Area Climatology — Page 422, Lesson 385

I want to walk you through the area climatology of the North Atlantic region, focusing on polar lows and the weather patterns that affect aviation across this area. Let's start with the big picture. Over the Atlantic, the polar front acts as the boundary between two distinct air masses. To the north we have mPc air — that's maritime Polar continental air, cold and relatively dry. To the south we have mTw air — maritime Tropical warm air, warm and moist. As travelling depressions — what we call low-pressure systems — develop along this front, a portion of that warm mTw air gets increasingly trapped between areas of mPc air on either side. This trapping process is what forms the warm and cold fronts that we study in frontal systems. Now, north of the polar front, we get a different type of system: polar air depressions. These are formed when Arctic air moves southward between about 65°N and 55°N over relatively warmer seas. That temperature contrast — cold air over warmer water — causes instability, and that instability produces the weather we associate with these polar lows. Let's look at a specific contrast that really matters for aviation planning: the difference between London and New York in winter. New York sits at 40°N, while London is at 52°N — London is twelve degrees of latitude farther north. Yet the winter weather is worse in New York. Why? The answer lies in the source of the air. Cold continental outflow from the North American High — that's a large area of high pressure over the continent — moves out over the adjacent but warmer Atlantic Ocean. That air becomes unstable over the warmer sea, forming low pressure. The resulting instability can then swing back inland, bringing snow to the New York area. London in winter can also be affected by cold continental outflow, but from a different source: the Siberian High. The key difference is that this air has a long land track before it reaches London, so it remains dry. Secondly, if the wind in London is from the prevailing west, it's flowing off the Atlantic and therefore is relatively warm, possibly giving rain but not snow. So the same type of mechanism — cold continental outflow — produces very different outcomes depending on the path the air takes. Now let's talk about cloud. In the north of this region, cloud averages 6 oktas — that's six-eighths of the sky covered, so mostly cloudy. This cloud is mostly associated with travelling depressions. Let me walk you through what a pilot flying west through a polar front depression would encounter, and I'll give you the distances from the warm front surface position. Starting farthest out: Cirrus — that's high, wispy ice-crystal cloud — appears 400 to 600 nautical miles ahead of the warm front surface position. Next, Cirrostratus — a thinner, layered high cloud — appears 300 to 500 nautical miles ahead. Then Altostratus — a mid-level layered cloud — appears 200 to 400 nautical miles ahead. Closer in, Stratus and Nimbostratus — low, layered rain-bearing cloud — appear 200 to 300 nautical miles ahead of the warm front surface position. Above the warm sector at low level, you'll find Stratus and Stratocumulus — low layered and lumpy cloud. And finally, at the cold front surface position and extending 100 to 200 nautical miles beyond it, you'll find Cumulus and Cumulonimbus — the towering, unstable cloud that brings showers and thunderstorms. Let me also describe what happens with the air masses. As mTw air moves north over colder seas toward the polar front, extensive stratus and stratocumulus forms, especially while that air is trapped in the warm sector of polar front depressions. Cumulus and cumulonimbus occur on cold fronts, with cumulus also forming in the following unstable northwesterly air behind the front. Down in the Caribbean, the moist northeast trade winds produce orographic cloud and rainfall on windward slopes — that's cloud formed when moist air is forced upward by rising terrain. That figure shows you the North American weather details in January and the typical cross-section through a polar front depression. The cross-section illustrates exactly what I just described: the sequence of cloud types as you approach and pass through the depression from west to east. So to summarize for your aviation planning: when flying across the North Atlantic in winter, expect the polar front to separate cold mPc air north from warm mTw air south. Travelling depressions along this front trap warm air between cold air masses, forming warm and cold fronts. North of the polar front, polar air depressions form from Arctic air moving over warmer seas. And the cloud sequence I gave you — from cirrus 400-600 NM out, down to cumulonimbus at and beyond the cold front — is your practical guide to what you'll see on the weather radar and out the windscreen as you transit a polar front depression.