212 BIOLOGICAL EFFECTS OF RADIATION 



followed the Smithsonian Institution in adhering to the 1913 standard. 

 In this paper radiation intensities are expressed in units of this scale 

 unless some other scale is specified. 



MEASUREMENTS OF SOLAR-RADIATION INTENSITY AT NORMAL 



INCIDENCE 



Figure 1 illustrates the marked increase in solar-radiation intensity 

 with altitude above sea level. The horizontal lines indicate the propor- 

 tion of the radiation reaching the outer limit of the atmosphere that is 

 transmitted to a place of observation, with the sun at different zenith 

 distances; or, conversely, the depletion of radiation intensity in passing 

 through the atmosphere is shown. With the sun in the zenith, the solar 

 rays reach the earth by the shortest possible path, and the air mass passed 

 through is called 1.0. With the sun at zenith distance 60°, the path is 

 twice as long and the air mass is 2.0. With the sun at zenith distance 

 70.7°, the air mass is 3.0, and for greater zenith distances the air mass 

 increases much more rapidly, reaching a value of approximately 30 when 

 the sun is just above the horizon.^ 



The path is through much less atmosphere on a high mountain than 

 at sea level, and in some investigations unit air mass is considered to be 

 with the sun in the zenith with air pressure 760 mm. If, therefore, the 

 air pressure is only 684 mm., with the sun in the zenith, and at zenith 

 distances 60.0° and 70.7°, respectively, the corresponding air masses will 

 be 0.9, 1.8, and 2.7, and with atmospheric pressure 456 mm., the air 

 masses will be 0.6, 1.2, and 1.8, etc. 



The curved lines in Fig. 1 clearly show the decrease in atmospheric 

 depletion with increase in altitude at the stations named, and its increase 

 with solar-zenith distance. The effect of increase in altitude is much 

 more marked near sea level than on high mountains, partly because at 

 elevated stations the pressure decrease for a given increase in elevation is 

 less, and partly because the moisture and dust content of the atmosphere 

 is much less. Dust has a marked diffusing effect, and water vapor 

 absorbs strongly in the infra-red, or long-wave radiation (see Brackett, 

 pages 171-175). 



Curves 25 and 27, Fig. 1, for Washington in February and June, 

 respectively, show much greater depletion in summer than in winter, 

 as also do curves 21 and 26 for Lincoln, Neb., for February and August. 

 For Davos, Switzerland, at an elevation of 1600 meters, lines 19 and 20, 

 show for February and June, respectively, much less difference in radia- 

 tion intensities than has been indicated for stations at lower elevations. 



^ For a more detailed statement of the relation between solar-zenith distance and 

 air mass, see Smithsonian Meteorological Tables, 5th revised Ed., 1931, p. Ixxix, and 

 Table 100, p. 226. 



