22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 7I 



Summary of zone observations. — As might be expected, it is rather 

 difficult to arrive at any very definite conclusions from observa- 

 tions of this character, owing to the many factors involved and 

 the wide variation in these factors themselves. For instance the 

 horizontal zones are a measure of the radiation from a strip of sky 

 of uniform width and extending entirely around the sky, whereas 

 the vertical zones are measures of circular areas of sky and varying 

 in altitude. It is of course evident that the horizontal zones have 

 not equal intensity per unit area in different azimuths of the sky, 

 and also, as shown by the vertical zone observations, the zone in 

 which the sun happens to be will show a larger sky radiation than 

 it would were the intense sky radiation surrounding the sun not 

 present. Hence the rough averages given immediately following 

 the table of horizontal zone observations must not be regarded as 

 very accurate if the sun and the ring of intense sky radiation sur- 

 rounding it be all within one zone. But as a rough average, the 

 values given are to be relied upon. There is even wider variation in 

 the vertical zones, but it may be stated that if the sky be divided 

 into four quadrants in azimuth, numbered i, 2, 3, and 4, respectively, 

 with the sun in the center of i, then 2 and 4 will have an intensity 

 of 50 to 80 per cent of i, and 3 will have from 25 to 40 per cent 

 of I, under ordinary cloudless conditions of sky. The zenith region 

 will show approximately 30 per cent of that surrounding the sun. 

 These values are also dependent largely upon the condition of the 

 sky, as regards haze, dust, smoke, etc., for the prevalence of all these 

 impurities tends to increase the sky radiation in the zone, either 

 horizontal or vertical, in which they are found. This is especially 

 noticeable in the region immediately surrounding the sun, for with 

 a clear blue sky, one can, by hiding the sun's disk, look practically 

 up to the limb of the sun. On the other hand, if the sky be hazy, 

 the glare is very intense, often several degrees from the sun's 

 limb, and the eye can scarcely distinguish exactly where the sky 

 leaves off and the sun's disk begins. 



RADIATION FROM A CLEAR SKY AT 30° AND 19° SUN 

 AT CALAMA 



Quite an extensive set of observations have been taken at Calama 

 on the radiation from cloudless skies with the sun exactly at air 

 mass 2, or at an altitude of 30° above the horizon. A table follows 

 which gives these results, with a description of the condition of the 

 sky prevailing at the time. 



