﻿FOGS 
  AND 
  CLOUDS 
  — 
  HUMPHREYS. 
  189 
  

  

  as 
  explained, 
  this 
  idea 
  is 
  wholly 
  wrong. 
  The 
  only 
  appreciable 
  effect 
  

   of 
  the 
  presence 
  of 
  the 
  other 
  gases 
  of 
  the 
  atmosphere 
  on 
  the 
  moisture 
  

   is 
  that 
  of 
  slowing 
  the 
  rate 
  of 
  its 
  spread 
  or 
  diffusion. 
  Temperature 
  

   and 
  temperature 
  alone, 
  to 
  within 
  a 
  negligible 
  amount, 
  determines 
  

   the 
  quantity 
  of 
  vapor 
  per 
  any 
  given 
  volume 
  necessary 
  to 
  produce 
  

   saturation, 
  or, 
  of 
  course, 
  any 
  definite 
  percentage 
  or 
  fraction 
  of 
  

   saturation. 
  

  

  CONDENSATION. 
  

  

  The 
  percentage 
  of 
  saturation 
  produced 
  by 
  a 
  given 
  amount 
  of 
  

   water 
  vapor 
  may, 
  therefore, 
  as 
  is 
  obvious 
  from 
  the 
  above 
  discussion, 
  

   be 
  varied 
  by 
  altering 
  the 
  volume 
  it 
  occupies 
  or 
  its 
  temperature, 
  or 
  

   both; 
  and 
  as 
  rapidly 
  as 
  saturation 
  tends 
  to 
  be 
  exceeded 
  moisture 
  

   condenses 
  out 
  onto 
  any 
  water 
  surface 
  or 
  solid 
  that 
  may 
  be 
  present. 
  

   Thus, 
  the 
  deposition 
  of 
  dew, 
  the 
  formation 
  of 
  hoarfrost, 
  and 
  the 
  

   sweating 
  of 
  ice 
  pitchers 
  all 
  are 
  examples 
  of 
  condensation 
  owing 
  to 
  

   passing 
  the 
  saturation 
  point 
  or 
  dew 
  point, 
  as 
  it 
  commonly 
  is 
  called. 
  

   In 
  these 
  cases 
  just 
  mentioned 
  the 
  temperature 
  of 
  the 
  water 
  vapor 
  

   and, 
  of 
  course, 
  of 
  the 
  other 
  constituents 
  of 
  the 
  atmosphere, 
  which, 
  

   however, 
  play 
  no 
  part 
  in 
  the 
  condensation, 
  is 
  lowered 
  through 
  con- 
  

   tact 
  with 
  cold 
  objects, 
  and 
  the 
  volume 
  of 
  this 
  vapor, 
  as 
  well 
  as 
  that 
  

   of 
  the 
  accompanying 
  chilled 
  gases, 
  decreased 
  — 
  decreased 
  because 
  in 
  

   the 
  open 
  air 
  the 
  pressure 
  remains 
  constant, 
  or 
  nearly 
  so, 
  whatever 
  the 
  

   degree 
  of 
  cooling. 
  

  

  Similarly, 
  whenever 
  the 
  temperature 
  of 
  the 
  open 
  air 
  passes 
  below 
  

   the 
  dew 
  point 
  condensation 
  occurs 
  in 
  the 
  form 
  of 
  innumerable 
  water 
  

   droplets 
  or 
  tiny 
  ice 
  crystals 
  throughout 
  the 
  chilled 
  volume, 
  and 
  

   thereby 
  produces 
  a 
  fog 
  or 
  a 
  cloud, 
  as 
  determined 
  by 
  location 
  — 
  a 
  fog 
  

   if 
  on 
  the 
  surface, 
  a 
  cloud 
  if 
  only 
  in 
  mid-air. 
  

  

  The 
  natural 
  processes 
  by 
  which 
  a 
  given 
  body 
  of 
  the 
  atmosphere 
  

   may 
  be 
  sufficiently 
  cooled 
  to 
  lose 
  a 
  portion 
  of 
  its 
  water 
  vapor 
  by 
  con- 
  

   densation 
  are: 
  (1) 
  Contact 
  with 
  objects 
  colder 
  than 
  itself; 
  (2) 
  mix- 
  

   ing 
  with 
  colder 
  air 
  ; 
  (3) 
  radiation; 
  (4) 
  expansion. 
  

  

  Condensation 
  as 
  a 
  result 
  of 
  contact 
  cooling 
  is 
  well 
  illustrated, 
  as 
  

   already 
  explained, 
  by 
  the 
  deposition 
  of 
  dew 
  — 
  the 
  bedewed 
  objects 
  

   having 
  been 
  cooled 
  by 
  radiation; 
  by 
  the 
  formation 
  of 
  hoarfrost, 
  

   which 
  occurs 
  under 
  the 
  same 
  conditions, 
  except 
  at 
  a 
  lower 
  tempera- 
  

   ture, 
  as 
  does 
  dew 
  ; 
  and 
  by 
  the 
  sweating, 
  during 
  warm 
  humid 
  weather, 
  

   of 
  all 
  cold 
  objects. 
  It 
  is 
  further 
  illustrated 
  by 
  the 
  formation 
  of 
  

   fog, 
  generally 
  light, 
  when 
  relatively 
  warm 
  humid 
  air 
  drifts 
  over 
  a 
  

   snow 
  bank 
  or 
  other 
  cold 
  surface. 
  

  

  The 
  second 
  of 
  the 
  above 
  processes 
  of 
  inducing 
  condensation 
  — 
  that 
  

   is, 
  the 
  mixing 
  of 
  masses 
  of 
  humid 
  air 
  of 
  different 
  temperatures 
  — 
  is 
  

   not 
  very 
  effective. 
  Indeed, 
  an 
  accurate 
  calculation, 
  based 
  on 
  the 
  

  

  