NO. 3 



SOLAR VARIATION AND WEATHER — ABBOT 



Fig. 6. — Record pyrheliometry to 15 miles altitude. 



wished to devise a method whereby several values of the solar 

 constant could be obtained per day, by intervals of observing too short 

 for hurtful changes of transparency. 



2. THE SHORT METHOD 



Alfred F. Moore, observing at Calama, Chile, showed me in 1920 

 a long series of observations with our sky-radiation instrument, the 

 pyranometer (fig. 9, p. 13), on the brightness of a limited zone of sky 

 surrounding the sun. When the transparency of the atmosphere is low, 

 the sky gets brighter, and vice versa. Comparing Moore's pyranometry 

 with simultaneous determinations of atmospheric transparency at 

 40 wavelengths, made by Langley's method, I was able to draw 

 families of curves throughout the spectrum of the sun, giving trans- 

 mission coefficients suited to all states of sky brightness at Calama. 

 (fig. 8, p. 11). 



This is the basis of the "short method" of solar-constant observmg. 

 It requires only about 10 minutes of observing by spectrobolometer, 

 pyranometer, and pyrheliometer. We became accustomed to making 



