jg2 Tables 177-180. 



ATMOSPHERIC TRANSPARENCY AND SOLAR RADIATION. 



TABLE 177.— Transmission ol Radiation Through Moist and Dry Air. 

 This table gives the wave-length, A. ; a the transmission of radiation by dry air above Mount 

 Wilson (altitude = 1730 m. barometer, 620 mm.) for a body in the zenith ; finally a correction fac- 

 tor, aw, due to such a quantity of aqueous vapor in the air that if condensed it would form a layer 

 I cm. thick. Except in the bands of selective absorption due to the air, a agrees very closely with 

 what would be expected from purely molecular scattering, a^ is very much smaller than would be 

 correspondingly expected, due possibly to the formation of ions by the ultra-violet light from the 

 sun. The transmission varies from day to day. However, values for clear days computed as fol- 

 lows agree within a per cent or two of those observed when the altitude of the place is such that 

 the effect due to dust may be neglected, e. g. for altitudes greater than 1000 meters. If B = 



the barometric pressure in mm., w, the amount of precipitable water in cm., then 3.^ = 3.^'^ aZ- w is 

 best determined spectroscopically (Astrophysical Journal, 35, p, 149, 1912,37, p. 359, 1913) other- 



wise by formula derived from Hann, w^ 

 station, h, the altitude in meters. 



2.3ewio 



h 



2i000 



Cw being the vapor pressure in cm. at the 



Fowle, Astrophysical Journal, 38, 1913. 

 TABLE 178.— Brightness ol (radiation from) Sky at Mt. Wilson (1730 m.) and Flint Island (sea level). 



Zenith dist. of zone .... 



10* X mean ratio sky/sun Mt. Wilson 



" " Flint Island 



Ditto X area of zone Mt. Wilson 



" " " Flint Island 



0-15^ 

 1500* 



i"5 

 51.0 

 3-9 



'5-35 

 400 



58.8 

 17.9 



35-50° 

 520 

 128 

 91.5 



22-5 



50-60° 

 610 

 150 

 87.2 

 21.4 



60-70'- 



660 



185 



104.3 



29.2 



70-80^ 

 700 



117.6 

 35-3 



80-90^ 

 720 

 460 



125-3 

 80.0 



Sun. 



636 

 210 



Altitude of sun . 



Sun's brightness, cal. per cm.2 per min. . 

 Ditto on horizontal surface .... 

 Mean brightness on normal surface sky X lo'/sun 

 Total sky radiation on horizontal cal. per cra.^ . 



per m. . . 



Total sun + sky, ditto 



5" 

 •533 

 .046 



423 



.056 

 .102 



IS" 

 .900 

 • 233 

 403 



.110 

 •343 



25" 

 1.233 

 .524 



■385 



.162 

 .686 



35" 



1.358 



.780 



365 



47*" 



1. 413 



1. 04 1 



346 



.205 

 1.246 



65° 

 1.496 

 1-355 



326 



.226 



1. 581 



82i° 



1.521 



1.507 



310 



.240 

 1-747 



* Includes allowance for bright region near sun. For the dates upon which the observation of the upper portion of 

 table were taken, the mean ratios of total radiation sky/sun, for equal angular areas, at normal incidence, at the island 

 and on the mountain, respectively, were 636 X 10—8 and 210 X 10— «, on a horizontal surface, 305 X 10— Sand 77 X 10—8; 

 for the whole sky, at normal incidence, 0.57 and 0.20; on a horizontal surface 0.27 and 0.07. Annals of the Astro- 

 physical Observatory of the Smithsonian Institution, vols. II and III, and unpublished researches (Abbot). 



TABLE 179.— Relative Distribution In Normal Spectrum of Sunlight and Skylight at Mount Wilson. 



Zenith distance about 50°. 



TABLE 180— Air Masses. 



See Table 174 for definition. Besides values derived from the pure secant formula, the table 

 contains those derived from various other more complex formula, taking into account the curva- 

 ture of the earth, refraction, etc. The most recent is that of Bemporad. 



The Laplace and Bemporad values, Lindholra, Nova Acta R. Soc. Upsal. 3, 1913 ; the others, Radau's Actmo- 

 metric, 1877. 

 Smithsonian Tables. 



