First, let me set the scene — Page 325, Lesson 310

I want to walk you through the topic of occlusions. This is a brand-new chapter, so let's start from the very beginning. First, let me set the scene. You already know that in a typical mid-latitude depression, you have a warm front and a cold front. The cold front moves faster than the warm front. Because of that speed difference, the surface position of the cold front will eventually catch up with the surface position of the warm front. When that happens — when the cold front overtakes the warm front — we have what is called an occlusion. Now, what happens next depends entirely on the temperatures of two specific air masses: the air ahead of the warm front, and the air behind the cold front. Those temperature relationships determine what type of occlusion forms. The point on the weather map where the occluded front meets both the warm front and the cold front is given a specific name. It is called the point of occlusion, or alternatively, the triple point. That's the junction where all three fronts come together. Let's move into the first type: the warm (front) occlusion. The condition for a warm occlusion is this: the air ahead of the warm front is colder than the air behind the cold front. So, the coldest air is out in front of the warm front, and the air behind the cold front is not as cold — it's relatively warmer. In a warm occlusion, the cold air behind the cold front actually rises over the colder air that is ahead of the warm front. Think about that carefully: the air behind the cold front is cold, but it is less cold than the air ahead of the warm front. So the less-cold air behind the cold front overrides the colder air ahead. That is the defining mechanism of a warm occlusion. This type of occlusion is most likely to occur in the winter months. Why? Because as a depression approaches the European coast, or the North American coast, cold continental air is drawn in ahead of the warm front. That continental air is very cold, making the air ahead of the warm front colder than the air behind the cold front. In plan view — that is, looking down at the weather map from above — the line of the occlusion follows the line of the warm front. And importantly, the line of the cold front becomes discontinuous. So on the map, you see the cold front line stops at the triple point, and the occlusion line continues along the path of the warm front. That figure shows you exactly what a warm occlusion looks like in cross-section and in plan view. You can see the cold air behind the cold front riding up over the colder air ahead of the warm front. So to summarise the key points so far: an occlusion forms when the faster-moving cold front catches the warm front. The triple point is where all three fronts meet. A warm occlusion occurs when the air ahead of the warm front is colder than the air behind the cold front, and the cold air behind rises over that colder air. It's most common in winter, and on the map the occlusion follows the warm front line while the cold front becomes discontinuous.