WEATHER BUREAU. 97 



Washington durin*^: 12 months ending with April 6, 1922, except that 

 for about four weeks in July and August, 1921, and for a like period' 

 in January, 1922, the measurements were made at Chicago, 111. 



From the sky brightness measurements thus obtained charts have 

 been prepared showing the resulting illumination on vertical sur- 

 faces, such as the walls of buildings, facing the eight principal points 

 of the compass, for each hour of each day throughout the year for 

 httitude 42° north, wath the sky free from clouds and also com- 

 l^letely covered with clouds. The charts are based on the measure- 

 ments made in practically smoke-free atmosphere in a suburb of 

 AVashington, but the percentage of diminution in illumination result- 

 ing from a smoke cloud such as usually prevails at Chicago has been 

 determined. 



Even with skies entirely free from clouds the brightness and the 

 resulting illumination may vary by as much as 50 per cent from the 

 mean value represented b}' the charts. This is due principally to the 

 presence of haze or dust in the atmosphere to a greater or less degree. 

 A clear blue sky, which is rare at Washington, is dark as compared 

 with a white, hazy sky. Thin clouds increase the brightness of the 

 sky markedly, and esj^ecially in the vicinity of the zenith. 



A preliminary report on the sky brightness measurements has been 

 published in the Monthly Weather Review, and also in the Transac- 

 tions of the Illuminating Engineering Society. A more complete 

 rejjort is now in preparation. 



These results are of practical importance to illuminating engineers 

 and others having to do w^th questions of natural lighting, especially 

 as applied to school rooms, office buildings, and industrial plants. 



There yet remain to be considered the effect upon interior illumina- 

 tion of substituting for skylight the daylight reflected from the walls 

 of neighboring buildings, and, also, the interior illumination from 

 skylight wdien the so-called saw-tooth roof construction is emploj^ed. 

 It is hoped to solve these two problems during the coming year. 



It is also the expectation to take part in an international investiga- 

 tion of the dust content of the atmosphere that has been planned by 

 the meteorological section of the International Geodetic and Geo- 

 physical Union. 



The official in charge of solar radiation investigations took advan- 

 tage of a ti'ip to Europe as a delegate to the International Institute 

 of Agriculture and to the International Geodetic and Geophysical 

 Union, both meeting in Eome in May, 1922, to consult with European 

 meteorologists on various topics, more particularly those relating to 

 solar radiation studies. 



