NO. 10 SOLAR RADIATION AND WEATHER STUDIES ABBOT 3 



I. SOLAR RADIATION MEASUREMENTS =■ 



I. Objects and Stations 



We measure at the earth's surface the total intensity of solar radia- 

 tion, its spectral distribution, the losses its various rays meet in travers- 

 ing the atmosphere ; and we compute its intensity and spectral dis- 

 tribution outside the atmosphere, and the variations of its intensity 

 from day to day as they occur in the sun itself before the rays enter 

 the atmosphere. At present, the Smithsonian Institution carries on 

 these measurements at three high-altitude desert stations chosen for 

 their cloudlessness and other favorable conditions. They are Table 

 Mountain, Calif. ; Montezuma, Chile ; and Mount St. Katherine. 

 Egypt. Their respective altitudes are 7,500, 9,000, and 8,500 feet, 

 approximately. Other Smithsonian stations formerly occupied have 

 included Washington, D. C. ; Hump Mountain, N. C. ; Mount Wilson 

 and Mount Whitney, Calif. ; Mount Harqua Hala, Ariz. ; Bassour, 

 Algeria ; and Mount Brukkaros, South-West Africa. Plate i shows 

 the station at Mount Montezuma. Besides these terrestrial stations, 

 a self-recording instrument for measuring total solar radiation was 

 raised by sounding balloons from Omaha, Nebr., July 1914, to a 

 level of over 15 miles. It made good records of the intensity of solar 

 radiation at that high level where only 1/25 of the atmospheric pres- 

 sure remained above. The mean value of the solar constant of radia- 

 tion as computed from mountain stations is 1.94 calories per square 

 centimeter per minute. Balloon pyrheliometry indicated 1.84 calories 

 at 15 miles elevation. Correction of balloon pyrheliometry for loss 

 in the highest atmosphere gives 1.88 calories, which agrees with moun- 

 tain solar-constant results within the experimental error of the balloon 

 observations. 



2. Instruments and Methods 



For measuring total solar radiation at the earth's mountain surface 

 we have hitherto depended ^ on the silver-disk pyrheliometer and the 

 water-flow pyrheliometer. The former is a secondary instrument 

 whose readings are converted into absolute units (calories per square 

 centimeter per minute) by comparisons with the water-flow pyr- 

 heliometer." These instruments are shown diagrammatically in figures 



^ This section is for the most part abbreviated from vols. 1-5, Annals of the 

 Astrophysical Observatory of the Smithsonian Institution. 



* We are now (1935) introducing the Angstrom electrical compensation pyr- 

 heliometer as a cooperating instrument. 



^ See improved water-flow pyrheliometer as described in Smithsonian Misc. 

 Coll., vol. 87, no. 15, 1932, and vol. 92, no. 13, 1934. 



