THE REFLECTING POWER OF CLOUDS 

 By L. B. ALDRICI1 



INTRODUCTION 



In the spring of 1918, the War Department established an observa 

 tion balloon school at Arcadia, California. ( )n clear days the balloons 

 of this school are in full view from the Smithsonian ( observing Sta 

 tion on Mt. Wilson. The valley to the south and west of :\lt. Wilson 

 is often filled in the early morning with dense fog, and from the 

 mountain-top one looks down upon a surface of white, billowy clouds 

 remarkably level and unbroken as a whole. Usually after several 

 hours the fog is dissipated, but on rare occasions it lasts until noon 

 or later. From this combination of circumstances it appeared evident 

 that one of these observation balloons sent up through such a fog 

 sea offered an unusual opportunity for determining the reflecting 

 power of a cloud surface practically filling a hemisphere of soli. I 

 angle. The top of the mountain, to be sure;, would cut off a portion 

 of the horizon, but being in the quarter opposite the sun, several miles 

 distant and with intervening haze itself supplying nearly as much 

 radiation as the small solid angle of cloud it took the place of, no 

 correction would be needed to allow for the presence of the moun- 

 tain. Accordingly Dr. Abbot obtained from the hirector of Military 

 Aeronautics, General Kenly, permission to use a balloon and detail 

 of officers and men for cloud reflection work on the fust favorable 

 day. Preliminary arrangements were made with the Commanding 

 Officer at Arcadia, and a favorable day awaited. 



On September 16, 1918, a very heavy fog filled tin- valley, pel 

 sisting all day and its top level almost reaching the summit of 

 Mt. Wilson. Prospects seemed excellent for a similar heavy fog at 

 a lower level on September 17, and final arrangements for the- experi- 

 ments were made. The sky conditions of September 17 more than 

 fulfilled expectations. A dense, homogeneous fog, unusually level 

 and even on top, filled the valley. Its upper surface was about X<x> 

 meters (2,600 feet) from the ground. It was 500 meters (i,'6oo 



Smithsonian Miscellaneous Collections, Vol. 69 No. 10 



