EEPORT OP THE SECRETARY. 15 



IV. An increase in tlie water-vapor pressure will cause a decrease in the 

 effective radiation from tlie earth to every point of the sky. The fractional 

 decrease is much larger for large zenith angles than for small ones. 



V. The total radiation which would be received from a perfectly dry atmos- 

 phere would be about 0.28 Zp.;^ with a temperature of 20° C. at the place of 

 observation. 



VI. The radiation of the upper, dry atmosphere would be aboHt 50 per cent 

 of that of a black body at the temperature of the place of observation. 



VII. There is no evidence of maxima or minima of atmospheric radiation dur- 

 ing the night that can not be explained by the influence of temperature and 

 humidity conditions. 



VIII. There are indications tbat the radiation during the daytime is subject 

 to the same laws that hold for the radiation during the nighttime. 



IX. An increase in altitude causes a decrease or an increase in the value of 

 the effective radiation of a blackened body toward the sky, dependent upon the 

 value of the temperature gradient and of the humidity gradient of the atmos- 

 phere. At about 3,000 meters altitude of the radiating body the elTective radia- 

 tion generally has a maximum. An increase of the humidity or a decrease of 

 the temperature gradient of the atmosphere tends to shift this maximum to 

 higher altitudes. 



X. The effect of clouds is very variable. Low and dense cloud banks cut 

 down the outgoing effective radiation of a blackened surface to about 0.015 

 calorie per cm.^ per minute ; in the case of high and thin clouds the radiation is 

 reduced by only 10 to 20 per cent. 



XI. The effect of haze upon the effective radiation to the sky is almost in- 

 appreciable when no clouds or real fog are formed. Observations in Algeria 

 in 1912 and in California in 1913 show that the great atmospheric disturbance 

 caused by the eruption of Mount Katmai in Alaska, in the former year, can only 

 have reduced the nocturnal radiation bj' less than 3.0 per cent. 



XII. Conclusions are drawn in regard to the radiation from large water sur- 

 faces, and the probability is indicated that this radiation is almost constant at 

 different temperatures, and consequently in different latitudes also. 



Another grant was made to Prof. Angstrom in October, 1915, for a 

 study of nocturnal radiation in the far north during the long 

 Arctic night. Concerning this study he wrote to the Institution on 

 February 16, 1916, as follows : 



Through this grant I have been able to make observations on nocturnal 

 radiation during the Arctic night in the north of Sweden, at a place named 

 Abisko, at about 68° 30' latitude. The observations were extended during 

 about a month (Jan. 1-26) and were obtained under various atmospheric con- 

 ditions. One night observations were taken at a temperature of — 30° C. 

 (—20° F.), when consequently the absolute humidity must have been very low. 

 In general, these observations confirm the views expressed in my paper ^ in 

 regard to the influence of temperature and humidity upon the nocturnal radia- 

 tion and the radiation of the atmosphere. 



In connection with the named measurements observations were also made on 

 the cooling of snow surfaces under the temperature of the surrounding air as a 

 consequence of nocturnal radiation. As was to be expected, a linear relation 

 was found to exist between the radiation and the named temperature difference. 



1 Smithsonian Misc. Coll., Vol. 65, No. 3, 1915. 



