SOLAR RESEARCH AT MOUNT WILSON, CALIFORNIA. 



Cloudinkss. 



165 



on which observations could be made amount to 132 out of 135. The 

 long periods of perfectly clear weather, permitting observations of 

 the sun to be made without interruption from day to day, should 

 prove of the greatest importance in the study of many solar problems 

 which require daily observations for their solution. From the rec- 

 ords so far obtained, it seems probable that observations of the sun 

 could be made at Mount Wilson on more than 300 days in a year. 

 In Los Angeles, during the past twenty-three years, the average num- 

 ber of "clear" days in the year is 317. 



The cloudiness in July and August was due almost entirely to 

 thunderstorms over the desert to the north and east. The clouds 

 rarely reached our zenith and almost never interfered with the regu- 



r solar observations (cf. table of Seeing, pp. 170, 171). 



HUMIDITY. 



The question of humidity is of special importance in connection 

 with the measurement of the solar constant, since water-vapor in the 

 atmosphere absorbs very strongly the solar beat. The results obtained 

 with a standard sling psych rometer, Weather Bureau pattern, are 

 given in the following table. 



