fronts, occluded fronts can still bring long-lasting rain or snow, especially in colder months. The combination of low visibility, significant moisture, and wind can pose hazards and impact transportation and outdoor operations.

How do occluded fronts affect aviation?

The trowal region in particular can be hazardous for aviation, with low ceilings, steady to heavy precipitation, and icing conditions. Pilots are trained to recognize and avoid these areas when possible.