NO. 4 TEMPERATURE VARIATIONS IN THE NORTH ATLANTIC 261 



EIGHT MONTHLY PERIODS IN THE SUN SPOTS AND IN THE AIR 

 PRESSURE DIFFERENCE OVER THE NORTH ATLANTIC 



Prof. Birkeland has pointed out that one might expect an eight- 

 monthly period in the sun spots on account of the combined action 

 of Venus and Jupiter, according as these stand in conjunction or in 

 opposition. Such an eight-monthly period we have actually found 

 in curve I for the sun spots which we give in figure 97. The 

 curve shows the difference between the observed relative numbers 

 and the twelve-monthly smoothed relative numbers fo'r the sun 

 spots as determined in Wolfer's publications in the Astronomische 

 Mittelungen. The curve shows particularly great variations in 

 the neighborhood of sun spot maxima and the greater excursions 

 seem to have a regular time interval. This holds especially in the 

 years 1904 to 1910 when the average time interval between these 

 excursions amounted to eight months. As earlier remarked Krog- 

 ness had found a similar eight-monthly period in the daily varia- 

 tions of the declination in Christiania. 



Curve IV, figure 92, for the air pressure difference in the North 

 Atlantic Ocean, shows also great excursions, with intervals between 

 which correspond to the excursions we have noted in the curve 

 of the sun spots. As the reader will see most clearly, in the latest 

 maximum period there come, from one to two and occasionally three 

 months after the eight-monthly excursions in the sun spot curve, 

 corresponding excursions in the curve of air pressure difference. 

 The same will also be found to a certain degree in the earlier maxi- 

 mum periods from 1891 to 1898, while on the other hand in the first 

 maximum period in the years 1884, 1885, and 1886, no indication of 

 such an agreement between the sun spot curve and the air pressure 

 difference curve appears to be found. However, the observed 

 agreements are as good as we could have expected in considera- 

 tion of the scanty observational material and the faulty treatment 

 of it. Furthermore the six-monthly and twelve-monthly periods 

 which are found in meteorological phenomena tend partly to hide 

 these assumed eight-month periods. 



As we have sa:id, the variations in the sun spots are particularly 

 great at sun spot maximum. They are occasionally at sun spot 

 minimum very small. Nevertheless the variations during sun spot 

 minimum are associated with fairly great variations in the atmos- 

 pheric and magnetic phenomena upon the earth. This can in part 

 be explained from the fact that it is not clear that it is the greater 

 or less absolute degree of intensitv in the solar activitv which influ- 



