Because the study was made of non-occluded fronts we find a relatively low 

 frequency of fronts south of Greenland. The majority of the low pressure systems 

 near this region will be deep occluded cyclones. 



The Atlantic Polar Front, in middle latitudes, oscillates within a wide zone. 

 The main source of temperature contrast is the difference in temperature between 

 the polar continental air of North America and the tropical maritime air of the 

 ocean. The_frontogenetical wind field is the field of deformation between the cold 

 continental and warm subtropical anticyclones. It will be seen that the mean 

 position of the front is such that the prevailing wind systems are nearly parallel 

 to the front. When the wind is tangential to the front, the front will be quasi- 

 stationary, and this probably accounts for the observed region of maximum front 

 occurrence. The most intense fronts will most likely coincide with the region of 

 maximum temperature contrast near the east coast of North America when the 

 synoptic situation is favorable for a quasi-stationary front in this vicinity. Since 

 most fronts move off the land with a moderate speed, this region of occasional 

 intense frontogenesis will not be a statistical maximum of front occurrence. 



The Atlantic Arctic Front derives its temperature contrast from the difference 

 in the temperature of the cold Arctic air and the warmer air of the western Atlantic. 

 The frontogenetical wind field is the convergent wind system located in the exten- 

 sive trough from Iceland to northern Scandinavia. Petterssen^ has stated that 

 frontogenesis is most active in this region when the Icelandic low is located further 

 to the east than normal and that it is non-existent when the cold air from the Euro- 

 pean continent streams toward the northwest. In the latter case the front will 

 disappear, due to the absence of a warm air mass. 



In the eastern Atlantic from the subtropics to about 55° N we find frequent 

 invasions of tropical maritime air. As this air flows northward, it will be cooled 

 from below and the resulting hydrometeors will be fog and drizzle. In the north- 

 western Atlantic the principal air mass will be modified polar continental air, cold 

 air which has been rapidly heated from below. Due to the instability which arises 

 from this heating, cumulus type clouds and showers should prevail. As this air 

 streams further eastward and finally northeastward, the heating from below will 

 cease, and consequently the shower frequency will be considerably diminished. 

 Orographic lifting along northwestern Europe will give rain over the land. 



The high frequency of fogs east of Newfoundland is mainly due to the advec- 

 tion of warm maritime air ovei the cold Labrador current. The cooling of the 

 maritime air will result in an extensive region of fog. 



No attempt has been made to produce a similar map for the summer. In 

 the summer the fronts are much weaker, and we have seen that a vast area of the 

 Atlantic is under the influence of the subtropical anticyclone. The Atlantic Polar 

 Front will be indistinct and the main frontal system will be the Arctic Front which 

 extends from Iceland along the northern coast of Europe and Asia. 



9. Average Cloudiness. 



The cloudiness maps. Figures 17 and 18, show the average cloudiness in 

 tenths of the sky covered, in winter and summer respectively. The data for these 

 charts was obtained from the Climatic Charts of the Oceans^ (to 60° N) , the Kbppen- 

 Geiger Handbuch'^ (beyond 60° N and Scandinavia) , and Climatic Maps of North 

 America.* 



Figure 17 shows that north of 40° N the average cloudiness exceeds 7/lOths 

 throughout the Atlantic. This relatively high cloudiness is the result of the intensive 

 cyclonic circulation which will result in frontal and air mass clouds. South of 35° N 



