﻿SOLAR 
  RADIATION 
  AS 
  A 
  POWER 
  SOURCE 
  1 
  

  

  Bv 
  C. 
  G. 
  Abbot 
  

   Secretary, 
  Smithsonian 
  Institution 
  

  

  [With 
  3 
  plates] 
  

  

  This 
  major 
  power 
  source 
  of 
  the 
  world 
  is 
  as 
  yet 
  almost 
  unused 
  by 
  

   engineers. 
  The 
  intensity 
  of 
  solar 
  radiation 
  at 
  mean 
  solar 
  distance 
  

   outsidQ 
  the 
  earth's 
  atmosphere 
  is 
  about 
  1.94 
  calories 
  per 
  square 
  centi- 
  

   meter 
  per 
  minute. 
  Expressed 
  in 
  English 
  measures, 
  this 
  is 
  7.15 
  B. 
  t. 
  u. 
  

   per 
  square 
  foot 
  per 
  minute. 
  Actual 
  supplies 
  of 
  sun 
  heat, 
  however, 
  

   vary 
  with 
  the 
  season. 
  Owing 
  to 
  the 
  ellipticity 
  of 
  the 
  earth's 
  orbit, 
  

   we 
  are 
  about 
  3 
  million 
  miles 
  nearer 
  the 
  sun 
  in 
  January 
  than 
  in 
  July, 
  

   and 
  the 
  actual 
  intensity 
  of 
  the 
  sun's 
  rays 
  outside 
  the 
  atmosphere 
  is 
  

   about 
  6 
  percent 
  greater 
  in 
  January 
  than 
  in 
  July. 
  As 
  there 
  is 
  less 
  hu- 
  

   midity 
  and 
  clearer 
  air 
  in 
  winter, 
  it 
  follows 
  that 
  the 
  sun's 
  heat 
  at 
  the 
  

   earth's 
  surface 
  in 
  the 
  Northern 
  Hemisphere 
  is 
  a 
  good 
  deal 
  more 
  intense 
  

   in 
  winter 
  than 
  in 
  summer 
  for 
  equal 
  solar 
  altitudes 
  above 
  the 
  horizon. 
  

   The 
  reason 
  winter 
  is 
  cold 
  is 
  because 
  the 
  sun 
  lies 
  so 
  far 
  south 
  that 
  its 
  

   rays 
  shine 
  very 
  obliquely, 
  so 
  that 
  the 
  average 
  intensity 
  on 
  a 
  horizontal 
  

   surface 
  is 
  thereby 
  greatly 
  reduced. 
  

  

  Thick 
  clouds 
  reflect 
  away 
  about 
  75 
  percent 
  of 
  the 
  sun 
  rays 
  which 
  

   strike 
  them. 
  Much 
  of 
  the 
  area 
  east 
  of 
  the 
  Mississippi 
  is 
  50 
  percent 
  

   cloudy, 
  so 
  it 
  follows 
  that 
  in 
  these 
  sections 
  a 
  third 
  of 
  the 
  sun's 
  radia- 
  

   tion 
  is 
  reflected 
  out 
  to 
  space. 
  Besides 
  this 
  cloud 
  loss 
  there 
  is 
  an 
  

   actual 
  absorption 
  by 
  the 
  water 
  vapor 
  and 
  other 
  ingredients 
  of 
  the 
  

   atmosphere. 
  This 
  amounts 
  in 
  humid 
  localities 
  to 
  from 
  15 
  to 
  25 
  

   percent. 
  Accordingly, 
  solar 
  power 
  propositions 
  would 
  operate 
  at 
  

   great 
  disadvantage 
  in 
  most 
  of 
  the 
  States 
  east 
  of 
  the 
  Mississippi, 
  ex- 
  

   cepting 
  Florida, 
  as 
  compared 
  to 
  the 
  arid 
  and 
  generally 
  high-altitude 
  

   regions 
  of 
  the 
  Southwest. 
  

  

  Measurements 
  of 
  solar 
  radiation 
  made 
  at 
  the 
  earth's 
  surface 
  upon 
  

   a 
  receiver 
  at 
  right 
  angles 
  to 
  the 
  beam, 
  and 
  with 
  the 
  sun 
  at 
  15° 
  or 
  more 
  

   above 
  the 
  horizon, 
  range 
  from 
  1.5 
  calories 
  per 
  square 
  centimeter 
  per 
  

   minute 
  down 
  to 
  1.0 
  calorie, 
  or 
  even 
  less, 
  depending 
  on 
  the 
  clearness 
  

   of 
  the 
  atmosphere. 
  In 
  favorable 
  localities 
  a 
  value 
  of 
  about 
  1.35 
  

  

  1 
  Reprinted 
  by 
  permission 
  from 
  The 
  Military 
  Engineer, 
  vol. 
  35, 
  No. 
  208, 
  February 
  1943. 
  

  

  99 
  

  

  