﻿FOGS 
  AND 
  CLOUDS 
  — 
  HUMPHREYS. 
  211 
  

  

  simple 
  manner, 
  as 
  is 
  also 
  obvious 
  from 
  the 
  fact 
  that 
  a 
  cloud 
  may 
  last 
  

   for 
  many 
  hours 
  without 
  giving 
  any 
  rain 
  whatever. 
  

   The 
  actual 
  processes 
  in 
  the 
  formation 
  of 
  rain 
  seem 
  to 
  be: 
  

  

  1. 
  For 
  some 
  reason, 
  such 
  as 
  surface 
  heating, 
  a 
  mountain 
  in 
  the 
  

   wind's 
  path, 
  or 
  convergence 
  of 
  different 
  currents, 
  the 
  surface 
  air 
  is 
  

   forced 
  up 
  to 
  considerable 
  heights 
  ; 
  during 
  which 
  rise 
  it 
  does 
  work 
  — 
  

   gives 
  up 
  energy 
  — 
  by 
  expansion 
  against 
  the 
  surrounding 
  pressure 
  

   and 
  thereby 
  cools. 
  

  

  2. 
  As 
  soon 
  as 
  the 
  dew 
  point 
  is 
  passed 
  condensation 
  begins 
  on 
  the 
  

   innumerable 
  nuclei 
  present 
  and 
  a 
  cloud 
  is 
  formed, 
  the 
  particles 
  of 
  

   which, 
  being 
  heavier 
  than 
  equal 
  volumes 
  of 
  air, 
  slowly 
  fall 
  with 
  

   reference 
  to 
  the 
  atmosphere 
  itself. 
  That 
  is, 
  the 
  rising 
  current 
  passes 
  

   by 
  the 
  cloud 
  particles 
  to 
  a 
  greater 
  or 
  less 
  extent 
  however 
  high 
  they 
  

   may 
  be 
  carried. 
  

  

  3. 
  The 
  lower 
  cloud 
  particles 
  filter 
  the 
  air 
  rising 
  through 
  them 
  and 
  

   thereby 
  more 
  or 
  less 
  clean 
  it 
  of 
  dust 
  motes 
  and 
  other 
  nuclei. 
  Hence 
  

   the 
  droplets 
  formed 
  in 
  the 
  rising 
  air 
  after 
  this 
  filtration 
  grow 
  much 
  

   faster, 
  being 
  relatively 
  few, 
  than 
  they 
  otherwise 
  would. 
  

  

  4. 
  Presently 
  many 
  of 
  the 
  larger 
  droplets 
  coalesce 
  and 
  thus 
  be- 
  

   come 
  heavy 
  enough 
  to 
  fall 
  against 
  the 
  rising 
  current. 
  Nor, 
  indeed, 
  

   can 
  they 
  fall 
  (reach 
  lower 
  levels) 
  until 
  by 
  condensation, 
  or 
  coa- 
  

   lescence, 
  or 
  both, 
  they 
  have 
  attained 
  a 
  certain 
  minimum 
  size 
  deter- 
  

   mined 
  by 
  the 
  vertical 
  velocity 
  of 
  the 
  air 
  in 
  which 
  they 
  happen 
  to 
  be. 
  

  

  In 
  short, 
  a 
  rising 
  current, 
  essential 
  to 
  any 
  considerable 
  condensa- 
  

   tion, 
  that 
  sustains, 
  or 
  even 
  carries 
  higher, 
  cloud 
  droplets 
  until 
  they 
  

   have 
  grown 
  to 
  falling 
  size, 
  and 
  the 
  automatic 
  filtering 
  of 
  the 
  ascend- 
  

   ing 
  air 
  by 
  the 
  cloud 
  formed 
  in 
  it 
  (which 
  filtering 
  restricts 
  further 
  

   condensation 
  to 
  comparatively 
  few 
  particles 
  and 
  thus 
  insures 
  their 
  

   rapid 
  growth), 
  appear 
  to 
  be 
  necessary 
  and 
  sufficient 
  to 
  account 
  for 
  

   the 
  formation 
  of 
  rain. 
  

  

  5. 
  Most 
  of 
  the 
  drops, 
  as 
  they 
  emerge 
  from 
  a 
  cloud, 
  are 
  likely 
  to 
  

   have 
  substantially 
  the 
  same 
  size, 
  namely, 
  that 
  which 
  is 
  just 
  sufficient 
  

   to 
  overcome 
  the 
  upward 
  movement 
  of 
  the 
  air 
  in 
  which 
  they 
  were 
  

   formed. 
  Now, 
  drops 
  of 
  the 
  same 
  size 
  fall 
  with 
  the 
  same 
  speed, 
  

   hence 
  any 
  two 
  that 
  happen 
  to 
  be 
  close 
  together 
  are 
  likely 
  to 
  remain 
  

   so 
  much 
  longer 
  than 
  drops 
  of 
  unequal 
  size 
  and 
  thereby 
  have 
  more 
  

   chances 
  of 
  union 
  through 
  fortuitous 
  disturbances. 
  Furthermore, 
  

   when 
  falling 
  drops 
  are 
  side 
  by 
  side 
  the 
  air 
  tends 
  to 
  push 
  them 
  

   together 
  just 
  as 
  passing 
  boats 
  are 
  forced 
  toward 
  each 
  other. 
  Clearly, 
  

   though, 
  this 
  pressure 
  has 
  time 
  to 
  bring 
  closely 
  neighboring 
  drops 
  

   into 
  actual 
  contact 
  only 
  when 
  they 
  fall 
  with 
  the 
  same 
  or 
  very 
  nearly 
  

   the 
  same 
  speed. 
  From 
  these 
  considerations 
  it 
  seems 
  that 
  the 
  smallest 
  

   drops, 
  size 
  1, 
  say, 
  should 
  unite 
  to 
  form 
  size 
  2, 
  and 
  size 
  2 
  unite 
  with 
  

   each 
  other 
  to 
  form 
  size 
  4, 
  rather 
  than 
  with 
  size 
  1 
  to 
  form 
  size 
  3, 
  

  

  