NOTE ON FOWLE'S SPECTROSCOPIC METHOD 



FOR THE DETERMINATION OF AQUEOUS 



VAPOR IN THE ATMOSPHERE 



By L. B. ALDRICH 



Director, Astrophysical Observatory 



(With One Plate) 



In 1912 F. E. Fowle ^ developed a spectroscopic method for deter- 

 mining the amount of water vapor in a column of atmosphere verti- 

 cally above the observer. This quantity he called the "precipitable 

 water," and he defined it as "the depth of liquid water which if in the 

 form of vapor would be contained in a column of air of the same 

 cross-section reaching vertically to the limits of the atmosphere." 



Fowle's method uses three infrared water-vapor bands, viz, p{\ = 



•935/*). ^(^=i-I3/a) and </''(A=i.47/a). With spectrobolometric 



energy curves obtained at Washington by passing the radiation from 



a bank of Nernst glowers through a known quantity of water vapor, 



he studied the depths of these absorption bands as affected by changes 



in width of bolometer strip, slit, and other variables. The depths as 



, . . . deflection at bottom of band 



expressed ni the ratio 



deflection at smooth curve above the band 



(or in abbreviated form, p/psc, </>/</>sc, and }p'/\p'sc) he correlated with 

 known quantities of water vapor in the path of the beam. The 

 amount of water vapor, expressed in centimeters of precipitable water, 

 was determined from the length of path and from wet- and dry-bulb 

 readings taken at many points along the path of the beam. The 

 accuracy of these water-vapor determinations he checked by absorbing 

 with phosphorus pentoxide and calcium chloride the water vapor in 

 known volumes of air along the path. Thus he developed a table 

 giving for standard conditions of definition and purity of the spec- 

 trum, as controlled by slit width, bolometer strip width, time of swing 

 of galvanometer, etc., the precipitable water corresponding to given 

 values of p/psc and <^/^sc- This table has since been extensively 

 used in the solar-constant work of the Astrophysical Observatory, 

 particularly in daily determinations of the short-method function F. 

 (see Ann. Astrophys. Obs., vol. 6, p. 66). 



Astrophys. Journ., vol. 35, p. 149, 1912; vol. 37, p. 359, I9i3- 

 SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. Ill, NO. 12 



