﻿EFFECTS 
  OF 
  SOLAR 
  RADIATION 
  — 
  O'BRIEN 
  

  

  123 
  

  

  sure 
  required 
  would 
  be 
  more 
  than 
  7 
  hours 
  ! 
  Evidently 
  even 
  in 
  ideal 
  

   weather 
  one 
  cannot 
  rely 
  upon 
  direct 
  exposure 
  to 
  winter 
  sunlight 
  in 
  

   the 
  higher 
  latitudes, 
  and 
  it 
  is 
  upon 
  the 
  winter 
  food 
  supply 
  that 
  the 
  

   population 
  must 
  depend 
  except 
  in 
  tropical 
  or 
  semitropical 
  latitudes. 
  

   Actually 
  much 
  of 
  the 
  vtiamin 
  D 
  in 
  the 
  food 
  may 
  have 
  been 
  sunlight- 
  

   produced, 
  but 
  this 
  topic 
  lies 
  outside 
  of 
  our 
  present 
  discussion. 
  

  

  The 
  calculations 
  thus 
  far 
  have 
  been 
  limited 
  to 
  the 
  three 
  latitudes 
  

   33°, 
  38°, 
  and 
  43°. 
  In 
  figure 
  20 
  is 
  shown 
  a 
  reproduction 
  of 
  a 
  United 
  

   States 
  Geological 
  Survey 
  map 
  upon 
  which 
  these 
  three 
  latitudes 
  have 
  

  

  PRODUCT 
  CURVES 
  OF 
  SOLA.R 
  ENERGY 
  

   TIMES 
  ANTIRACHITIC 
  EFFICIENCY 
  

  

  2900 
  

  

  Z950 
  

  

  3000 
  .5050 
  

  

  WAVELENGTH 
  IN 
  ANGSTBOMS 
  

  

  3100 
  

  

  3150 
  

  

  Figure 
  10. 
  — 
  Product 
  curves 
  of 
  solar 
  spectral 
  energy 
  by 
  antirachitic 
  efficiency. 
  (2.8 
  

   mm. 
  ozone 
  in 
  zenith 
  atmosphere.) 
  

  

  been 
  drawn. 
  It 
  will 
  be 
  seen 
  that 
  these 
  latitudes 
  are 
  fairly 
  represent- 
  

   ative 
  of 
  the 
  United 
  States, 
  although 
  significant 
  areas 
  of 
  the 
  country 
  

   lies 
  above 
  43° 
  and 
  below 
  33°. 
  Similarly 
  in 
  figure 
  21 
  latitudes 
  33° 
  and 
  

   43° 
  north 
  and 
  south 
  of 
  the 
  Equator 
  have 
  been 
  drawn 
  on 
  a 
  Mercator 
  

   projection 
  of 
  the 
  earth's 
  surface. 
  It 
  will 
  be 
  seen 
  that 
  a 
  substantial 
  

   portion 
  of 
  the 
  populated 
  areas 
  is 
  included 
  within 
  these 
  latitude 
  belts. 
  

   If 
  one 
  desires 
  the 
  antirachitic 
  effect 
  of 
  sunlight 
  outside 
  these 
  belts 
  it 
  

   is 
  only 
  necessary 
  to 
  determine 
  the 
  zenith 
  angle 
  of 
  the 
  sun 
  for 
  the 
  

   place, 
  time 
  of 
  year, 
  and 
  time 
  of 
  day 
  with 
  the 
  aid 
  of 
  a 
  nautical 
  almanac. 
  

   The 
  secant 
  of 
  this 
  zenith 
  angle 
  determines 
  the 
  air 
  mass 
  from 
  which 
  

   the 
  relative 
  antirachitic 
  effect 
  may 
  be 
  determined 
  by 
  reference 
  to 
  

   figure 
  11. 
  

  

  