6 SMITHSONIAN' MISCELLANEOUS COLLECTIONS VOL. 82 



As a second step, we consider the spectral distribution and intensity 

 of emission of the perfect radiator at different temperatures/ By- 

 interpolation on large scale plots we have prepared table 3. This gives 

 the approximate ' intensity of emission of the perfect radiator at tem- 

 peratures corresponding to the mean temperatures of Simpson's lay- 

 ers, and to those of his selected latitudes of the earth's surface. 



Multiplying the values in table 2 by corresponding ones in table 3, 

 we obtain the emission of radiation outwards from each Simpson at- 

 mospheric layer towards a complete hemisphere. The values are given 

 in table 4. 



Again interpolating in the plots (fig. 3) we next obtained the trans- 

 mission coefficients for each superincumbent atmospheric mass lying 

 above the respective Simpson layers. Allowance is made for the ozone 

 absorption between 9/a and iifj.. These values are given in table 5. 



Multiplying these values by corresponding ones in tables 3 and 4, 

 we obtained the contributions of the Simpson atmospheric layers and 

 also of the earth's surface' at the latitudes 90°, 70°, 60°, 50°, 40°, 

 and 0° to the intensity of emission of the earth as a planet towards 

 outer space.* These results are given in table 6. 



All of these results apply to cloudless skies. We now assume, with 

 Simpson, that the earth is 50 per cent cloudy ; that the clouds totally 

 absorb all radiation arising from beneath them ; that they radiate quite 

 as efficiently as the perfect radiator ; and that their upper surfaces 

 maintain the saine average temperature as the earth at 70° latitude. 

 We are not able to compute their radiation in Simpson's manner, since 

 we have shown reasons to believe that the stratosphere is almost desti- 

 tute of water vapor, instead of containing 0.3 mm. of precipitable 

 water as he supposes. We simply assume that the combined emission 

 of clouds and atmosphere during one-half the time at all latitudes is 

 the same as that of the earth's surface and the superincumbent atmos- 

 phere at latitude 70°. That is: For the atmosphere 0.15 1 cal. per 

 cm.^ per min. ; for the cloud surface o.ioo cal., giving a total for com- 

 pletely overcast sky of 0.251 cal. During the other half of the time, 



^ See Smithsonian Physical Tables, p. 248. 



"We do not guarantee these values to within 2 per cent. 



^We assume, with Simpson, that the earth's surface may be regarded as a 

 perfect radiator. 



* Notwithstanding our previous evidence that the water-vapor content of the 

 stratosphere is vanishingly small, we have thought best to estimate 30 per 

 cent of black-body efficiencj- as appHcable to the stratospheric radiation in the 

 wave-length region 13^1 to 50^, where water vapor is so very active. We have 

 allowed 16 per cent of black body efficiency to the ozone band, g/i to 11 /jl. 



