Occlusions — Page 337, Lesson 313

I want to walk you through the topic of occlusions now. We’ve already covered the basic structure of a depression, and now we’re looking at what happens as it matures and begins to decay. Let’s start with the weather associated with an occlusion. The weather is usually bad. Why? Because the normal frontal depression weather — the cloud and rain you’d get from a warm front and a cold front separately — gets concentrated into a smaller horizontal band. So instead of having two separate belts of weather, you get a mixture of clouds all packed together. For example, you can have cumulonimbus clouds, which we abbreviate as Cb, embedded within nimbostratus, which is Ns. That’s a serious combination: Cb brings heavy showers, thunderstorms, and turbulence, while Ns gives steady, widespread rain or snow. Furthermore, an occlusion forms towards the end of the life cycle of a depression. At that stage, the depression is slow moving. Because the system is moving slowly, the weather can last for a lengthy period of time — it hangs around. Now, the book tells me that the situation I just described applies more particularly to the warm type occlusion. That’s because the warm type has a wider precipitation belt, and it’s more frequent in European winters. The reason for that frequency is the effect of Polar Continental air coming from the east. And there’s a specific hazard mentioned: rain ice. Rain ice is a particular hazard with this type of occlusion in those conditions. Occlusions can also become non-active. When that happens, they produce a little cloud and nothing more, as the depression itself dies. Let’s move on to back bent occlusions. As the occlusion forms, the first point of occlusion — the place where the cold front first catches up to the warm front — is at the depression centre. From there, the occlusion gradually moves south and west, forming what we call a back bent occlusion. It’s rather like a loop through the depression centre. This back bent portion is usually some 100 to 200 nautical miles long. It gives a belt of rain in the cold air behind the cold front, and that rain is often of a thundery nature. There’s a diagram for this. Now, movement. The precise forecasting of weather and movement of the occlusion is difficult. However, the point of occlusion — that meeting point — may be plotted for some time ahead. You do this by moving the warm front and cold front of a warm sector depression as described in the last chapter. Where those fronts meet will be the new point of occlusion. The movement of the depression itself is much more difficult to predict. But we do know that in the Northern Hemisphere, it will curve in an anticlockwise direction. Its speed is dependent upon isobar spacing — tighter spacing means faster movement, looser spacing means slower. Finally, growth and decay. The growth of a depression, from formation to the time it produces its lowest pressure at the centre, takes about 4 days. The dying away — as the depression fills — can take 10 days or more. Eventually, the depression is absorbed by some other pressure feature. For the British Isles specifically, the time sequence typically involves three stages. First, formation and growth near the eastern seaboard of the USA or in the mid-Atlantic. Second, the depression reaches its lowest pressure in the central to eastern Atlantic. Third, the depression fills — that’s the process of frontolysis — over East Europe, the Norwegian Sea, or Scandinavia. Eventually, these depressions fill and lose their identity in central Asia or the Arctic Ocean.