﻿FOGS 
  AND 
  CLOUDS. 
  

  

  By 
  W. 
  J. 
  Humphreys, 
  C. 
  E., 
  Ph. 
  D., 
  

  

  Professor 
  of 
  Meteorological 
  Physics, 
  United 
  States 
  Weather 
  Bureau, 
  

   Department 
  of 
  Agriculture. 
  

  

  [With 
  26 
  plates.] 
  

  

  EVAPORATION 
  AND 
  PRECIPITATION. 
  

  

  WHERE 
  FROM 
  AND 
  WHERE 
  TO. 
  

  

  Everyone 
  knows 
  that 
  rain 
  and 
  snow 
  come 
  out 
  of 
  the 
  clouds, 
  and 
  

   that 
  every 
  cloud 
  and 
  fog 
  particle 
  is 
  either 
  a 
  water 
  droplet 
  or 
  an 
  ice 
  

   crystal. 
  Naturally, 
  therefore, 
  one 
  asks 
  where 
  all 
  this 
  endless 
  supply 
  

   of 
  water 
  comes 
  from 
  ; 
  endless, 
  because 
  year 
  after 
  year, 
  century 
  after 
  

   century, 
  and 
  age 
  after 
  age, 
  rain 
  and 
  snow 
  have 
  descended 
  as 
  they 
  now 
  

   descend. 
  And 
  where, 
  too, 
  does 
  it 
  all 
  go, 
  this 
  world 
  average 
  ot 
  

   16,000,000 
  tons 
  a 
  second? 
  The 
  answers 
  are: 
  It 
  comes 
  from 
  the 
  soil 
  

   and 
  its 
  vegetation, 
  from 
  rivers, 
  lakes, 
  and 
  the 
  oceans 
  ; 
  and 
  to 
  them 
  it 
  

   returns 
  — 
  an 
  endless 
  cycle 
  of 
  evaporation 
  and 
  condensation. 
  

  

  " 
  The 
  inist 
  and 
  cloud 
  will 
  turn 
  to 
  rain, 
  

   The 
  rain 
  to 
  mist 
  and 
  cloud 
  again." 
  

  

  — 
  Longfellow. 
  

  

  EVAPORATION. 
  

  

  The 
  first 
  of 
  these 
  processes, 
  namely, 
  evaporation, 
  consists 
  in 
  the 
  

   change 
  of 
  water 
  (water 
  in 
  this 
  case, 
  though 
  many 
  substances 
  behave 
  

   similarly) 
  from 
  the 
  liquid, 
  or 
  even 
  solid, 
  state 
  to 
  that 
  of 
  an 
  invisible 
  

   gas, 
  in 
  which 
  condition 
  it 
  becomes 
  an 
  important, 
  though 
  always 
  rela- 
  

   tively 
  small, 
  part 
  of 
  the 
  air 
  we 
  breathe. 
  

  

  The 
  rate 
  of 
  this 
  evaporation 
  depends 
  on 
  a 
  number 
  of 
  things, 
  the 
  

   more 
  important 
  of 
  which 
  are 
  : 
  

  

  (a) 
  The 
  area 
  of 
  the 
  evaporating 
  surface. 
  The 
  larger 
  the 
  surface 
  

   the 
  more 
  rapid 
  the 
  total 
  evaporation. 
  It 
  is 
  in 
  recognition 
  of 
  this 
  

   law 
  that 
  we 
  spread 
  out 
  a 
  drop 
  or 
  even 
  a 
  puddle 
  of 
  water 
  to 
  hasten 
  

   its 
  disappearance. 
  

  

  1 
  Based 
  on 
  a 
  lecture 
  given 
  before 
  the 
  section 
  of 
  physics 
  and 
  chemistry 
  of 
  the 
  Franklin 
  

   Institute 
  on 
  Jan. 
  5, 
  1922. 
  Reprinted 
  by 
  permission 
  from 
  the 
  Journal 
  of 
  the 
  Franklin 
  

   Institute, 
  February-March, 
  1922. 
  

  

  187 
  

  

  