Air Masses — Page 305, Lesson 293

I want to walk you through the concept of air masses. This is a foundational topic in meteorology, and as a professional pilot, understanding air masses is essential for anticipating weather conditions you'll encounter on a route. Let's start with the definition. An air mass is a large volume of air where the humidity and temperature in the horizontal are more or less constant. That means if you were flying through the middle of an air mass, the temperature and moisture content would be fairly uniform across a huge area. How does an air mass get those properties? The air acquires its temperature and humidity by remaining roughly stationary over a surface where conditions are generally constant for some length of time. That surface is typically a high pressure area. Because the air is sitting over a stable, uniform surface, at its source, all air masses must be stable. Now, those basic properties — stability, temperature, and humidity — can change as an air mass moves over surfaces with different properties. Let's look at two scenarios. First, if an air mass moves to a warmer area, it will become heated in the lower layers. As a result, it should become unstable, warmer, and have a lower relative humidity. The heating from below makes the lower layers less dense, promoting upward motion — that's the instability. Second, if an air mass moves to a colder region, it should become more stable, colder in the lower layers, and have an increased relative humidity. The cooling from below makes the lower layers denser and more stable, and since cooler air holds less moisture, the relative humidity goes up. Next, let's talk about how we identify and classify air masses. They are identified by two main characteristics: temperature or latitude, and humidity or sea/land source. For temperature or latitude, we have four categories: Equatorial, Tropical, Polar, and Arctic. For humidity or sea/land source, we have two categories: Maritime, meaning it formed over the sea, and Continental, meaning it formed over land. These are combined into a three-letter classification system. Let me walk you through each letter. The first letter indicates moisture content: 'c' for continental or 'm' for maritime. The second letter indicates the source region or type: 'E' for Equatorial, 'T' for Tropical, 'P' for Polar, and 'A' for Arctic. The third letter indicates temperature: 'c' for cold or 'w' for warm. So, for example, the five air masses that affect Europe are: - Arctic maritime, written as mAc. That's maritime, Arctic, cold. - Polar maritime, mPc. Maritime, Polar, cold. - Polar continental, cPc. Continental, Polar, cold. - Tropical maritime, mTw. Maritime, Tropical, warm. - Tropical continental, cTw. Continental, Tropical, warm. You can see the pattern: the first letter tells you if it's moist or dry, the second tells you where it came from, and the third tells you if it's cold or warm relative to the surface it's moving over. That figure shows the general source regions for these air masses. And there are also figures showing the source regions in January and July, which shift with the seasons. So to summarise: an air mass is a large, horizontally uniform body of air that acquires its properties from a source region, typically a high pressure area. It starts stable, but can change as it moves. And we classify it using a three-letter code that tells us its moisture content, source region, and temperature.