Iceland these lows tend to move northeastward and eastward toward Bear Island 

 and Barents Sea, usually with diminishing intensity. This strong circulation 

 pattern brings high temperatures, much rain and south and southwest storm winds 

 to northwestern Europe, and cold showery conditions with strong west and north- 

 west winds to the northwestern Atlantic. This is the weather pattern which is 

 most clearly reflected in the normal charts for the winter season. 



However, the normal charts do not convey any impression of the severity 

 of the conditions which normally prevail when this weather type is established. 

 The lowest pressure in the Icelandic low region on the daily synoptic charts when 

 this weather pattern is well established runs from 20 to 70 mb. lower than the 

 1000 mb. minimum indicated on the mean pressure chart. The reason for the 

 failure of the normal charts to indicate the normal severity of the strong circulation 

 pattern over the North Atlantic may be expressed rather obviously in the follow- 

 ing two facts, each of which has its synoptic implications: 



(i) The general circulation pattern of the strong type is subject to consid- 

 erable longitudinal shifts. The Polar Front may be shifted westward to the east 

 coast of North America, so that the Icelandic low lies in the vicinity of Newfound- 

 land and Labrador, or it may be shifted eastward to the mid-Atlantic at low 

 latitudes and extend thence northeastward into western Europe. In this latter case 

 the severe storms of the North Atlantic are felt with maximum severity over all 

 of western Europe. Such shifts have the effect on a mean or normal chart of smooth- 

 ing out the extremes which are present on a single synoptic chart. Either of these 

 two extreme longitudinal shifts of the strong circulation pattern usually represents 

 the first stage of the transition or breakdown of the strong type into a weak circula- 

 tion pattern which is radically different in its characteristics from the strong type. 



{2) The normal chart includes in its makeup a proportional number of the 

 weak circulation type patterns as well as the strong and the transitional types. 

 Although these weak circulation patterns are not sufficiently freciuent nor intense 

 to leave any clear imprint of their characteristics on the normal charts, neverthe- 

 less in extreme cases they nearly reverse the characteristics of the strong circulation 

 patterns. This occasional reversal of the more frequent strong type has the effect 

 on the normal chart of greatly weakening the characteristics of the normal patterns 

 from those of the strong circulation type. 



Since the weak circulation patterns are the ones which are least reflected 

 in the normal charts of the North Atlantic, they represent the maximum departures 

 of the North Atlantic weather from the conditions represented by the mean charts. 

 For this reason it is important to consider briefly the chief characteristics of the 

 weak circulation patterns, and to include an example of this type of weather in the 

 illustrative daily synoptic charts. The most serious large-scale errors in forecast- 

 ing the daily weather in the eastern Atlantic and western Europe are usually made 

 at the time that a weak circulation pattern is becoming established. 



The weak circulation weather types, or low index patterns, are characterized 

 essentially by the disappearance or reversal of the prevailing zonal westerlies of 

 middle latitudes. This can take place in either of two quite different ways. One of 

 these occurs with the general disappearance of strong pressure gradients. The 

 Icelandic and Aleutian lows over the northern ocean areas tend to fill up and dis- 

 appear, their places being taken by the northward extension of the subtropical 

 Atlantic and Pacific highs, which tend to be somewhat weakened at the lower 

 latitudes. This type of weak circulation pattern is characteristic of the warmer half 

 of the year, though it may occur in modified form during the winter season. How- 

 ever, it is characterized by the general absence of any very severe weather condi- 



44 



