On weather maps, you may notice a front that resembles a purple line with alternating spikes and semicircles. This front actually occurred not too long ago and is partially the reason for why the Twin Tiers got active weather this past Saturday and why the temperature dropped significantly. The front is known as an “occluded” front.

Occluded fronts happen when a strong and mature low pressure system leads to cold air masses overrunning the warm air mass. On a typical overhead depiction of the frontal system, the cold air mass behind the cold front and the cold air mass ahead of the warm front surround the much smaller warm air mass down south. The actual frontal system is more complex than this, but the basic Norwegian Cyclone Model is used for simplicity.

Overhead depiction of a basic Norwegian Cyclone Model, including a cold front, warm front, occluded front, streamlines, and low pressure system. Photo credit: SKYbrary.

When an occluded front is present, two different types of occlusion can occur. These occlusions are known as cold occlusions and warm occlusions. In a cold occlusion, the cold air mass that overtakes the warm front is colder than the cold air mass ahead of the warm front. Because of it’s high density, the cold air mass overtaking the warm front plows under both air masses.

Cross-section view of cold occlusion. Photo credit: SKYbrary.

In a warm occlusion, the cold air mass that overtakes the warm front is warmer than the cold air mass ahead of the warm front. The cold air mass in this case rises over the colder air mass while lifting the warm air.

Cross-section view of warm occlusion. Photo credit: SKYbrary.

A variety of weather features can be found along an occluded front, which can make forecasting more difficult. Thunderstorms are possible, but usually their passage is associated with a drying air mass. If wind speeds increase greatly with height along the front, then cold core funnel clouds are possible. Small isolated occluded fronts often remain for a short period of time after a decaying low pressure system, which can create cloudy conditions with patchy rain or showers.