Occluded Fronts: When Warm and Cold Fronts Meet

An occluded front is a composite of two frontal systems that merge as a result of occlusion. Cold fronts generally move faster than warm fronts. In fact, the speed of a cold front is about double that of a typical warm front. As a result, a cold front will sometimes overtake an existing warm front. Essentially, an occluded front forms as three air masses meet.

There are two types of occluded fronts
Warm occlusions
Cold occlusions
Cold air occluded fronts are more common than warm occluded fronts.

A front takes its name from two places: it is the literal front, or leading edge, of air that's moving into a region; it is also analogous to a war battle front, where the two air masses represent the two clashing sides. Because fronts are zones where temperature opposites meet, weather changes are usually found along their edge.

Related: Why do we have weather?

Fronts are classified depending on what kind of air (warm, cold, neither) is advancing onto the air in its path. The main types of fronts include:

Warm Fronts

If warm air moves in such a way that it advances onto and replaces the cooler air in its path, the leading edge of the warm air mass found at the earth's surface (the ground) is known as a warm front.

When a warm front passes through, the weather becomes noticeably warmer and more humid than it was before.

Cold Fronts

If a cold air mass spills onto and overtakes a neighboring warm air mass, the leading edge of this cold air will be a cold front.

When a cold front passes through, the weather becomes significantly colder and drier. (It isn't uncommon for air temperatures to drop 10 degrees Fahrenheit or more within an hour of a cold frontal passage.)

Occluded Fronts

Sometimes a cold front will "catch up" to a warm front and overtake both it and the cooler air out ahead of it.

If this happens, an occluded front is born. Occluded fronts get their name from the fact that when the cold air pushes underneath the warm air, it lifts the warm air up from the ground, which makes it hidden, or "occluded." 

Occluded fronts usually form with mature low pressure areas. They act like both warm and cold fronts.

The symbol for an occluded front is a purple line with alternating triangles and semi-circles (also purple) pointing in the direction the front is moving.

Sometimes a cold front will "catch up" to a warm front and overtake both it and the cooler air out ahead of it. If this happens, an occluded front is born. Occluded fronts get their name from the fact that when the cold air pushes underneath the warm air, it lifts the warm air up from the ground, which makes it hidden, or "occluded." 

Updated by Tiffany Means