AIR MASSES OVER EUROPE
Although, in general, the characteristics of air masses over Europe are much the same as those found over North America, certain differences do exist. One reason for this is that an open ocean extends between Europe and North America toward the Arctic. This allows an influx of mA air to reach Europe. This type of air is not encountered over North America. The location of an extensive mountain range in an east-west direction across southern Europe is an additional influence not present over North America, where the prevailing ranges are oriented in a north-south direction.
If the trajectory of the air is observed carefully and the modifying influences of the underlying surface are known, it is easy to understand the weather and flying conditions that occur in an air mass over any continent or ocean.
Maritime Arctic (mA) Air in Winter
Maritime arctic air is observed primarily over western Europe. Strong outbreaks of this air, originating in the Arctic between Greenland and Spitsbergen, usually follow a cyclonic trajectory into western Europe.
Because of their moisture content and in-stability, cumulus and cumulonimbus clouds are typical of this air mass, frequently producing widespread showers and squalls. Visibility is generally good, but moderate to severe icing often affects aircraft operations.
With the presence of a secondary cyclonic system over France or Belgium, mA air occa-sionally sweeps southward across France to the Mediterranean, giving rise to the severe mistral winds of the Rhone Valley and the Gulf of Lyons. Heavy shower and thunderstorm conditions are typical in this situation.
Maritime Arctic (mA) Air in Summer
In summer, this air is so shallow that in mov-ingsouthward from its source region, it modifies to the point where it can no longer be identified and is then indicated as mP air.
Maritime Polar (mP) Air in Winter
Maritime polar air observed over Europeusually originates in the form of cP air over North America. It reaches the west coast of Europe by various trajectories and is found in different stages of modification; it produces weather similar to mP air over the west coast of North America.
Maritime Polar (mP) Air in Summer
Maritime polar air observed over Europe is similar to mP air observed on the west coast of North America. The weather conditions associated with this air are generally good. Oc-casionally, because of surface heating, a shower or thunderstorm is observed in the daytime over land.
Continental Arctic (cA) and Continental Polar (cP) Air in Winter
The source region for cA and cP air is over northern Russia, Finland, and Lapland. The cA and cP air are observed over Europe in connec-tion with an anticyclone centered over northern Russia and Finland. Occasionally they reach the British Isles and at times extend southward to the Mediterranean.
Because of the dryness of cA and cP air, clouds are usually absent over the continent. Fair-weather cumulus are the typical clouds when cA and cP air are observed over the British Isles. Over the Mediterranean, cA and cP air soon become unstable and give rise to cumulus and cumulonim-bus clouds with showers. Occasionally these air masses initiate the development of deep cyclonic systems over the central Mediterranean. Visibility is usually good; however, after this type becomes modified, haze layers form and reduce the visibility.