﻿FOGS 
  AND 
  CLOUDS 
  — 
  HUMPHREYS. 
  193 
  

  

  Fogs 
  also 
  frequently 
  occur 
  when 
  cold 
  air 
  slowly 
  flows 
  in 
  over 
  

   warm 
  water. 
  This 
  explains 
  the 
  " 
  frost 
  smokes 
  " 
  of 
  polar 
  seas, 
  and 
  

   the 
  " 
  steaming 
  " 
  of 
  rivers 
  and 
  lakes 
  on 
  cold 
  frosty 
  mornings. 
  In 
  

   these 
  cases 
  the 
  relatively 
  warm 
  water 
  goes 
  on 
  evaporating 
  into 
  the 
  

   cold 
  air 
  even 
  after 
  it 
  becomes 
  saturated 
  and 
  thereby 
  produces 
  a 
  

   water 
  droplet 
  or 
  a 
  minute 
  ice 
  crystal 
  about 
  every 
  one 
  of 
  the 
  myriad 
  

   millions 
  of 
  nuclei 
  present. 
  If, 
  however, 
  the 
  cold 
  air 
  comes 
  in 
  with 
  

   a 
  rush 
  — 
  that 
  is, 
  if 
  it 
  enters 
  as 
  a 
  strong 
  wind 
  — 
  no 
  fog 
  is 
  produced, 
  

   simply 
  because 
  the 
  vapor 
  is 
  distributed 
  by 
  the 
  accompanying 
  turbu- 
  

   lence 
  through 
  too 
  large 
  a 
  volume 
  to 
  produce 
  saturation. 
  

  

  kinds. 
  

  

  In 
  respect 
  to 
  the 
  ways 
  in 
  which 
  they 
  are 
  formed, 
  fogs 
  may 
  be 
  

   divided 
  into 
  two 
  classes 
  : 
  

  

  1. 
  Radiation 
  fog 
  (pi. 
  1, 
  fig. 
  1), 
  due 
  to 
  the 
  cooling 
  of 
  the 
  iower 
  

   air 
  below 
  the 
  dew 
  point, 
  partly 
  by 
  its 
  own 
  radiation 
  and 
  partly 
  by 
  

   contact 
  with 
  the 
  surface 
  which 
  itself 
  had 
  cooled 
  by 
  radiation. 
  This 
  

   type 
  of 
  fog 
  is 
  common, 
  as 
  already 
  explained, 
  along 
  streams 
  and 
  in 
  

   valleys 
  where, 
  through 
  the 
  summer 
  and 
  autumn, 
  it 
  is 
  apt 
  to 
  occur 
  

   on 
  any 
  calm, 
  clear 
  night. 
  

  

  2. 
  Advection 
  fog 
  (pi. 
  1, 
  fig. 
  2, 
  and 
  pi. 
  2, 
  fig. 
  1), 
  produced 
  by 
  the 
  

   advection 
  or 
  horizontal 
  movement 
  of 
  air 
  from 
  one 
  place 
  to 
  another 
  

   such 
  as 
  the 
  drifting 
  of 
  relatively 
  mild 
  air 
  from 
  the 
  ocean 
  inland 
  over 
  

   snow 
  banks, 
  or 
  from 
  a 
  warm 
  current 
  to 
  a 
  cold 
  one; 
  and 
  the 
  flow 
  of 
  

   frosty 
  air 
  over 
  open 
  water. 
  Furthermore, 
  any 
  fog 
  when 
  shifted 
  to 
  

   a 
  new 
  position 
  may 
  then 
  be 
  called 
  advection 
  fog. 
  Usually, 
  too, 
  the 
  

   shifted 
  fog, 
  like 
  that 
  so 
  common 
  on 
  many 
  leeward 
  coasts, 
  is 
  advective 
  

   also 
  in 
  origin. 
  

  

  There 
  also 
  are 
  several 
  other 
  classifications 
  of 
  fogs, 
  less 
  scientific 
  

   perhaps, 
  but 
  often 
  very 
  convenient. 
  Thus 
  we 
  speak 
  of 
  dry 
  fog, 
  mean- 
  

   ing 
  a 
  fog 
  which, 
  because 
  of 
  the 
  small 
  amount 
  of 
  water 
  content, 
  does 
  

   not 
  wet 
  our 
  clothing 
  — 
  evaporates 
  as 
  fast 
  as 
  caught 
  up 
  — 
  or 
  else 
  mean- 
  

   ing, 
  as 
  we 
  often 
  do, 
  a 
  haze 
  caused 
  by 
  a 
  forest 
  fire, 
  dust 
  storm, 
  or 
  vol- 
  

   canic 
  explosion 
  ; 
  wet 
  fog, 
  meaning 
  one 
  containing 
  so 
  much 
  water 
  that, 
  

   like 
  a 
  Scotch 
  mist, 
  it 
  makes 
  at 
  least 
  the 
  surface 
  of 
  one's 
  clothing 
  dis- 
  

   tinctly 
  damp; 
  sea 
  fog, 
  fog 
  originating 
  on 
  the 
  ocean, 
  whether 
  re- 
  

   maining 
  there 
  or 
  drifting 
  on 
  shore; 
  land 
  fog, 
  one 
  occurring 
  in 
  the 
  

   country 
  and 
  which, 
  as 
  its 
  nuclei 
  are 
  but 
  slightly 
  hygroscopic, 
  quickly 
  

   evaporates 
  ; 
  city 
  fog, 
  one 
  occurring 
  over 
  a 
  city, 
  especially 
  a 
  city 
  that 
  

   uses 
  a 
  large 
  amount 
  of 
  soft 
  coal 
  and 
  has 
  but 
  few 
  smoke 
  consumers, 
  

   generally 
  slow 
  to 
  evaporate, 
  owing 
  jointly, 
  presumably, 
  to 
  the 
  hy- 
  

   groscopic 
  nature 
  of 
  the 
  nuclei 
  and 
  to 
  the 
  oil 
  in 
  the 
  unburned 
  sooty 
  

   smoke 
  ; 
  black 
  fog, 
  one 
  containing 
  a 
  great 
  amount 
  of 
  soot, 
  such 
  as 
  occa- 
  

   sionally 
  forms 
  over 
  large, 
  smoky 
  cities; 
  pea-soup 
  fog, 
  a 
  local 
  name 
  

  

  