Air Masses — Page 305, Lesson 294

I want to walk you through the air masses that affect the British Isles and northwestern Europe. The principles here apply to other parts of the world in a similar way, but we'll focus on the specific types that influence this region. Let's start with Arctic Maritime, abbreviated as mAc. The 'm' stands for maritime, meaning it comes from over the sea, and 'Ac' stands for Arctic. Its source region is the polar ice cap. In its source region, this air mass is stable, very cold, and dry. We usually experience it between September and May. During the summer months, mAc has characteristics so similar to Polar Maritime — which we'll cover next — that no distinction is made; we just treat them the same in summer. Now, what happens to mAc as it moves? As it travels down the Norwegian Sea, its relative humidity and absolute humidity increase significantly. The air mass becomes very unstable. This leads to the development of large CU — that's cumulus clouds — and CB — cumulonimbus clouds. These give heavy showers of snow, often with blizzard conditions. Next is Polar Maritime, abbreviated mPc. The 'Pc' stands for Polar, and again 'm' for maritime. Its source is the northwest area of the North Atlantic. In its source region, it is stable, cold, with absolute humidity low but relative humidity high. The weather it brings is cold and moist, with a northwesterly airflow. As it approaches the UK, it becomes unstable, producing Cu and Cb clouds with heavy showers, sometimes hail and thunderstorms. These clouds are most likely over northwest coasts and inland in summer. Visibility is good except in showers. Flying through it is described as bumpy flying. At night, inland, the cloud dissipates. The clearing skies cause a low level inversion with stable air below — that's ideal conditions for radiation fog. Then we have Polar Continental, abbreviated cPc. The 'c' stands for continental, meaning it comes from over land, and 'Pc' for Polar. Its source is Siberia, and this only occurs in winter. In its source region, it is stable, very cold, and dry. The weather depends on the path it takes. If the airflow is mainly from the east via continental Europe, then it's very cold, very dry, with no cloud and no precipitation. It becomes unstable, giving good visibility. If instead the airflow comes over the Baltic or North Seas, the air becomes unstable, with large Cu and heavy snow showers on the east coast of Sweden and the UK. It remains very cold. Visibility is good except in showers. Let me also mention the other air masses shown on the diagram for context. You'll see mPw — Polar Maritime returning — mTw — Tropical Maritime — and cTw — Tropical Continental. We'll cover those in detail when we get to them, but for now, the key ones affecting the British Isles are the three we just discussed: Arctic Maritime, Polar Maritime, and Polar Continental.