Air Masses — Page 305, Lesson 295

I want to walk you through the air masses that affect Europe, starting with a summer situation that's quite specific. In summer, the air mass virtually disappears — but there's an exception. When high pressure sits over Scandinavia in early to mid summer, we get a north-easterly flow across the North Sea into the eastern UK. That air originates as dry, warm, and stable. But as it crosses the North Sea, it picks up moisture and cools down. The result is what we call Haar conditions along the east coast of northern England and Scotland — very low stratus cloud, drizzle, advection fog, and poor visibility. Now let's move to Tropical Continental air, abbreviated cTw. The 'cT' stands for continental tropical, and the 'w' indicates it's warmer than the surface it's moving over. Its source is North Africa and southeastern Europe. It's mainly a summer air mass — warm, dry, and stable. The weather it brings is a warm, dry southerly or south-easterly flow. There's no cloud or precipitation, and it's warm or very warm. Visibility is moderate, except when dust haze occurs, which it can. Next is Tropical Maritime air, mTw — maritime tropical, warmer than the surface. Its source is the Azores anticyclone. This air is warm, stable, has high absolute humidity, and high relative humidity. The weather is a warm, moist south-westerly air flow. As the air moves north, its temperature reduces — but it stays warm. Stability and relative humidity both increase. We get low cloud, stratus and stratocumulus, drizzle or light precipitation, and poor visibility. Advection fog appears over the sea in late spring and early summer, and over land in winter and early spring. In high summer, insolation — that's heating from the sun — and convection break down the stability, giving clear skies or possibly a few small cumulus clouds. Then we have Returning Polar Maritime air, abbreviated mPw — maritime polar, warmer than the surface. This is polar maritime air that has moved south of the North Atlantic and approaches from the west or south-west. In its lower layers, it acquires and retains tropical maritime conditions. In north-west Europe during summer, it tends to give typical mPc conditions — that's maritime polar cold — particularly by day. In winter, conditions in north-west Europe tend to be similar to those of an mTw air mass. Finally, Equatorial air, mEw — maritime equatorial, warmer than the surface. These air masses do not affect Europe. The weather associated with equatorial regions is discussed later in Chapters 19 and 20.