Occlusions — Page 325, Lesson 312

I want to walk you through occlusions now — specifically the two main types: the warm occlusion and the cold occlusion. Let's start with what happens in general when an occlusion forms. The warm sector — that wedge of warm air between the cold front and the warm front — gets lifted off the ground entirely. Once that happens, the cumuliform cloud associated with the cold front gets pushed into the stratiform cloud of the warm front. That merging creates a specific hazard: embedded cumulonimbus, or embedded CB. That means you can have thunderstorms hidden inside a layer of stratiform cloud, which is a serious concern for any pilot flying through it. Now, for a warm occlusion specifically: most of the precipitation occurs ahead of the surface position of the occlusion. So if you're flying toward the occlusion from the west, you'll encounter the rain or snow before you cross the surface front line. There's an important point about identifying occlusion types. Figure 18.3 in the book is labelled as a warm occlusion, but it might look like a cold occlusion at first glance. The only way to be certain which type you're dealing with is to check the temperatures — specifically, the temperature ahead of the warm front compared to the temperature behind the cold front. Let me walk through the example from the chart. At the station circle at 56° North, 10° West — that's in the 10 o'clock position relative to the occlusion — you see the number "06", which means the temperature there is 6°C. Then to the east of that position, at another station circle near Tiree, the temperature is 3°C. So the air ahead of the occlusion is colder than the air behind it. That tells us it must be a warm occlusion — and that's why the diagram is correctly labelled as such. Now let's move to the cold occlusion, sometimes called a cold front occlusion. If the air behind the cold front is colder than the air ahead of the warm front, then a cold occlusion will form. In this case, the cold air behind the cold front undercuts the less cold air that's ahead of the warm front. That undercutting action is the key mechanism. This type of occlusion is most likely to occur in the summer months. Why? Because the continental air being pulled in ahead of the warm front is warmer than the Atlantic air or Pacific air behind the cold front. So you get that colder air from the ocean rushing in behind the front, undercutting the warmer continental air ahead. Just like with the warm occlusion, the warm sector is raised off the ground, and you again have the hazard of embedded CB. But here's the difference in precipitation: with a cold occlusion, most of the precipitation occurs behind the surface position of the occlusion. So the rain or snow falls after you cross the front line, not before. In plan view — looking down on the weather chart from above — the line of a cold occlusion follows the line of the cold front, and the line of the warm front becomes discontinuous. So the cold front's signature dominates the surface pattern. So to summarise the key distinction: warm occlusion — air ahead is colder, precipitation ahead of the surface position. Cold occlusion — air behind is colder, precipitation behind the surface position, and it's more common in summer. Both lift the warm sector and create the hazard of embedded CB.