NO. 4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT 



25 



Comparison of whole sky at M2 and ilf j. — By averaging 64 values 

 at 30° sun and 51 values at 19° sun, it will be seen that increased sky 

 radiation attends higher sun „ for approximately 0757 of a calorie is 

 radiated by the whole sky at 30° sun and .0642 of a calorie at 19° 

 sun. With a narrow zone of sky around the sun, the opposite is 

 true — that is, the radiation decreases as the sun's altitude becomes 

 greater. This is probably due to a clearing of the sky as the sun 

 gets farther from the horizon. The radiation from the region next 

 to the sun is much greater than for other equal areas of sky, as 

 is shown by the vertical zone observations described in this paper. 

 Hence as this region of greater intensity gets higher in the sky, its 

 radiation falls more nearly vertical on a horizontal surface and this 

 causes the total sky radiation to increase as the altitude of the sun 

 increases. 



On January 24, 1919, at Calama, a series of sky observations was 

 made which may prove of some interest and illustrate what has 

 just preceded. The sky was very hazy. Beginning with the sun at 

 about 9° altitude, the measurements were alternated between the 

 30° zone arrangement pointed at the sun, but with the sun shaded ofif 

 the strips, and the whole sky, the sun being again shaded off. Follow- 

 ins: are the results. 



