258 Prof. S. Arrhenius on the Influence of Carbonic Acid 



temperature which is caused by the variation of /3 to the 

 following valnes to be 



13=0-60 t=- 5°C. 

 0-80 + 5-6 



0*90 +11-7 



1-00 +18-6. 



These values are calculated for v=l, i.e. for the solid crust 

 of the earth's surface, except the snowfields. For surfaces 

 with another value of v, as for instance the ocean or the 

 snowfields, we have to multiply this value t by a fraction 

 given above. 



We have now shortly to consider the influence of the 

 clouds. A great part of the earth's surface receives no heat 

 directly from the sun, because the sun's rays are stopped by 

 clouds. How great a part of the earth's surface is covered 

 by clouds we may find from Teisserenc de Bort's work* on 

 Nebulosity. From tab. 17 of this publication I have deter- 

 mined the mean nebulosity for different latitudes, and found: — 

 Latitude. . 60. 45. 30. 15. 0. -15. -30. -45. -60. 

 Nebulosity. 0-603 0-48 0*402 0-511 0-581 0-463 0-53 0-701 



For the part of the earth between 60° S. and 60° N. we 

 find the mean value 0'525, i. e. 52'5 per cent, of the sky is 

 clouded. The heat-effect of these clouds may be estimated in 

 the following manner. Suppose a cloud lies os 7 er a part of 

 the earth's surface and that no connexion exists between this 

 shadowed part and the neighbouring parts, then a thermal 

 equilibrium will exist between the temperature of the cloud 

 and of the underlying ground. They will radiate to each 

 other and the cloud also to the upper air and to space, and 

 the radiation between cloud and earth may, on account of the 

 slight difference of temperature, be taken as proportional to this 

 difference. Other exchanges of heat by means of air-currents 

 are also, as a first approximation, proportional to this dif- 

 ference. If we therefore suppose the temperature of the 

 cloud to alter (other circumstances, as its height and compo- 

 sition, remaining unchanged), the temperature of the ground 

 under it must also alter in the same manner if the same supply 

 of heat to both subsists — if there were no supply to the 

 ground from neighbouring parts, the cloud and the ground 

 would finally assume the same mean temperature. If, therefore, 

 the temperature of the clouds varies in a determined manner 



* Teisserenc de Bort, " Distribution moyence de la nebulosite," Ann. 

 du bureau central meteorologique de France, Ann6e 1884, t. iv. 2 de partie, 

 p. 27, 



