normal path (cf. Figures 1 and 16). However, it is only over western Europe, on 

 the frontal system which remains from the disturbance now filling off the east coast 

 of Greenland, that a development which is not consistent with a strong circulation 

 condition is to be noted. The eastward advance of this front is obviously being 

 checked by the strong continental anticyclone to the east, which shows little tend- 

 ency to give ground. In fact, the negative pressure tendencies and the rain in the 

 warm sector over Germany and northern Italy suggest cyclogenesis along the south- 

 ern portion of this front and very rapid southward displacement of the cold front 

 over northern France. Such a development does not occur when the strong westerly 

 circulation extends eastward across northern Europe into Asia. It represents the 

 first blocking action of the weak circulation pattern which is already established 

 over Eurasia, as evidenced by the abnormally high pressure over western Russia. 



No significant change in the weather pattern is to be noted on the map of 

 January 10. The frontal system over west central Europe has remained nearly 

 stationary, with rapid fiUing of most of the low pressure trough except for the 

 active cyclogenesis in the central Mediterranean. This development, a conse- 

 quence of the anticy clonic blocking, was clearly indicated on the preceding chart. 

 The rapid northeastward movement and intensification of the complex disturbance 

 near Cape Hatteras on the preceding chart also proceeded in accordance with the 

 circulation pattern. Also to be noted is the rapid eastward movement nearly half- 

 way across Canada of a strong disturbance from the Pacific Coast of the so-called 

 Alberta low type. Such rapid movement across the North American Continent of 

 intense disturbances which follow a course lying north of the Canadian border is 

 again highly typical of a strong circulation weather pattern extending from the 

 Pacific to the Atlantic. These disturbances usually move eastward to Davis 

 Strait or Greenland and merge with the Icelandic low. 



One fact that is worth remarking about the North Atlantic air masses during 

 periods of strong circulation, a fact which holds quite generally in middle latitudes, 

 is the small contrast of properties shown by the air masses which follow eastward 

 in rapid succession. Direct polar and tropical outbreaks do not occur. The rapid 

 eastward motion tends to prevent any marked convergence of polar and tropical 

 currents, and the fronts, especially warm fronts, are correspondingly weak. _ Usually, 

 however, the typical showery convectively unstable condition of the maritime polar 

 air which follows any cold or occluded front is clearly m evidence, but the tropical 

 air mass characteristics are frequently less clearly in evidence. Much of the warm 

 air is only returning maritime polar which has not become entirely tropical in its 

 characteristics. 



The following synoptic chart (January 11) shows no important change in 

 the trend of the weather development from the two preceding days. The eastern 

 European anticyclone holds its position and successfully blocks all eastward 

 progress of the fronts which try to move inland from the North Atlantic, while 

 the central Mediterranean disturbance remains active and stationary. The dis- 

 turbance which originated two days previously in the vicinity of Cape Hatteras 

 has already reached the southern tip of Greenland with greatly increased intensity, 

 while the Alberta low has moved rapidly to eastern Canada. Thus the strong cir- 

 culation pattern continues unabated over North America and the North Atlantic, 

 while the low index blocking pattern continues in Eurasia. 



The map for January 12 shows little change in the situation, unless perhaps 

 that the Eurasian anticyclone has extended its influence slightly westward in 

 western Europe. The rapid eastward motion continues across the North Atlantic. 

 The Alberta low is approaching southern Greenland with the suggestion of a tend- 



47 



