SUNLIGHT AS A SOURCE OF RADIATION 



10.3 



where io and i are the intensities of a collimatod beam of lij2;ht in the 

 wave-length region from X to X + dX, entering and emerging from a 

 layer of ozone x cm in thickness at normal temperature and pressure 

 (NTP), i.e., at 0°C and a pressure of 1 atm. The long-wave-length region 

 of the ozone absorption from about 3400 to 3100 A has a band structure 

 known as the Huggins bands; the smooth absorption below 3100 A is 



lOOOrr 



2200 2400 2600 2800 3000 



WAVE LENGTH, A 



J200 



3400 



''Fig. 3-5. Ultraviolet absorption coefficients of some atmospheric gases. 



known as the Hartley continuum. The absorption in the Huggins bands 

 varies with the temperature in a complicated way, which is of no interest 

 here; roughly, a decreases between 10 and 30 per cent when the tempera- 

 ture falls from 18° to -50°C (Vigroux, 1950). It is seen from the ozone 

 curve of Fig. 3-5 that, below 3100 A, a rises to very high values; therefore 

 it is not surprising that even the few millimeters of ozone which e.xists 

 in the upper atmosphere is sufficient to prevent the detection of ultra- 

 violet wave lengths from 2915 to 2150 A at the surface of the earth. 

 In the standard atmosphere the pressure is 1.0132 X lO*' dynes /cm-, 

 and the total air in a vertical column of the standard atmosphere from 

 sea level to space amounts to 8 km of air at XTP; 20.75 per cent of this is 

 oxygen. In Fig. 3-5, curve II is the attenuation coefficient a per atnios- 



