Most people understand the basics of cold and warm fronts: Passage of a cold front cools things down, and passage of a warm front warms things up. But warm fronts and cold fronts don’t simply follow one another in orderly procession. They can also merge in what’s known as an occluded front, an important stage in the development of many of the great weather-making low-pressure systems known as midlatitude cyclones.

Midlatitude (or extratropical) cyclones – which shouldn’t be confused with tropical cyclones or hurricanes – form along weather fronts, which are boundaries between air masses of different temperatures and associated conditions. A wave along the front creates a low-pressure disturbance, which draws in winds that – because of the rotation of the Earth – spiral around the low. The leading edge of the warm air mass, where it rises over denser cold air, creates a warm front; that of the cold air mass behind, which shoves under the warm sector behind the warm front, creates a cold front.

In an occluded front, the trailing cold front overtakes the preceding warm front. This is conventionally described as the cold front “catching up” to the warm front. However, while it’s true that cold fronts tend to move faster than warm fronts, recent research suggests more underlying cyclonic processes cause the frontal mashup. Regardless, an occluded front involves the warm air behind the warm front behind forced aloft, the low-pressure center of the cyclone moving away from the frontal boundary, and the cold front coming into contact with the cold air mass originally downwind (so to speak) of the warm front.

Two types of occluded front exist: the warm-type and the cold-type. They’re distinguished by the relative temperatures of the air mass ahead of the occlusion – in other words, the air mass ahead of the original warm front – and the air mass behind the cold front. If the air behind the cold front is colder than the air ahead of the occlusion, it shoves beneath that air (because it’s denser) to form a cold-type occluded front. If the air behind the cold front is warmer than the air ahead, it rides over it to form a warm-type occluded front – which appears to be the more common case. In either situation, the lighter warm air representing the air mass originally between the warm and cold fronts sits above the boundary between the two cooler air masses.

Colored weather maps represent cold fronts with blue lines studded with blue triangles pointing in the direction of the front’s movement. Warm fronts appear as red lines marked with red half-circles that also point toward the direction of frontal movement. An occluded front shows on the map as a combination of these symbols: a purple line alternating with purple triangles and half-circles.

A forward-moving front, whether warm or cold, causes one air mass to lift above another; by forcing the air mass to its condensation level, this creates clouds and often precipitation. Weather along an occluded front can take many forms, but some combination of cold-front and warm-front effects often takes place, with anything from light to heavy precipitation often diminishing to clear skies after the front’s passage.