Occluded Fronts

An occluded front is the front where the warm air has been lifted away. A cold front has overtaken a warm front, and what meets you at the boundary is the cold air that was behind the one and the cold air that was ahead of the other. The warm, moist air has been progressively removed from the low's centre, and the front you see drawn in purple on a surface map is the seam where two cold air masses now argue over which one is colder.

A composite of two fronts

The National Weather Service defines an occluded front as a composite of two fronts, formed as a cold front overtakes a warm or quasi-stationary front. That word — composite — is doing real work. This is not a new boundary. It is two existing boundaries that have collided and merged, with the warm sector that used to sit between them squeezed upward and out of the picture.

Understanding why requires the ordinary definitions. A cold front is a zone separating two air masses in which the cooler, denser mass is advancing and replacing the warmer. A warm front is the reverse: a transition zone where warm air replaces the colder air ahead of it. In a young cyclone these two run out from the low centre like the arms of a wedge, with warm air occupying the space between them. The cold front moves faster. Given enough time, geometry does the rest.

How the cyclone gets there

The life cycle was worked out by Norwegian meteorologists in the 1910s and 1920s, and the sequence they described still holds. It begins with a boundary separating warm air to the south from cold air to the north, frequently stationary — a front between two air masses that is moving very slowly or not at all.

A wave develops on that front when an upper-level low embedded in the jet stream moves overhead. The front kinks. What was one stationary boundary becomes two moving ones: a cold front and a warm front, with the air masses now in motion and precipitation developing, heaviest along the front itself. As the wave intensifies, both fronts organise, marked by a sharper temperature gradient and a distinct change in wind direction across each.

Then the cold front catches the warm front and overtakes it. That is the mature stage, and the occluded front is its signature. These are large systems — an extratropical cyclone is often 2,000 kilometres across, with the cold front extending toward the equator for hundreds of kilometres.

Cold occlusions and warm occlusions

Once the warm air is aloft, the front at the surface separates cold from cold, and the question becomes which cold is colder. Two types result.

A cold occlusion forms when the coldest air is behind the cold front. The advancing air is the densest in the system, so it does what dense air always does: it undercuts what is ahead of it and forces that air upward. A warm occlusion forms when the coldest air lies ahead of the warm front. In that case the air arriving behind the cold front, though cold, is less cold than the air it meets, and it must ride up and over instead of burrowing beneath.

The distinction matters for where the lifting happens and therefore where the heaviest weather sits relative to the surface boundary. Which type you have depends on the relative coldness of the air behind the cold front and the air ahead of the warm front — and that coldness is a matter of history: whether an air mass arrived as continental polar air, which stays very dry as it moves south from Canada, or as maritime polar air, arctic air that crossed the ocean and picked up some warmth and moisture on the way.

Why the weather turns messy

Front slope explains most of what falls. Warm fronts have a gentle slope, so air rises along the frontal surface gradually over a broad area, producing widespread layered cloud and precipitation along and north of the boundary. Cold fronts are much steeper and force air upward abruptly, which favours a narrow band of showers and thunderstorms along or just ahead of the front.

An occluded front is, by definition, a composite of those two fronts, and both kinds of lifting are still in play: the gradual rise that gives a warm front its widespread layered precipitation, and the abrupt lift that gives a cold front its narrow band of showers and thunderstorms. Add a vertical temperature profile with cold air at the surface and lifted warm air above it, and the precipitation type becomes a function of exactly which layers a falling particle passes through. Rain, drizzle, snow, snow grains, graupel and ice pellets — transparent pellets of frozen raindrops, or largely melted then refrozen snowflakes — can all come out of the same system within a few hours of each other.

What to notice from the ground

Here is the recognition that actually works without a map. When a conventional cold front passes, you have spent the previous hours in warm air. With an occlusion, that warm interval is brief or missing altogether. The rain and cloud arrive, the wind shifts direction, the precipitation may change character more than once, and the surface temperature often stays cold throughout, because the warm air has been lifted off the surface. Prolonged, mixed, cold precipitation with no warm sector in front of it is the fingerprint.

The beginning of the end

The occlusion is also the cyclone's obituary, written in advance. The storm is powered by warm moist air feeding into its centre. As the cold front continues advancing on the warm front, the occlusion lengthens and progressively removes that warm moist air from the low's core. Without the warm air mass to sustain it, the low-pressure system gradually dissipates.

This is why an occluded front on a surface chart is a timing cue as much as a weather feature. The system has reached the mature stage of its life cycle, and the next stage in the model is dissipation. What remains can still be substantial — mixed precipitation, thunderstorms along the cold-frontal segment, hail of an inch or more indicating a severe storm, squalls in which wind speed increases by at least 18 mph and holds at 25 mph or more for a minute — but the engine is being disconnected from its fuel.