frontal clouds will be less frequent, due to the absence of strong frontal systems. 

 Likewise, as the result of subsidence, air mass clouds will be less frequent in the 

 lower latitudes. It is evident that the degree of cloudiness increases considerably 

 in the vicinity of the North American coastline. The presence of polar continental 

 air over the land in winter results in comparatively clear skies over the land, but 

 when this air passes over a warmer water surface the subsecjuent heating from below 

 will cause an increase in the amount of cloudiness. This effect is present over the 

 Great Lakes, where we find over 7/lOths cloudiness. Observations over the Atlantic 

 have shown that stratus and strato-cumulus clouds are the most frequent north 

 of 40° N. 



Over the ocean in summer the amount of cloudiness is slightly less than in 

 winter. Moreover, the type of clouds differs somewhat from winter to summer. 

 In summer we find a higher percentage of cumulus type clouds, especially in the 

 low latitudes. Furthermore, over land the summer heating causes a considerable 

 increase in the cumulus cloud types. 



The total amount of cloudiness over the ocean is found to change only slightly 

 from one season to another. The most pronounced change is perhaps the decrease 

 in the cloud frequency near latitude 30° N in summer. This minimum of cloudi- 

 ness is the result of subsidence within the extensive subtropical anticyclone. 



10. Precipitatiox Frequency. 



The frequency of occurrence of precipitation is indicated in Figures 19 and 

 20. This data was obtained from the Climatic Charts of the Oceans- and from the 

 Koppen-Geiger Handbuch.^ The most frequent region of winter precipitation in 

 the Atlantic coincides with the mean position of the Atlantic Polar Front. Unfor- 

 tunately there was no data available for the Atlantic beyond 60° X. However, 

 it is quite likely that the maximum region of precipitation frequency is between 

 southern Greenland and Iceland, i.e., near the mean low pressure center of Figure 1. 

 Likewise, precipitation should be quite frequent in the vicinity of the Arctic Polar 

 Front. It is evident from Figure 19 that precipitation is more frequent along 

 western Europe than along eastern North America. This is due not only to the 

 deep cyclones approaching Europe but also to the prevailing winds which give 

 orographic precipitation along northwestern Europe. 



As in the case of cloudiness, the frecjuency of precipitation in summer is less 

 than in winter, but the general features of the two charts vary only slightly from 

 season to season. It is important to note that precipitation is less frequent in low 

 latitudes than in high latitudes. The precipitation of low latitudes is generally in 

 the form of heavy convective showers. 



11. Fog Frequency. 



In Figures 21 and 22 the frequency of fog has been illustrated for winter 

 and summer respectively. The oceanic data was obtained from the Climatic Charts 

 of the Oceans,^ whereas the land station data was obtained from the Koppen- 

 Geiger Handbuch.' 



It is obvious from the maps that fog occurs more frequently over the ocean 

 in summer than in winter. In summer the fog frequency along the northeast coast 

 of North America is very high. This is due to the presence of cold water. Advection 

 of air from the land or the neighboring ocean will result in a cooling from below, 

 and consequently in the formation of fog. Over the North Sea the principal cause 

 of fog is the advection of warm air from Europe. This gives a maximum fog 



10 



