SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. IIO 



NW. to SE., has become still more steep and has reached the rela- 

 tively high value of 5.0 mb. The anomalous plus area of + 2.5 mb. 

 is situated over Iceland and the Strait of Denmark as before. The 

 area with positive departures ^2.0 mb. is covered with little crosses. 

 The minus area of —2.5 mb. has shifted a little toward the SE., and is 

 now situated over the Baltic States. The area with negative departures 

 ^ 2.0 mb. is cross-hatched with horizontal lines. The beginning of a 

 flattening of this ionosphere-controlled sea-level pressure field can 



Average Absolute Sea-Level Pressure - 



Distribution ( mb ) during ttie Wintermonttis of the 16 Years with 

 low Sun-Activity (r«40) of the Period 1906 to 1937 



("Long- Period Means") 

 Fig. 4. 



be recognized as early as 3 days after the ionospheric storms in figure 

 9. The plus-area (with -I-2.0 mb.) has shifted somewhat toward E., 

 and the minus-area, with extreme values diminished to —1.7 mb., 

 has broken up into two parts, during displacement toward the S. 

 The greatest pressure difference is only just 3.7 mb. 



Figure 10, representing the situation 4 days after the ionospheric 

 storms, and figure 11, showing these conditions 5 days after, dem- 

 onstrate how the flattening of the additional pressure field progresses 

 slowly but steadily, with maximum gradients of 3.3 and 3.1 mb. 

 From figure 12, 6 days after, it can be seen that the plus-area with 



