APPENDIX 7 

 EEPORT ON THE ASTROPHYSICAL OBSERVATORY 



Sir: I have the honor to submit the following report on the ac- 

 tivities of the Astrophysical Observatory for the fiscal year ended 

 June 30, 1936 : 



As heretofore, our principal work has been the determination of 

 the variability of the intensity of the suii's radiation as it would be 

 found outside of our atmosphere ; i. e., changes in the so-called solar 

 constant of radiation. Measurements of the solar constant of radia- 

 tion have been continued daily when possible at three stations: 

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

 Egypt. Although to the eye the sky at Table Mountain seems 

 equally as favorable to the work as that at the two foreign stations, 

 the measurements at Table Mountain show much greater irregularity. 

 Days that appear excellent there sometimes yield solar-constant 

 values which are quite impossible. We suppose this is caused by 

 invisible clouds of water vapor sweeping over the station through 

 the canyons of the Sierra Madre Mountains. 



We have attempted in the past year to develop a criterion based 

 on the measurements which should inform us, before reducing an 

 observation, whether it is affected by this invisible prejudicial sky 

 condition, and in this search we have measurably succeeded. Find- 

 ing it so useful for Table Mountain work, we have applied it also to 

 the work of Montezuma and Mount St. Katherine, where much less 

 frequently than at Table Mountain the results are similarly inex- 

 plicably bad. These investigations have occupied Mr. Aldrich and 

 several computers at Washington for most of the year, but are now 

 completed. They lead to a recomputation of all solar-constant 

 measurements of recent years at all stations — a heavy task now 

 under way. 



Of the criterion itself, its technical character forbids a full descrip- 

 tion here. In brief, it comprises constructing families of standard 

 curves representing average relationships of pyrheliometry to pyra- 

 nometry at different air masses and different values of precipitable 

 water. Wlien the observations on any occasion depart from these 

 standard curves beyond a certain tolerance, such observations are to 

 be viewed with suspicion. In this way single observations of a given 

 day may be rejected from the mean of the day. Also, suspicious days 

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