nights if fog-free terminals are available. Tropical air aloft in- 

 vades the Arctic only on rare occasions, and only in the Atlantic 

 sector, coming from the Azores high in advance of an unusually 

 deep low. Surface tropical air would be even more rare north 

 of 60° N. 



The temperature distribution in individual air masses in the 

 Arctic tends to be more uniform than in the temperate zones, but 

 less homogeneous than the tropical air masses. In the fall and 

 spring with rapidly moving of cold types of air masses, the coldest 

 air may be just behind the front, so that extreme cold is coincident 

 with the most stormy weather conditions such as strong gusty 

 winds, rain, snow, and blizzards. The cold in such cases may pre- 

 clude human activity even though the turbulence associated with 

 the front keeps the temperature from dropping to 50° to 60° below 

 zero, as it does in stagnant air masses. Usually if the wind is at 

 all strong, the temperature will be above 20° below zero. 



Exceptions to this rule will be noted in places like Wrangel 

 Island, where local drainage off of the continent may give tempera- 

 tures of minus 40° F. with strong winds. In the middle of the 

 winter (December through February), on the other hand, the 

 coldest air is to be found a day or more after the frontal passage, 

 even in rapidly moving air masses, since turbulence in the frontal 

 zone is about the only warming influence to be found in the Arctic 

 at that season. 



WIND CIRCULATION OF THE NORTH POLAR REGION 



From the limited data available, a generalized picture of the 

 wind system of the north polar region can be constructed. Of 

 primary importance is the anticyclonic circulation. As little 

 winds-aloft data are available for the areas north of the eightieth 

 parallel, conclusion about the upper-wind circulation in the Polar 

 Anticyclone can be based, to a limited extent, on the knowledge 

 of the circulation in anticyclones of lower latitudes. 



The Polar Anticyclone, as is the case with the continental anti- 

 cyclone of winter in lower latitudes, is dependent on a net loss of 

 heat. However, in the case of the continental anticyclone of 

 lower latitudes, sufficient insolation is received during the warmer 

 half of the year to not only end its existence, but even to replace 

 it with low pressure. In the summer the Polar Anticyclone is not 

 destroyed but pressures are considerably reduced by the added 

 insolation. During the long winter little heat is received from 

 the sun, and there is a continual loss of heat through radiation, 



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