F. W. Very — Shy Radiation and Isothermal Layer. 331 



The sky, which at first was a uniform cirro-stratus with strong 

 halo, had changed at the last observation to banded cirro-stratus 

 and cirro-cumuli. Halo fainter. The effective temperature of 

 the sky was 



at 5:15 a, — 53-1/-65 = 81-7° F. lower than the in- 

 strument, or _ — 22°F. 



at 10 : 48 a, — 37*l/'6o = 57*1 lower than the instru- 

 ment, or.. — 3° F. 



Here, with little change in the amount of cloud, the sky deflec- 

 tion was persistently that of a surface whose effective tempera- 

 ture was some 20° F. colder than the air. A progressive air 

 movement from the south was going on, and the sky deflection 

 registers the increase in the temperature of the cloud layer. 



No attempt was made to pick out the times of best sky 

 (except for the altitude measures), but the observations were 

 taken at irregular intervals when it happened to be convenient 

 to secure them. Each recorded galvanometer deflection is the 

 mean of five successive readings which were quite consistent 

 except in the cases noted of rapidly changing sky. 



I will now compare a few of the best of these readings with 

 some made many years ago at Allegheny Observatory, in 

 which only the clearest nights, suitable for refined lunar meas- 

 ures of radiation, were used. The altitude of the Westwood 

 Observatory is some 200 feet above sea-level, and that of the 

 (old) Allegheny Observatory is 1145 feet. Temperatures are 

 Centigrade. 



Allegheny, Pa. 



Westwood, Mass. 



Winter 



Winter 



Summer 



Sky 

 tem- 

 pera- 

 ture 



Air 

 temper- 

 ature 



Dew- 

 point 



Sky 

 temper- 

 ature 



Air 



temper- 

 ature 



Dew- 

 point 



Sky 

 tem- 

 pera- 

 ture 



Air 



temper- 

 ature 



Dew- 

 point 



-54° 



— 67 



— 44 



— 1-5° 



— 12-4 



— 9'2 



— 7-7° 



— 22-6 



— 14-9 



-61'5° 

 -30-2 



— 24*8 



-13-3° 

 -12-2 

 — 15-8 



— 22'2° 



— 13-9 

 -16-1 



-6-9° 

 — 2-4 



o-o 



+ 13-9° 

 + 19-4 



+ 21.6 



+ 10-0° 

 + 14-2 

 + 15-0 



On the whole, the effective temperature of the sky is lower 

 when the dew-point is lower and the aqueous obstruction of 

 radiation to space is least; but it will be noticed that the 

 observed effective temperature has never been found lower 

 than that of the isothermal layer whose behavior in this 



