NO. 4 TEMPERATURE VARIATIONS IN THE NORTH ATLANTIC I49 



About the same time that Koppen's treatise already referred to 

 was published, spectroscopic investigations that were made, particu- 

 larly by Lockyer, indicated that the sun is probably hotter at the 

 time of sun spot maximum. The results of Koppen and others that 

 the air temperature of the earth is colder at maximum than at mini- 

 mum seemed therefore to be paradoxical. This was explained by 

 Blanford (1875) by suggesting that the air temperature of the land 

 stations such as those which Koppen investigated must be deter- 

 mined not by the quantity of heat that falls on the exterior of the 

 planet but by that which penetrates to the earth's surface, chiefly to 

 the land surface of the globe. The greater part of the earth's sur- 

 face being, however, one of water, the principal immediate effect of 

 increased heat must be the increase of evaporation and therefore as 

 a subsequent process the cloud and to rain fall. Now a cloudy 

 atmosphere intercepts the greater part of the solar heat, and the 

 re-evaporation of the fallen rain lowers the temperature of the 

 surface from which it evaporates and that of the stratum of air 

 in contact with it. The heat liberated by cloud condensation doubt- 

 less raises the temperature of the air at the altitude of the cloudy 

 stratum, but at the same time we have two causes at work equally 

 tending to depress that of the lowest stratum. Accordingly it must 

 be expected that an increase of the evaporation and of the rain by 

 increased solar activity would cause a diminution of the tempera- 

 ture over the earth's surface. 



S. A. Hill (1879) investigated the absolute yearly temperature 

 variation in the mean of different stations in North India and found 

 that the greatest variation occurred in the neighborhood of the mini- 

 mum of sun spots and the smallest variation in the neighborhood 

 of the sun spot maximum. The agreement was not particularly 

 good. Great departures occurred and the investigation embraced 

 only the years 1866 and 1878. More trustworthy results he thought 

 to obtain by investigating the mean yearly variation of the monthly 

 mean of the temperature of different stations in North India for 

 the years 1863 to 1878. He found that the greatest yearly varia- 

 tion occurred one or two years after the minimum of sun spots and 

 the smallest variation in the year after the sun spot maximum. 

 This relation, if such relation exists, seems more clearly to occur the 

 further we go toward the northwest in India. He himself, however, 

 notes that the observational material is very fragmentary. He 

 appears to be of the opinion that since an increase of the amplitude of 

 yearly heat variation probably is more associated with a greater 



