﻿FOGS 
  AND 
  CLOUDS 
  — 
  HUMPHREYS. 
  191 
  

  

  the 
  lowering 
  of 
  the 
  temperature, 
  and 
  that 
  convectional 
  expansion, 
  

   therefore, 
  whether 
  incident 
  to 
  the 
  blowing 
  of 
  wind 
  up 
  and 
  over 
  

   mountains, 
  or 
  to 
  local 
  heating, 
  is 
  a 
  most 
  effective 
  means 
  of 
  inducing 
  

   condensation 
  and 
  the 
  formation 
  of 
  clouds. 
  

  

  CONDENSATION 
  NUCLEI. 
  

  

  Whenever 
  ordinary 
  air, 
  kept 
  humid 
  by 
  the 
  presence 
  of 
  water, 
  

   is 
  suddenly 
  expanded 
  in 
  a 
  closed 
  vessel, 
  it 
  instantly 
  becomes 
  filled 
  

   throughout 
  with 
  a 
  miniature 
  cloud, 
  precisely 
  as 
  occurs 
  on 
  an 
  incom- 
  

   parably 
  larger 
  scale 
  in 
  nature. 
  Subsequent 
  expansions 
  of 
  the 
  same 
  

   air, 
  otherwise 
  undisturbed, 
  induce 
  less 
  and 
  less 
  cloud, 
  and 
  presently 
  

   none 
  at 
  all. 
  If 
  filtered 
  air 
  — 
  that 
  is, 
  air 
  drawn 
  through 
  several 
  inches 
  

   of 
  cotton 
  wool, 
  or 
  other 
  substance 
  of 
  similar 
  texture 
  — 
  is 
  used, 
  con- 
  

   densation 
  by 
  moderate 
  expansion 
  is 
  impossible 
  from 
  the 
  first. 
  The 
  

   admission, 
  however, 
  of 
  a 
  little 
  smoke 
  restores 
  to 
  the 
  exhausted 
  air, 
  

   and 
  endows 
  the 
  filtered 
  air 
  with, 
  full 
  powers 
  of 
  condensation. 
  There 
  

   are, 
  therefore, 
  condensation 
  nuclei 
  in 
  the 
  atmosphere 
  — 
  hundreds 
  and 
  

   often 
  thousands 
  of 
  them 
  per 
  cubic 
  inch 
  — 
  which 
  can 
  be 
  filtered 
  out; 
  

   and 
  microscopic 
  examination 
  shows 
  that 
  they 
  consist 
  essentially 
  of 
  

   dust 
  particles. 
  Hence, 
  dust, 
  moisture, 
  and 
  some 
  cooling 
  process 
  are 
  

   the 
  three 
  essential 
  factors 
  in 
  all 
  natural 
  fog 
  and 
  cloud 
  formation. 
  

  

  It 
  is 
  true 
  that 
  a 
  few 
  substances 
  other 
  than 
  dust, 
  such 
  as 
  the 
  

   oxides 
  of 
  nitrogen, 
  act 
  as 
  condensation 
  nuclei, 
  but 
  they 
  seem 
  gen- 
  

   erally 
  to 
  be 
  negligible 
  in 
  quantity. 
  Furthermore, 
  condensation 
  can 
  

   be 
  obtained 
  in 
  air 
  wholly 
  free 
  from 
  any 
  such 
  nuclei 
  provided 
  it 
  is 
  

   ionized 
  and 
  forced 
  to 
  at 
  least 
  a 
  fourfold 
  supersaturation, 
  a 
  degree 
  

   of 
  humidity 
  that 
  probably 
  never 
  occurs 
  in 
  nature. 
  Indeed, 
  under 
  

   very 
  great 
  supersaturation, 
  eight 
  or 
  nine 
  fold, 
  condensation 
  occurs 
  

   even 
  in 
  perfectly 
  clean 
  nonionized 
  air. 
  But 
  this, 
  too, 
  is 
  only 
  a 
  lab- 
  

   oratory 
  experiment, 
  and 
  not 
  a 
  process 
  by 
  which 
  clouds 
  are 
  formed 
  

   in 
  nature. 
  

  

  i 
  FOGS. 
  

  

  DISTINCTION 
  BETWEEN 
  FOG 
  AND 
  CLOUD. 
  

  

  As 
  already 
  explained, 
  whenever 
  the 
  air 
  is 
  cooled, 
  by 
  any 
  means 
  

   whatever, 
  below 
  its 
  dew 
  point, 
  a 
  portion 
  of 
  the 
  water 
  vapor 
  present 
  

   separates 
  out 
  on 
  such 
  dust 
  particles 
  or 
  other 
  condensation 
  nuclei 
  

   as 
  happen 
  to 
  be 
  present. 
  If 
  this 
  process 
  occurs 
  only 
  at 
  a 
  con- 
  

   siderable 
  distance 
  above 
  the 
  surface 
  of 
  the 
  earth, 
  leaving 
  the 
  lower 
  

   air 
  clear, 
  the 
  result 
  is 
  some 
  form 
  of 
  cloud. 
  If, 
  on 
  the 
  other 
  hand, 
  

   it 
  extends 
  quite 
  to, 
  or 
  occurs 
  at, 
  the 
  surface 
  of 
  the 
  earth 
  it 
  is 
  then 
  

   called 
  a 
  fog, 
  no 
  matter 
  how 
  shallow 
  nor 
  how 
  deep 
  it 
  may 
  be. 
  The 
  

   distinction, 
  therefore, 
  between 
  fog 
  and 
  cloud 
  is 
  that 
  of 
  position. 
  

   Fog 
  is 
  a 
  cloud 
  on 
  the 
  earth 
  ; 
  cloud, 
  a 
  fog 
  in 
  the 
  sky. 
  

  

  