DISCUSSION OF SYNOPTIC WEATHER MAPS 

 OVER THE NORTH ATLANTIC FOR SELECTED PERIODS 



BY H. C. WILLETT 



A. Inteoductory Remarks. 



Familiarity with the chmate of a region as expressed by monthly or seasonal 

 charts of the normals of the meteorological elements gives little knowledge of the , 



variety of the day to day weather. This variety of the day to day weather condi- '^^jp^^*^ Ijl 

 tions is seen best in the daily synoptic charts. To illustrate this variety of weather 

 over the North Atlantic, by way of supplementing the preceding discussion of the 

 climate of this region, two periods of the daily weather maps were selected for 

 discussion. These two periods, one in January and one in August, were selected 

 as including a rather wide range of weather patterns which are fairly typical of 

 their respective seasons. Obviously, all extremes of weather in the North Atlantic 

 area could not be represented without the inclusion of a prohibitively large number 

 of maps. The inclusion of eastern North America and western Europe is necessary 

 for any intelligent discussion of the synoptic conditions over the ocean. In par- 

 ticular, basic changes in the general circulation pattern tend to progress from Europe 

 to the Atlantic. The selected periods were chosen from the Second Polar Year 

 because the network of surface observations was most complete that year. 



The following discussion of the daily synoptic maps is restricted primarily 

 to the departure of the daily weather patterns from the normal conditions as well 

 as to the degree to which, and conditions under which, such departure may be 

 expected. The details of the analysis are accepted without discussion as presented cut^ajl' n ij 



by the maps, although the 24-hour interval between maps and the extremely rapid ^j^^ u^^ 

 movement which characterizes part of the winter period especially render the * 



synoptic analysis somewhat uncertain at a few points. 



B. The Winter Period, January 9-24, 1933. 



The normal distribution of pressure, winds and air masses over the North 

 Atlantic in midwinter is shown in Figures 1 and 16. However, as is so frequently 

 the case in regions where the short period weather patterns fluctuate greatly, the 

 normal or average seasonal pattern is rarely present. The normal charts show most 

 clearly the influence of what is probably the most frequent and strongest circulation 

 pattern over the North Atlantic. That is the pattern which is normally present 

 with a high zpnaljiidex, or in other words, an active state of the general circulation 

 as shown by strong zonal westerlies in middle latitudes. Under this condition the 

 Polar Front usually lies in the mid-Atlantic, extending eastward and northeastward 

 from the vicinity of Florida toward the northwest coast of Europe, with a well 

 developed Azores high centered south of 40° in the eastern Atlantic. The lows which 

 develop on the Polar Front off the southeast coast of the United States move 

 rapidly northeastward with increasing intensity. They tend to reach occlusion and 

 maximum intensity somewhere in the area south of Greenland northeastward to 

 Iceland, thus producing and maintaining the so-called Icelandic low, which 

 appears on all mean pressure charts of the North Atlantic. From the vicinity of 



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