Air Masses — Page 305, Lesson 296

I want to walk you through the concept of air masses and fronts. Let's start with the air masses we haven't covered yet. We have Polar Maritime air, which is given the code mPc. This air mass also has a source in the other oceans in temperate regions — so not just the North Atlantic, but other ocean areas at similar latitudes. Then there's Tropical Maritime air, coded mTw, which has sources in the other subtropical oceans. And Polar Continental air, coded cPc, has a source in Canada, and this air mass considerably affects North American weather. Now, let's move to the core of this section: fronts. A front is a zone or surface of interaction between two air masses of different temperature. That's the formal definition. When two air masses meet, the warmer one will rise over the top of the colder one because of the difference in density. The warmer air is less dense, so it's forced upward. The frontal surface — that's the sloping boundary where they meet — is frequently, but not always, active with much cloud and precipitation. The ground position of that frontal surface is what we see shown on synoptic charts, the weather maps you'll use in flight planning. A front is usually only a few miles wide. But if the term "zone" is used, then the region of interaction is much wider — up to 300 nautical miles. So "zone" means a broader area of transition, not a sharp line. Now, the main global fronts are four. I'll list them: the Polar Front, the Arctic Front, the Mediterranean Front, and the Intertropical Convergence Zone, which we abbreviate as ITCZ. Let's take the Polar Front first. The Polar Front is the boundary between polar and tropical air masses. In the Northern Hemisphere, the polar front is found between latitude 35° North and 65° North. In the North Atlantic, the front extends from mid-Florida to southwest UK in winter, and from Newfoundland to northwest UK in summer. In the North Pacific, the polar front is found in similar latitudes. In the Southern Hemisphere, it is found between about 50° South and 55° South throughout the year — so it's more constant in latitude down there. There are numerous waves on the front, and these waves cause depressions — low-pressure systems — which contain their own portions of the polar front. Next, the Arctic Front. The Arctic Front is the boundary between the Arctic and the Polar air masses. It may have an associated jet stream. It lies at higher latitudes than the polar front, but sometimes moves into temperate latitudes — specifically from south Greenland to north of Norway — in winter and spring. That figure shows the general source regions for air masses, which helps you visualise where these fronts sit geographically. And Figure 17.7 in the book illustrates the frontal surface itself — the sloping boundary between the two air masses. So to summarise: a front is the interaction zone between two different-temperature air masses, the warmer rises over the colder, and the main global fronts are the Polar Front, Arctic Front, Mediterranean Front, and the ITCZ. The Polar Front separates polar from tropical air, and the Arctic Front separates Arctic from polar air, sitting at higher latitudes but sometimes dipping south in winter and spring.