All the physical properties of air masses that govern the weather and climate of Europe are determined by its shape and geographical location. Europe, which forms the western part of the great Eurasian continent, is broader in the south and narrow in the north.
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Because of the peculiarity of its shape the maritime influence is observed in higher latitudes. The wintertime air masses originating in the north are relatively warmer, while the summertime air masses produced there are relatively colder than their counterparts in North America.
Another salient feature of the continent, which influences the air masses, is the east-west alignment of the mountain chains.
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The Alps, the Pyrenees and the Caucasus, all the mountain chains extend from west to east and do not obstruct the passage of westerlies which, therefore, enter the continent more readily and more deeply.
In many respects the maritime polar Atlantic air masses in the Western Europe are similar to the maritime polar Pacific air masses along the Pacific coastal regions of North America. Similarly, the maritime tropical Atlantic air masses invading Portugal and Spain resemble the mT Pacific air masses in California.
There is a lot of resemblance between the continental polar air masses of the Western Europe and the western United States of America. Maritime tropical air masses of Great Britain and British Columbia have similar characteristics.
In Europe the trend of mountains is almost parallel with the westerly circulation, while in North America the highlands lay athwart the westerlies. That is why the maritime air masses have free access to the interior of the European landmass and retain their initial characteristics much longer than their counterparts in North America.
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Whereas the east-west mountain chains of Europe do not allow the large sub-tropical bodies of warm water to influence the air masses beyond the great southern peninsulas, the Gulf of Mexico exerts a marked influence on the North American weather.
According to Haurwitz and Austin, the outstanding feature of Europe is the absence of a real source region. According to them, the major portion of Europe may be broadly classified as a zone of transition where the various air masses are subject to transformations. The principal air masses of Europe and their physical properties can be summarised briefly as follows according to seasons:
Winter Air Masses:
(1) Continental polar (cP) air masses:
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The winter source region of the continental polar air masses is located over the snow-covered regions of Eastern Europe and Asia from about 45 o north latitude to the North Pole. These air masses are characterized by low humidities, very low surface temperatures and stable lapse rates in their source regions.
These air masses affect the weather of Russia and Central Europe. Because of the general west-to-east atmospheric circulation, the cP air masses rarely reach Western Europe.
Since the cP air masses move into Western Europe from Asia, they pick up heat and moisture from the warmer underlying surface, and yet they are rather cold. However, because they absorb moisture and there is turbulent mixing in their lower layers, stratocumulus clouds form.
Air masses that originate in western Russia or Fenno-Scandinavia invade central and western Europe. Generally the outbreaks of cold cP air masses occur in the rear of cyclones centered over northern Europe.
Since the Arctic air masses are modified by crossing the open water, they arrive over Europe with steep lapse rates and become modified into mP air masses. Clear skies and low night temperatures are the rule rather than the exception. These air masses are similar to those found in the Central United States.
(2) Maritime polar (mP) air masses:
These air masses are chiefly found in Western Europe. They originate in a source region located in the North Atlantic. Because of the oceanic influence they are relatively warmer. They are humid and unstable.
When mPK air masses invade the continental land mass, they yield light precipitation. However, because of their contact with the cold surface, stability is produced in them. In the absence of any physical barrier, the maritime polar air masses freely enter up to the Alps.
Whenever these air masses are involved into a cyclone, their frontal uplift produces light precipitation in the plains and heavy precipitation in the mountainous regions. They invade the Mediterranean region through Southern France. Sometimes, these maritime air masses travel from over the Mediterranean Sea via Syria and the valleys of Euphrates and Tigris.
The physical properties of these air masses are determined by the ocean routes followed by them. If they come from the Greenland-Spitz-Bergen region, they reach northwestern Europe as cool unstable air masses because of their initial low temperature and shorter route over the open water.
In these air masses, winds are strong and gusty, lapse rate steep and humidity in the lower layers high. Because of the above properties, these air masses produce heavy snow showers.
On the contrary, if mP air masses follow a longer ocean route, they arrive as relatively warm humid air masses that are in a neutral or slightly stable state of equilibrium. Their surface temperatures are almost 10″ to 15″ higher.
Their lapse rate is not so steep. However, rain and snow showers are not uncommon. When the mP air masses take a still more southerly route and enter Europe from a southwesterly direction, their surface temperatures may be higher than those of the winter mPW, so that stability develops in their lower layers.
They usually produce stratus clouds and drizzle. Sometimes the interaction between different mP air masses produces widespread frontal disturbances, so common in Western Europe during the winter.
(3) Maritime tropical (mT) air masses:
They originate from the sub-tropical high pressure cell in the tropical part of Atlantic Ocean. They are cooler, drier and more stable than their counterparts in North America. Because of their long trajectories, they undergo substantial modification before invading Europe.
