CHAPTER VII 



RADIATION BETWEEN THE SKY AND THE EARTH DURING THE 



DAYTIME 



I must include here some observations which, in spite of their pre- 

 liminary nature, yet may be of use in throwing a certain light upon 

 questions nearly connected with the problem especially in view. 



In the daytime, the radiation exchange between the sky and the 

 earth is complicated by the diffuse sky radiation of short wave length 

 that is present in addition to the temperature radiation of the sky. If 

 this diffuse radiation is stronger than the effective temperature ra- 

 diation to the sky, a black body like the instrument will receive heat. 

 In the contrary case it will lose heat by radiation. 



If one attempts to measure this positive (from sky to earth) or 

 negative radiation with the instrument used in the present investi- 

 gation, the sun itself being carefully screened off, such an attempt 

 meets with the difficulty arising from the introduction of a systematic 

 error. The bright metal strip has a smaller reflecting power for 

 the diffuse radiation of short wave length than for the longer heat 

 waves and we can no longer make use of the instrumental constant k, 

 which holds only for long waves such as we have to deal with in the 

 measurements of the nocturnal radiation. The reflecting power of 

 the strips being about 97 per cent for waves longer than 2 fi, and 

 only about 70 per cent for waves of 0.5 /x length (a mean value of the 

 wave length of the diffuse sky radiation), the introduction of the 

 constant k into daylight measurements will evidently give a value 

 of the sky radiation that is about 30 to 35 per cent too low. 



On several occasions during the summer of 1912, I had the 

 opportunity of making skylight measurements as well with my own 

 instrument as with an instrument constructed on the same principle, 

 but modified for the purpose of making day observations. This 

 latter instrument is briefly described by Abbot and Fowle 1 in their 

 interesting paper, " Volcanoes and Climate," where the effect of 

 the diffusing power of the atmosphere on the climate is fully dis- 

 cussed. Both the strips employed in this instrument are blackened. 



1 Smithsonian Miscellaneous Collections, Vol. 60, No. 29, 1913. (Reprinted 

 in Annals of the Astrophysical Observatory of the Smithsonian Institution, 

 Vol. 3.) 



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