NO. 4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT 1 3 



If the logarithm of H be plotted against air mass in the six obser- 

 vations for each day, it will nearly always occur that the points for 

 I, II, III, and IV will be practically in a straight line, while V and VI 

 will be somewhat higher than the line, VI being considerably higher 

 than V . There is no theoretical reason why this relation should hold 

 exactly, but its approximate indications may prove convenient to 

 the reader. The divergence of the two later observations may be 

 partially due to two causes, (i) An increase of water vapor in 

 the air late in the morning with attendant increase in haziness causes 

 decrease of the intensity of direct sun light and increase of the in- 

 tensity of scattered sky light. (2) Sky light is much stronger 

 proportionately in the shorter wave lengths than sun light. The 

 longer wave lengths have a very much higher transmission coefficient 

 than the shorter ones — or in other words, the shorter ones increase 

 much more rapidly with a decrease of air mass than the longer 

 ones. Thus the sky light may tend to increase faster than the direct 

 sun light at high sun. 



rcLe 



Fig. I 



The table shows that haze has a very marked effect in increasing- 

 sky radiation, and to a somewhat lesser extent in decreasing the 

 solar radiation. 



Sky radiation by zones. — In order to investigate the proportion of 

 the sky radiation falling on a horizontal surface from various sec- 

 tions of the sky, two kinds of observations were made: (i) with 

 thimbles placed over the pyranometer glass so as to divide the hemi- 

 sphere into zones each 30° in width, and (2) with the thimble ad- 

 mitting a 60° cone of light, measuring the intensity of the ring con- 

 centric with the sun, and comparing it with cones of equal size and 

 altitude, and located in azimuth 60°, 120°, 180°, respectively, from 

 the sun, starting toward the east. 



First will be discussed the horizontal zones, giving the theoretical 

 intensity for an equal sky. and then the experimental results obtained 

 under varying conditions. 