When compared with the polar air masses they are warmer and moister with a marked stability in the upper layers. The lower layers are characterized by a distinct inversion layer. Since there is a continual cooling from below, these air masses develop stable layers in their lower parts.
Beneath the inversion layer widespread sheets of stratus cloud develop from which there some is drizzling. The amount of precipitation from these air masses is only moderate in the plains. Moreover, because of the upper layer stability, even the frontal activity does not produce heavy rainfall.
The cyclonic storms visiting Western Europe have maritime tropical air masses in their warm sectors. It is, therefore, natural that these air masses are more important during the winter months than during the warmer seasons in Europe.
Having originated in the tropical east Atlantic these wintertime maritime tropical air masses are not convectively unstable like their counterpart in eastern America.
(4) Continental tropical (cT) air masses:
During the winter season they influence the weather of only the Mediterranean region. These air masses originate over the great desert of Sahara and over the dry South-Western Asia. The cyclones moving over the Mediterranean draw cool air masses from the north which move southward toward Africa, and warm air from over north Africa is transported northward across the Mediterranean.
The northwestern Mediterranean region is invaded by the modified mP air masses which are cold and dry. They are designated as mistral. Over the warm Mediterranean, these air masses pick up heat and moisture from below.
Similarly, cold modified cP air masses enter the eastern Mediterranean. They are cold and dry. They are called bora. They come to this region in the rear of extra-tropical cyclones. Being heated from below, they become unstable in the lower layers.
On the contrary, the North African dry and hot air is cooled from below while moving across the Mediterranean. These modified cool and moist air masses affect the wintertime weather of Italy and the Aegean Sea.
In view of the above three modified air masses; the Mediterranean region may be called a transition zone. Thus, winter precipitation of the cyclonic origin in the southern and central Europe is due to the continental tropical air masses that have been modified over the Mediterranean Sea.
Summer Air Masses:
(1) Continental polar (cP) air masses:
The continental air masses during summer season originate over Central Europe, south of the Arctic front. In the opinion of Byers, these air masses which lie between the Arctic and tropical air masses are neutral. But their trajectory does not lie over the ocean. According to Trewartha, these air masses are simply the modified version of mP air.
However, in summer they are more humid than their counterpart in North America. Their temperatures are lower than those of the west European cP air masses. But cP air masses are decidedly warmer than the mP air. This accounts for the less changeable weather in the continent during the summer season.
(2) Maritime polar (mP) air masses:
The main source region of mP air masses during the summer season is the extensive region of the North Atlantic. These air masses which move into the continent of Europe develop either in the northern or the southern parts of the above source region and thus follow different trajectories.
According to the location of their source regions as well as the path followed by them, these air masses demonstrate different physical properties. Air masses forming in the southern part of the source region reach Europe after travelling a longer distance over the open water. Naturally they are marked by stability.
When these air masses are involved in a fully developed cyclone and undergo uplift along the fronts, they yield summer rainfall. On the contrary, air masses originating in the northern part of the Atlantic source region enter Europe by a shorter route. Such air masses are fairly unstable. Some convective activity may also be found in them.
When they are drawn in a cyclone and are made to rise along the fronts, heavy shower is the result. Since there is no mountain barrier blocking the movement of maritime air from the North Atlantic, the prevalence of such air masses produces mild weather conditions over northwestern Europe. Cool summers are the rule in the western part of the continent.
(3) Maritime tropical (mT) air masses:
During the summer season, the influence of mT air masses over the weather of the continent is limited. Since mT air masses form in the northern and the eastern parts of the Azores High, there is pronounced stability aloft.
That is why very stable mTs air mass is found south of 40° north latitude along the west coast of Europe. As these air masses come into contact with the cooler waters of the underlying ocean surface, they become all the more stable in their lower parts.
They are, therefore, not producers of precipitation. Whenever there is a mountain barrier blocking their movement, they are forced to rise which results in precipitation. However, the mT air masses are cooler and more stable over Europe than similar air masses over eastern North America.
(4) Continental tropical (cT) air masses:
In summer, the cT air masses originate over the extensive lowlands of North Africa and Asia Minor. Since the continental region south of 45° north latitude is a source region for these air masses, and since there are many seas there, the cT air masses differ from one region to another in respect of their moisture content.
However, they do not spread widely beyond their source regions and, therefore, they are rather unimportant except in those locations. However, all the cT air masses are very warm and dry.
The continental tropical air masses in summer cross the Mediterranean Sea where they are called sirocco. When the trajectory of these air masses over the water surface is long, they get modified and produce humid and sultry weather. The summer precipitation of the central and southern Europe is partly caused by these modified air masses.
The summertime cT air masses of the southeastern Europe and the adjacent regions of Asia develop in association with the summer anticyclones. They are relatively more stable and do not produce such stormy weather as is produced by the cT air masses originating in Sahara.