﻿120 
  

  

  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1943 
  

  

  The 
  significant 
  feature 
  of 
  the 
  curves 
  of 
  figure 
  11 
  is 
  the 
  rapid 
  falling 
  

   off 
  of 
  the 
  antirachitic 
  effectiveness 
  of 
  sunlight, 
  with 
  increase 
  in 
  atmos- 
  

   pheric 
  ozone 
  and 
  with 
  increase 
  in 
  the 
  air 
  mass 
  resulting 
  from 
  obliquity 
  

   of 
  the 
  sun's 
  rays. 
  For 
  air 
  mass 
  2 
  corresponding 
  to 
  the 
  sun 
  30° 
  above 
  

   the 
  horizon 
  it 
  will 
  be 
  noted 
  that 
  the 
  antirachitic 
  effect 
  is 
  less 
  than 
  

   one-tenth 
  that 
  of 
  the 
  sun 
  in 
  the 
  zenith 
  under 
  otherwise 
  identical 
  con- 
  

  

  3000 
  

   WAVELENGTH 
  IN 
  ANGSTROMS 
  

  

  Figtjbb 
  7. 
  — 
  Solar 
  spectral-energy 
  for 
  various 
  air 
  masses. 
  (2.0 
  mm. 
  ozone 
  in 
  

  

  zenith 
  atmosphere.) 
  

  

  ditions. 
  For 
  air 
  mass 
  3 
  corresponding 
  to 
  the 
  sun 
  about 
  20° 
  above 
  

   the 
  horizon 
  the 
  effect 
  has 
  fallen 
  to 
  about 
  one-eightieth 
  that 
  of 
  the 
  

   zenith 
  sun. 
  

  

  It 
  now 
  becomes 
  possible 
  to 
  predict 
  the 
  antirachitic 
  effectiveness 
  of 
  

   sunlight 
  for 
  a 
  elear 
  day 
  at 
  any 
  point 
  on 
  the 
  earth's 
  surface 
  at 
  any 
  

   season 
  and 
  at 
  any 
  time 
  of 
  day, 
  providing 
  only 
  that 
  the 
  zenith 
  ozone 
  

   over 
  the 
  station 
  be 
  known. 
  This 
  has 
  been 
  carried 
  out 
  for 
  latitudes 
  

   33°, 
  38°, 
  and 
  43°, 
  and 
  for 
  the 
  conditions 
  of 
  2.0 
  and 
  2.8 
  millimeters 
  of 
  

   ozone, 
  representing 
  average 
  low 
  and 
  average 
  high 
  ozone 
  quantities 
  

  

  