ARCTIC AIR MASSES 



Generally speaking, air masses are classified according to source 

 and recent life history. The simplest method of designating them 

 allows for four types, which apply to all parts of the earth. These 

 four broad types are : 



cP : continental polar. 



mP: maritime polar. 



cT : continental tropical. 



mT : maritime tropical. 



However, in a discussion of air masses over a particular region, 

 it is desirable to use a more specialized scheme. This is because 

 air from different geographical regions of the same general type 

 assumes characteristics peculiar to the source region, which help 

 materially in analysis and forecasting. For example, polar air 

 which has lain for days over the almost unbroken pack ice north 

 of Bering Strait, in the total darkness of winter, will show, in 

 general, marked similarity to a mass of polar air from the subpolar 

 plateau of Outer Mongolia. But in the arctic air, in addition to 

 the strong inversion of temperature in the lower 5,000-foot layer, 

 there will be found, in most cases, a characteristic 500-foot layer at 

 the very surface, in which the temperature falls rapidly with height 

 and the surface layer shows higher humidity. At the same time, 

 at the top of the troposphere (at about 30,000 feet) , the tempera- 

 ture will be somewhat lower than at the same altitude in the sub- 

 polar air. So it is customary to differentiate the two types of air 

 by adding the following types to the above classifications : 



cA : continental arctic. 



mA : maritime arctic. 



By adding the letter W (warm) or K (cold) to any of the above 

 types, it can be shown whether the air is warm or cold with respect 

 to the surface over which it is moving (stable or unstable). 



Thus in the arctic and subarctic regions we have the following 

 types, together with the general source region and the season or 

 seasons of occurrence: 



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