NO. 4 TEMPERATURE VARIATIONS IN THE NORTH ATLANTIC l8l 



CLOUDINESS AND SUN SPOTS 



The relation between the variations in the cloudiness and the solar 

 activity is less accurately investigated. Klein has, however, shown 

 that the highest clouds in the atmosphere, the cirrus, the cirro- 

 stratvis, and cirro-cumulus increase with the developmicnt of sun 

 spots. Since these highest clouds produce the halos and similar 

 phenomena around the sun and moon, such as mock suns, mock 

 moons, and similar optical phenomena, it would be expected that 

 these would be more prevalent at sun spot maximum than at sun 

 spot minimum. This is actually the case, as is shown by the statistics 

 of such phenomena over a considerable interval of time made by 

 Tromholt. Even Tyco Brahe's diaries show that halos round the 

 sun and moon are most prevalent in times of Northern Lights. 



DUST IN THE ATMOSPHERE AND SUN SPOTS 



Reference should be made here to the remarkable agreement found 

 by Busch in the year 1891, between the variations of the sun spots 

 and the variations in the polarization of the sky light. He found 

 that the height of the neutral point (Arrago's point and Babinet's 

 point) above the horizon at sunset rose and fell along with the fre- 

 quency of sun spots, but the maximum and minimum of the neutral 

 point came on the whole a year later than the maximum and mini- 

 mum of sun spots. Earlier it was shown that after great volcanic 

 eruptions, such as the Krakatao eruption, the recorded heights of 

 the neutral points were increased. This depends upon the volcanic 

 dust which is thrown out to the higher layers of the earth's atmos- 

 phere. From this we may conclude that at sun spot maximum 

 the higher layers of the earth's atmosphere are filled with more 

 dust than at sun spot minimum (see Arrhenius, 1903, p. 873). 



THEORIES ON THE RELATION BETWEEN VARIATIONS OF THE SOLAR 

 ACTIVITY AND METEOROLOGICAL VARIATIONS 



After the above summary of the earlier investigations, it must be 

 admitted that there is a dependence between the variations of 

 the meteorological elements, such as the temperature and pressure, 

 and the variations in the solar activity. 



For the explanation of this dependence various hypotheses have 

 been put forward. These may be divided principally into five classes, 

 as follows: 



I. The direct, that is to say, that the variations in the tempera- 

 ture of the earth are caused directly by variations in the outgoing 

 radiation from the sun. 



