SUNLIGHT AS A SOURCE OF KADIATION 107 



(1930) indicate that a diurnal variation, if it exists, is small. Regarding 

 the change of ozone thickness with sunspots, there are few long-contiiuied 

 series of ozone data. Fowle (1934a, 1935) obtained yearly a\'erage values 

 at north latitude 34° in Arizona and California. Ilis values from 1921 to 

 1928 varied with the sunspots, but the correspondence was not main- 

 tained from 1928 to 1934. A coamection between ozone and sea-level 

 barometer changes is complicated and has not been clearly established. 

 From the data of six ozone stations in Europe, Dobson concluded that the 

 smallest amount of ozone occurred to the southwest of a barometric high 

 and that the passage of a barometric low over a station was accompanied 

 by an increase in ozone content. In general, since most of the ozone lies 

 between 15 and 35 km, its variations would be expected to be correlated 

 with changes of pressure and winds in the stratosphere rather than in the 

 troposphere, and at this time the relation between stratospheric and 

 tropospheric weather cannot be said to be completely known and 

 understood. 



CALCULATED ULTRAVIOLET INTENSITY AT THE EARTH'S SURFACE 



A complete calculation of the solar ultraviolet radiation falling on the 

 surface of the earth from the sun and sky is very complicated and will not 

 be attempted. It is, however, instructive to calculate the ultraviolet 

 intensity of the direct rays of the sun at the surface of the earth for vari- 

 ous zenith angles of the sun. Let in and i be the intensities, respectively, 

 of the rays of the sun outside the atmosphere and at the surface of the 

 earth. Then 



where a = the absorption coefficient of ozone per centimeter, 

 X = the thickness of the ozone in centimeters at NTP, 

 a„ = the attenuation per atmosphere below the ozone region due 



to pure air and haze, and 

 Z = the zenith angle of the sun. 

 The symbols i, in, a, and a^ refer to the wave-length region from X to 

 X + r/X. In Fig. 3-7 the curve labeled to is the Naval Research Labora- 

 tory curve of Fig,. 3-4 and is plotted in arbitrary units against the wave 

 length for the ultraviolet region of the solar spectrum where the absorp- 

 tion of ozone becomes important. The values of i for ozone of thickness 

 4 mm, for no atmospheric attenuation, and for various zenith angles were 

 calculated from Eq. (3-6) with .r = 0.4 cm, «„ = 0, and values of a 

 obtained by drawing a smooth curve through the fluctuations of curve I, 

 Fig. 3-5. The i vs. X curves are plotted in Fig. 3-7 for six values of Z. 

 The total ultraviolet energy E in wave lengths less than 3200 A was 



