﻿192 
  ANNUAL 
  REPOKT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  In 
  some 
  cases 
  the 
  only 
  basis 
  of 
  distinction 
  between 
  fog 
  and 
  cloud 
  

   is 
  that 
  of 
  viewpoint. 
  For 
  example, 
  the 
  mist 
  that 
  sometimes 
  covers 
  

   only 
  the 
  crest 
  of 
  a 
  mountain 
  generally 
  is 
  called 
  fog 
  by 
  those 
  actually 
  

   in 
  it, 
  and 
  cloud, 
  at 
  the 
  same 
  time, 
  by 
  those 
  in 
  the 
  valley 
  below. 
  

  

  WHERE 
  AND 
  HOW 
  FORMED. 
  

  

  Fogs 
  are 
  likely 
  to 
  form 
  along 
  rivers 
  and 
  large 
  creeks 
  and 
  in 
  the 
  

   mountain 
  valleys 
  of 
  all 
  but 
  arid 
  regions 
  in 
  the 
  latter 
  part 
  of 
  any 
  

   still 
  cloudless 
  night 
  of 
  summer 
  or 
  autumn. 
  During 
  these 
  seasons 
  

   the 
  streams 
  and 
  the 
  humid 
  soil 
  are 
  warm, 
  especially 
  when 
  exposed 
  

   to 
  sunshine, 
  and 
  hence 
  evaporate 
  much 
  moisture 
  into 
  the 
  lower 
  

   atmosphere, 
  where, 
  in 
  great 
  measure, 
  it 
  remains 
  when 
  there 
  are 
  no 
  

   winds 
  to 
  blow 
  it 
  away. 
  

  

  Throughout 
  the 
  night, 
  however, 
  the 
  surface 
  of 
  the 
  soil 
  and 
  the 
  

   adjacent 
  humid 
  air, 
  by 
  virtue 
  of 
  its 
  humidity, 
  lose 
  heat 
  rapidly 
  by 
  

   radiation 
  to 
  the 
  colder 
  atmosphere 
  above 
  and 
  to 
  the 
  sky, 
  or 
  empty 
  

   space, 
  beyond. 
  This 
  loss 
  of 
  heat 
  by 
  radiation 
  is 
  no 
  greater, 
  of 
  

   course, 
  by 
  night 
  than 
  in 
  the 
  daytime, 
  but 
  when 
  there 
  is 
  no 
  sunshine 
  

   to 
  make 
  good 
  such 
  loss, 
  or 
  do 
  better 
  — 
  as 
  generally 
  is 
  the 
  case 
  through 
  

   the 
  forenoon 
  — 
  the 
  inevitable 
  consequence 
  is 
  a 
  lowering 
  of 
  the 
  tem- 
  

   perature. 
  Hence, 
  during 
  calm, 
  clear 
  nights 
  the 
  temperature 
  of 
  the 
  

   humid 
  surface 
  air 
  often 
  falls 
  below 
  the 
  dew 
  point 
  and 
  a 
  fog 
  of 
  cor- 
  

   responding 
  depth 
  and 
  density 
  is 
  formed. 
  

  

  If 
  the 
  sky 
  is 
  overcast 
  there 
  commonly 
  is 
  enough 
  radiation 
  from 
  

   the 
  clouds 
  back 
  to 
  the 
  earth, 
  especially 
  if 
  they 
  happen 
  to 
  be 
  low 
  

   (hence 
  warm) 
  , 
  to 
  prevent 
  the 
  cooling 
  of 
  the 
  surface 
  air 
  to 
  the 
  dew 
  

   point 
  and 
  the 
  consequent 
  formation 
  of 
  fog. 
  Neither 
  does 
  fog 
  form 
  

   when 
  there 
  is 
  considerable 
  wind, 
  partly 
  because 
  the 
  more 
  humid 
  

   lower 
  air 
  is 
  then 
  mixed 
  with 
  the 
  drier 
  upper 
  air 
  and 
  the 
  surface 
  

   dew 
  point 
  thereby 
  lowered, 
  and 
  partly 
  because 
  this 
  mixing 
  prevents 
  

   much 
  fall 
  in 
  the 
  surface 
  temperature 
  by 
  distributing 
  the 
  loss 
  of 
  

   heat 
  through 
  a 
  relatively 
  large 
  amount 
  of 
  air 
  instead 
  of 
  leaving 
  it 
  

   confined 
  essentially 
  to 
  that 
  near 
  the 
  ground. 
  Hence 
  fogs 
  of 
  the 
  

   kind 
  under 
  consideration 
  — 
  radiation 
  fogs, 
  summer 
  fogs, 
  land 
  fogs, 
  

   valley 
  fogs 
  — 
  seldom 
  occur 
  either 
  when 
  the 
  sky 
  is 
  cloudy 
  or 
  the 
  night 
  

   windy. 
  

  

  Another 
  source 
  of 
  numerous 
  fogs 
  is 
  the 
  drifting 
  of 
  relatively 
  

   warm 
  humid 
  air 
  over 
  places 
  much 
  colder, 
  such 
  as 
  the 
  drifting 
  of 
  

   on-shore 
  winds 
  over 
  snow 
  banks. 
  In 
  this 
  way 
  the 
  humid 
  air 
  

   frequently 
  is 
  cooled 
  below 
  its 
  dew 
  point 
  and 
  fog 
  — 
  "winter 
  fog" 
  

   or 
  "sea 
  fog" 
  — 
  produced. 
  Likewise, 
  heavy 
  fogs 
  often 
  are 
  formed 
  

   when 
  the 
  wind 
  is 
  from 
  warm 
  to 
  cold 
  water 
  — 
  from 
  the 
  Gulf 
  Stream 
  

   to 
  the 
  Labrador 
  Current, 
  for 
  instance. 
  

  

  