TABLES 553-556. 419 



ATMOSPHERIC TRANSPARENCY AND SOLAR RADIATION. 



TABLE 553. Transmission of 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, a w , 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 w 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 clue to dust may be neglected, e.g. for altitudes greater than 1000 meters. If B=s 



the barometric pressure in mm., w, the amount of precipitable water in cm., then a B = a* 20 a*, w is 

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

 wise by formula derived from Ilann, w= 2-3e w io 2200 , e w being the vapor pressure in cm. at the 

 station, h, the altitude in meters. See Table 377 for long-wave transmission. 



Fowle, Astrophysical Journal, 38, 1913. 



TABLE 554. -Brightness of (radiation from) Sky at Mt. Wilson (1730 m.) and Flint Island (sea level). 



* 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 & and 210 X 10 8, on a horizontal surface, 305 X 10 *> and 77 > 

 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 II and III, and unpublished researches (Abbot).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 555. -Relative Distribution In Normal Spectrum of Sunlight and Sky-light at Mount Wilson. 



Zenith distance about 50. 



TABLE 556. -Air Masses. 



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

 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, Lindholm, Nova Acta R. Soc. Upsal. 3, 1913 ; the others, Radat 

 metric, 1877. 

 SMITHSONIAN TABLES. 



