﻿190 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  temperature 
  and 
  volume 
  changes 
  and 
  other 
  factors 
  of 
  the 
  problem, 
  

   shows 
  that 
  while 
  a 
  fog 
  or 
  cloud 
  may 
  be 
  obtained 
  by 
  this 
  process 
  no 
  

   appreciable 
  amount 
  of 
  rain 
  or 
  snow 
  is 
  likely 
  to 
  result. 
  Layer 
  or 
  

   stratus 
  clouds 
  at 
  the 
  boundary 
  between 
  relatively 
  warm 
  air 
  above 
  

   and 
  cold 
  beneath, 
  a 
  phenomenon 
  of 
  occasional 
  occurrence, 
  are 
  at 
  

   least 
  partially 
  due 
  to 
  mixing, 
  as 
  are 
  also 
  the 
  fogs 
  that 
  so 
  frequently 
  

   occur 
  over 
  adjacent 
  warm 
  and 
  cold 
  ocean 
  currents. 
  

  

  The 
  third 
  process 
  by 
  which 
  air 
  loses 
  heat, 
  namely, 
  by 
  radiation, 
  

   probably 
  is 
  of 
  little 
  importance 
  in 
  the 
  production 
  of 
  clouds, 
  since 
  

   the 
  most 
  chilled 
  portions 
  of 
  free 
  air 
  sink 
  to 
  lower 
  levels 
  and 
  thereby 
  

   become 
  even 
  warmer 
  and 
  drier 
  than 
  they 
  were 
  at 
  first. 
  Near 
  the 
  

   surface, 
  however, 
  where 
  descent 
  and 
  the 
  consequent 
  dynamical 
  

   warming, 
  as 
  it 
  is 
  called, 
  are 
  impossible, 
  radiation 
  often 
  is 
  very 
  effec- 
  

   tive 
  in 
  the 
  production 
  of 
  fog. 
  

  

  The 
  last 
  of 
  the 
  cooling 
  processes 
  mentioned 
  above 
  — 
  that 
  is, 
  expan- 
  

   sion 
  — 
  is 
  exceedingly 
  effective, 
  and 
  to 
  it 
  is 
  due 
  the 
  great 
  bulk 
  of 
  cloud 
  

   formation. 
  Now, 
  expansion 
  of 
  the 
  air 
  may 
  be 
  produced 
  either 
  by 
  

   heating 
  it 
  or 
  by 
  reducing 
  the 
  pressure 
  to 
  which 
  it 
  is 
  subjected. 
  It 
  

   is 
  the 
  latter 
  process, 
  obviously, 
  and 
  not 
  the 
  former, 
  even 
  though 
  

   heating 
  generally 
  is 
  the 
  initial 
  cause, 
  that 
  produces 
  the 
  cooling 
  of 
  

   free 
  air 
  and 
  the 
  formation 
  of 
  cloud. 
  To 
  make 
  these 
  points 
  clear, 
  

   consider 
  the 
  results 
  when 
  a 
  given 
  mass 
  of 
  humid 
  air 
  is 
  heated, 
  as 
  it 
  

   may 
  be, 
  over 
  a 
  region 
  warmed 
  by 
  sunshine 
  or 
  otherwise. 
  With 
  

   increase 
  of 
  temperature 
  it 
  expands 
  and 
  thereby 
  becomes 
  lighter, 
  

   volume 
  for 
  volume, 
  than 
  the 
  surrounding 
  cooler 
  air. 
  The 
  surround- 
  

   ing 
  denser 
  air 
  then 
  underruns 
  the 
  lighter 
  and 
  lifts 
  it 
  up 
  to 
  higher 
  

   levels, 
  just 
  as 
  hot 
  air 
  is 
  driven 
  up 
  a 
  chimney. 
  As 
  the 
  warmed 
  air 
  

   is 
  thus 
  forced 
  up 
  (rises, 
  as 
  we 
  generally 
  say) 
  the 
  pressure 
  to 
  which 
  

   it 
  is 
  subjected 
  obviously 
  decreases 
  in 
  proportion 
  to 
  the 
  weight 
  of 
  the 
  

   air 
  left 
  below. 
  It 
  therefore 
  gradually 
  expands 
  as 
  it 
  rises 
  and 
  

   thereby 
  does 
  work, 
  and 
  since 
  the 
  only 
  energy 
  available 
  for 
  this 
  work 
  is 
  

   the 
  heat 
  of 
  the 
  ascending 
  air, 
  it 
  follows 
  that 
  as 
  its 
  height 
  increases 
  

   its 
  temperature 
  must 
  correspondingly 
  decrease. 
  As 
  a 
  matter 
  of 
  

   fact, 
  for 
  air 
  of 
  average 
  humidity, 
  the 
  rate 
  of 
  this 
  decrease 
  is 
  ap- 
  

   proximately 
  1° 
  F. 
  per 
  187 
  feet 
  increase 
  of 
  elevation 
  up 
  to 
  the 
  base 
  of 
  

   the 
  cloud, 
  if 
  there 
  be 
  one, 
  in 
  the 
  rising 
  air, 
  and 
  then 
  much 
  less 
  

   through 
  the 
  cloud. 
  In 
  any 
  event 
  the 
  ascending 
  air 
  attains 
  equi- 
  

   librium 
  only 
  when 
  it 
  has 
  cooled 
  by 
  expansion 
  to 
  the 
  temperature 
  

   of 
  the 
  air 
  that 
  finally 
  surrounds 
  it. 
  Hence, 
  when 
  it 
  comes 
  to 
  rest, 
  it 
  

   is 
  colder, 
  often 
  much 
  colder, 
  than 
  it 
  was 
  before 
  it 
  was 
  heated. 
  

  

  Of 
  course 
  the 
  expansion 
  of 
  the 
  rising 
  air 
  correspondingly 
  increases 
  

   the 
  vapor 
  capacity, 
  but 
  it 
  is 
  easy 
  to 
  show, 
  both 
  theoretically 
  and 
  

   experimentally, 
  that 
  this 
  increase 
  of 
  vapor 
  capacity 
  by 
  volume 
  ex- 
  

   pansion 
  is 
  small 
  in 
  comparison 
  with 
  its 
  simultaneous 
  decrease 
  through 
  

  

  