VISIBLE AND NEAR-VISIBLE RADIATION 



173 



In order to gain an idea of the influence of dust and smoke, we may 

 turn to the Weather Bureau's records for Chicago (Fig. 19). October 

 23 was a clear day with the strong wind purging the atmosphere of smoke. 

 On October 22, dense city smoke prevailed until 10:51, when it was swept 

 away by a shift of the wind. October 26 shows the effect of combined 

 smoke and clouds. 



From the high intensities attained on clear days in June, the midday 

 maximum falls to a lowest value in December. With the corresponding 

 shortening of the days, the total energy received per day may fall to 



22 

 20 



18 



\b 

 14 

 12 

 1 

 8 

 6 



6 7 8 9 10 11 Noon 1 2 3 4 5 6 



Apparent or True Solcir Ti'me 



Fig. 19. — Records of total solar radiation received on a horizontal surface at the University 



Observatory, Chicago, 111. 



one-tenth of its summer value for average sky in the city. [See Kimball 

 (44), also page 224, for a group of typical curves of seasonal fluctuation in 

 total daily energy.] For an average sky in summer, noon intensity com- 

 monly reaches 1 gm. cal./min./cm.^ at sea level. 



In order to form an idea of the spectral distribution of direct solar 

 radiation. Fig. 20 has been plotted, showing the extrapolated value for 

 radiation outside the atmosphere and the effect of successively greater 

 integral air masses. Air mass is defined as unity for the sun at zenith 

 (and approximately equals the secant of the zenith distance for the other 

 positions). Only the outstanding bands of atmospheric absorption are 

 shown. Those in the infra-red are chiefly due to water vapor, the amount 

 being moderately great, 1.37 cm. precipitable water. These curves have 

 been prepared from Abbot's data (4) and are intended to be representa- 

 tive of an average day in Washington. Note the marked decrease 



o 



between 4000 and 5000 A with increasing air mass, as well as the deepening 

 of the infra-red absorption bands. 



