. Compendium of meteorology. Meteorology. North America is favored by the reduced zonal circula- tion. The principal weakness in the Weather Bureau "normal" pressure map for April is found in its failure to indicate the southwestward displacement of the North Atlantic cyclonic activity to an annual minimum latitude. The arctic anticyclonic circulation rarely de- parts from the single-cell pattern indicated for April, but its center may be located over Eortheast Greenland or the Canadian Archipelago when large-scale circula- tion anomalies disrupt the average pressure distribu- tion.
. Compendium of meteorology. Meteorology. North America is favored by the reduced zonal circula- tion. The principal weakness in the Weather Bureau "normal" pressure map for April is found in its failure to indicate the southwestward displacement of the North Atlantic cyclonic activity to an annual minimum latitude. The arctic anticyclonic circulation rarely de- parts from the single-cell pattern indicated for April, but its center may be located over Eortheast Greenland or the Canadian Archipelago when large-scale circula- tion anomalies disrupt the average pressure distribu- tion. Average 700-mb Contour Patterns. A survey of pre- vious research on the upper-level circulation over the Arctic was presented earlier in this article. The use of mean circulation methods developed by Rossby, Willett, and Namias has emphasized the importance of major anomalies in flow patterns aloft. Extended forecasting for the Arctic and elsewhere requires almost continuous reference to the upper-level "normal" maps published by the U. S. Weather Bureau [12, 13]. As a result of this writer's research, it appears that many of the errors found in the 40-yr sea-level normals have been extra- polated up to the 700-mb charts. Namias [13] used many short-term radiosonde records, especially for high- latitude stations, by applying a correction based on the concurrent departure from normal of the local sea- level pressure and surface temperature. In preparing July and January average 700-mb contour charts for this article, the elimination of errors attributable to incorrect "normal" sea-level pressures has received pri- mary emphasis. July. The suggested revision of the Namias 700-mb contours for July is shown in Fig. 5. A comparison with. Fig. 4.—Average sea-level pressure (mb) during April. Fig. 5.—Average 700-mb contours (ft) and isotherms (°C) during July. April. By this time the mid-latitude continental warming has become well established although the Arc
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