382 F. W. Very — Shy Radiation and Isothermal Layer. 



respect is similar to that of a dense cloud which reflects back 

 all surface radiation of a particular sort ; or the isothermal 

 layer is like a black solid of absolute temperature approxi- 

 mately T = 218° Abs. C, \ max.=13*5^, which is opaque to 

 radiation similar to its own, but transmits a portion of the 

 radiation of shorter wave-length from a solid body at T = 289°, 

 and \ max. =10*2 fi. 



Taking a single day of clear sky (February 3d), the day sky 

 at 9 : 21 a. m. was 7°F. colder than the night sky at 8 : 30 p. m., 

 conditions of air temperature and humidity being not very 

 different. The change of apparent sky temperature from 

 — 22° F. at 9: 21 a. m. to 0° F. near noon is about like that of 

 the diurnal change of air temperature from +0*8° F. at 7 : 37 

 a. M. to + 20'2°F. at 4 p. m. The sky variation also follows 

 the changing air temperature into the evening hours : 



9:21a. m. 



Air +10° F. 



Sky-22° F. 



Difference 



= —32 



12 : 39 p. m. 



+ 16 









— 16 



8 : 30 p. m. 



+ 12 



— 15 





-27 



Thus the noon sky on a clear day appears warmer, that is, less 

 transmissive of telluric radiation, than the morning or evening 

 sky. The noon air, as we know, is also less transmissive of 

 solar radiation. 



When, however, we average a considerable number of day- 

 light observations of apparent sky temperature, including 

 skies which are more or less contaminated with cirrus, and 

 compare with similar night observations, there is not much 

 difference. Thus ten night readings gave an average galva- 

 nometer deflection of — 39'7 div. and twelve daylight observa- 

 tions gave a mean of — 38'8 div., which are nearly identical. 

 The temperature of the instrument varied only a few degrees 

 from 60° F. 



Finally, we may classify the observations according to the 

 clearness of the sky as follows : 



Very clear 



Clear 



Milky blue 



Cirro-stratus 



-61-5° C. 



— 30-2 



— 26'2 



— 24-8 



-23-5° C. 



— 25-2 



— 18-0 



-15-1° C. 



— 12-7 



— 13-5 



— 15-2 

 -17*9 



-14-7° C. 



— 3-5 

 -13-3 



— 10-8 



— 10-6 



—18-9° C. 

 —11-3 

 — 9'3 



Mean 

 -35-7° C. 



-22-2° C. 



-14-9° C. 





—11-5° C. 



