the cold off-shore waters. The frequency of this fog condition in summer is to be 

 seen from Figu: e 22. However, it will be noted from this figure that the fog frequencies 

 along the rugged coast lines of northeastern North America, Greenland and Iceland 

 are locally extremely variable. This is doubtless caused by great variation in the 

 shielding effects on the observation sites of the local terrain. The successful fore- 

 casting of fog for coastal sites in any of this region must depend upon an intimate 

 knowledge of local topography and winds. Especially during the daytime a very 

 short haul over land may be sufficient to dissipate an ocean fog or low stratus. 

 Generally, however, widespread fog is likely to be formed in summer over the cold 

 coastal waters off the northeast Atlantic coast of North America whenever the air 

 mass transport is persistently from between southeast and southwest. Even a 

 west wind suffices to produce fog when the continental air mass is moderately warm 

 and humid. Such widespread fogs may extend or be transported eastward as far 

 as longitude 40° W, or even further in exceptional cases. On the other hand, wide- 

 spread fog formation over the northeastern Atlantic east of 30° W occurs occasion- 

 ally in summer with the persistent westward transport of warm continental air 

 masses from northern Europe. This is a low index pattern which is usually charac- 

 terized by the extension of high pressure from northern Russia to northern Scan- 

 dinavia, while the Polar Front extends west-east across central Europe. In sum- 

 mer this low index pattern brings the warmest weather to Scandinavia, whereas the 

 corresponding winter pattern brings northwestern Europe its coldest weather. 



The map of August 21 shows no significant change from the conditions of 

 the preceding day. Note the extremely slow movement both of the frontal systems 

 and the numerous weak cyclonic centers which are associated with these fronts. 



On August 22 and 23 there is no further significant change of the weak cir- 

 culation pattern. Probably the low index characteristics are best shown on the 

 map of August 23, with the maximum development of the polar anticyclonic 

 centers over eastern North America and the northeastern Atlantic, while the fronts 

 in the Atlantic extend nearly north-south. The development of the anticyclonic 

 center in the northeastern Atlantic has displaced the Polar Front in western Europe 

 moderately southward. This development carries with it a return to more nearly 

 normal temperature in western Europe. Northwest of this high, however, in the 

 vicinity of Iceland and southwestern Greenland, temperatures are abnormally 

 high. Note that the slow eastward advance of the cold front along the east coast 

 of North America has displaced the fog-bearing tropical maritime air mass east- 

 ward so that the fog condition now extends to 40° W, and northward to southern 

 Greenland. 



On August 24 there appears for the first time a noticeable tendency towards 

 cyclogenesis in the vicinity of southern Greenland. This development might be 

 taken as the first sign of the impending gradual change of the general circulation to 

 a highly active state. There is no particular evidence in the current chart either as 

 to whether the trend will continue, or as to why it started at this particular time 

 and place. The location, however, is just that of the most frequent development of 

 the Icelandic low. Consequently, it is a spot which should be watched carefully in 

 the absence of an Icelandic low. 



During August 25 and 26 the slow development of cyclonic activity con- 

 tinues over a large area in the vicinity of southern Greenland, but this development 

 is not strong enough to change the weak circulation characteristics of the North 

 Atlantic weather pattern. In particular, the persistence of the large anticyclonic 

 cell over the northeastern Atlantic and northern Europe helps to maintain the low 

 index character of the circulation pattern. 



53 



