NO. 8 BAROMETRIC PRESSURE — DUEI.L AND DUELL I5 



imum gradients of the additional sea-level pressure field after iono- 

 spheric storms in a very condensed form. As can be seen from the 

 upper curve, the greatest increase of the gradient, from 1.6 to 3.7 mb., 

 takes place as early as the first day after the ionospheric storms. 

 However, the absolute peak of 5.0 mb. is reached only on the second 

 day after the disturbed days. Then occurs a decrease of 1.3 mb. to 

 the "third day after," and from that time a gradual decrease until, 

 on the "eigiith day after," the Imv value of 1.4 mb. is reached. The 

 lower curve of the same figure shows the iulcrdiurnal variation uf 

 the gradient, and accordingly has its peak, with f 2.1 mh., on the fir>t 

 day after the ionospheric storms. 



As has been cnijihasized above, this relatively clear relationship 

 between the invasions of solar particles and the behavior of sea-level 

 pressure can be demonstrated only for the winter months and, even 

 then, only for the years with low sun activity. This fact cannot yet 

 be explained in a really satisfactory manner. However, we shall enter 

 briefly into this question when we discuss the manner in which 

 solar-activity influences are transmitted to the troposphere. Here 

 attention can only be called to the fact that other authors, working 

 with similar statistics and subdividing these, also obtained very dif- 

 ferent results for the different seasons as well as for the years with 

 high and with low sun activity. Some few examples will make this 

 evident : A. Peppier (9) found, by using monthly mean values, that 

 there has been a positive correlation since 1906 between the relative 

 sunspot numbers and the course of sea-level pressure anomalies over 

 the Atlantic in the zone between 60° and 70° N. latitude, and a 

 negative correlation when the zone between 25°and 35° N. latitude 

 was considered. When subdividing his statistics according to the 

 dift'erent seasons, he found that relationship well developed during 

 the winter, but could not discover it during the summer and autumn. 

 J. M. Stagg (8) found that in Lerwick on days with geomagnetic 

 disturbances, the forenoon maximum of the diurnal variation of sea- 

 level pressure was lower, and the afternoon maximum was higher, 

 than on days without geomagnetic disturbances. This relationshi]) 

 was likewise particularly evident in the years with low suji activity. 

 O. Krogness (10) has found that a 27-day period, caused by the sun 

 rotation in some meteorological elements in the northern part of 

 Norway, could be observed regularly in the years with low stm 

 activity. 



