Other types of depression also exist, in temperate and other regions — Page 337, Lesson 318

I want to walk you through a new chapter — Chapter 19, "Other Depressions." Up to now, we've focused heavily on polar front depressions, which are the main low-pressure systems in temperate latitudes. But as a professional pilot, you need to know that other types of depressions exist, both in temperate regions and elsewhere. This chapter introduces those. Let's start with the introduction. The key point is that polar front depressions predominate in temperate latitudes — they're the most common — but they are not the only kind. Other types of depression also exist, in temperate and other regions. The chapter lists two main categories to begin with: orographic depressions and thermal depressions. We'll cover each in detail. Now, let's look at orographic depressions — also called lee depressions. The word "orographic" refers to mountains or high ground. Here's the mechanism: when a flow of air meets a mountain range at a large angle — meaning the wind is blowing more or less perpendicular to the mountain chain — there is a marked tendency for much of the air to flow around the end of the range instead of flowing over the top. This is an important point — the air doesn't just pile up and go over; it diverts horizontally around the obstacle. That diversion causes a comparative lack of air on the downwind side of the mountains — the lee side. "Lee" means the side sheltered from the wind. Because there is less air mass on that side, low pressure occurs. That low pressure is the orographic depression. The excerpt gives a specific real-world example: orographic depressions formed over Northern Italy when a cold front comes from the north into the Alps. In that situation, the colder air of the front is now coming over the Alps above the warmer air to the south. That creates very unstable conditions, which can produce significant convective weather — meaning cumulonimbus clouds, showers, thunderstorms, that kind of thing. Let me break that down. Normally, cold air is denser and stays near the surface, pushing under warm air. But here, the cold air is forced up and over the Alps by the mountain barrier, so it ends up riding above the warmer air on the Italian side. That's an inverted temperature structure — cold over warm — which is inherently unstable. The result is strong convection. The chapter then describes three weather situations that can occur with orographic depressions. The first one is: if the air is dry and stable, then any uplift caused by the depression will have little effect. The weather will be warm, clear, and dry. This is called the Föhn effect. The Föhn effect is a well-known phenomenon in mountain meteorology — dry, warm downslope winds on the lee side. So in this case, the orographic depression doesn't produce much weather because the air lacks moisture and is stable. The second situation: if the air is moist, then the uplift caused as it passes over the depression can ensure that cumulus and cumulonimbus clouds — abbreviated Cu and Cb — with showers and possibly thunderstorms and hail may develop. So here, the same orographic low-pressure mechanism, but with moist air, produces significant convective weather. There's a third situation mentioned in the chapter heading, but the excerpt cuts off before describing it fully. We'll get to that when we continue. Let me also note that there are figures referenced in the book. Figure 19.1 shows orographic depressions, Figure 19.2 illustrates the Föhn effect, and Figure 19.3 shows the moist, unstable case. These are available on your screen. So to summarise what we've covered: orographic depressions form when air flows around a mountain range instead of over it, creating a lee-side low. The resulting weather depends on air stability and moisture content — dry stable air gives the Föhn effect with clear weather, while moist air can produce cumulus, cumulonimbus, showers, thunderstorms, and hail.