Trough of Warm Air Aloft

When large masses of warm air and cold air meet, they do not mix because of density differences. Instead, they form a front, usually hundreds of miles long. An occluded front forms when a warm air mass gets caught between two cold air masses. The warm air mass rises as the cool air masses push and meet in the middle. The warm air mass is occluded, or “cut off,” from the ground and pushed upward, creating a trough of warm air aloft (TROWAL). Occluded fronts can bring strong winds and heavy precipitation. Occluded fronts usually form around mature low pressure areas. There are two types of occlusions, warm and cold.

Cold Occlusion

In a cold occlusion, the cold air mass overtaking the warm front is colder than the cool air ahead of the warm front, and plows under both air masses.

Warm Occlusion

In a warm occlusion, the cold air mass overtaking the warm front is warmer than the cool air ahead of the warm front, and rides over the colder air mass while lifting the warm air.