﻿FOGS 
  AND 
  CLOUDS 
  — 
  HUMPHREYS. 
  213 
  

  

  WHY 
  THE 
  BASE 
  OF 
  A 
  GIVEN 
  CLOUD, 
  OR 
  CLOUD 
  SYSTEM, 
  HAS 
  EVERYWHERE 
  

   NEARLY 
  THE 
  SAME 
  HEIGHT. 
  

  

  One 
  of 
  the 
  most 
  interesting 
  and 
  significant 
  things 
  learned 
  from 
  

   cloud 
  measurements 
  is 
  the 
  fact 
  that 
  the 
  base 
  of 
  any 
  cloud 
  sheet 
  — 
  that 
  

   is, 
  a 
  particular 
  sheet 
  at 
  a 
  particular 
  time, 
  whether 
  continuous 
  or 
  

   broken 
  and 
  scattered 
  — 
  is 
  everywhere 
  at 
  about 
  the 
  same 
  level. 
  The 
  

   reasons 
  for 
  this 
  are 
  very 
  simple: 
  (a) 
  Since 
  the 
  temperature 
  and 
  

   humidity 
  of 
  the 
  air 
  at 
  any 
  given 
  time 
  are 
  nearly 
  the 
  same 
  at 
  neigh- 
  

   boring 
  places 
  over 
  a 
  considerable 
  area, 
  the 
  height 
  throughout 
  such 
  

   region 
  is 
  approximately 
  constant 
  at 
  which 
  rising 
  air 
  will 
  have 
  cooled 
  

   to 
  the 
  dew 
  point 
  and 
  cloud 
  begun 
  to 
  form. 
  Hence 
  the 
  base 
  levels 
  

   of 
  a 
  series 
  of 
  detached 
  cumulus 
  clouds 
  are 
  about 
  the 
  same, 
  (b) 
  Since 
  

   rising 
  air 
  ascends 
  until 
  by 
  expansion 
  it 
  has 
  cooled 
  to 
  the 
  temperature 
  

   of 
  the 
  then 
  surrounding 
  air, 
  at 
  which 
  level 
  it 
  spreads 
  out 
  and 
  drifts 
  

   away 
  with 
  the 
  general 
  circulation, 
  and 
  as 
  convection, 
  whatever 
  its 
  

   cause, 
  applies 
  to 
  air 
  of 
  all 
  degrees 
  of 
  humidity, 
  and 
  occurs 
  at 
  differ- 
  

   ent 
  times 
  and 
  all 
  manner 
  of 
  places, 
  it 
  follows 
  that 
  the 
  atmosphere 
  is 
  

   always 
  more 
  or 
  less 
  stratified 
  in 
  respect 
  to 
  its 
  water 
  vapor. 
  In 
  

   general, 
  therefore, 
  clouds 
  must 
  also 
  form 
  in 
  these 
  layers. 
  There 
  

   may 
  be, 
  and 
  often 
  are, 
  two 
  or 
  more 
  cloud 
  layers 
  at 
  the 
  same 
  time 
  

   at 
  different 
  levels, 
  but 
  a 
  cloud 
  filling 
  the 
  whole 
  depth 
  of 
  the 
  

   atmosphere, 
  or 
  even 
  a 
  layer 
  5 
  or 
  6 
  miles 
  thick, 
  is 
  not 
  to 
  be 
  expected, 
  

   nor 
  is 
  any 
  such 
  layer 
  of 
  considerable 
  extent 
  often, 
  if 
  ever, 
  formed. 
  

  

  LEVELS 
  OF 
  MAXIMUM 
  CLOUDINESS. 
  

  

  Since 
  several 
  types 
  of 
  clouds 
  occur 
  at 
  various 
  elevations, 
  and 
  since 
  

   no 
  level, 
  from 
  the 
  surface 
  of 
  the 
  earth 
  up 
  to 
  the 
  highest 
  cirrus, 
  is 
  

   free 
  from 
  condensation, 
  it 
  might 
  seem 
  that 
  clouds 
  are 
  as 
  likely 
  to 
  

   have 
  one 
  height 
  as 
  another 
  throughout 
  their 
  possible 
  range. 
  How- 
  

   ever, 
  it 
  is 
  certain 
  that 
  there 
  are 
  levels 
  of 
  maximum 
  and 
  minimum 
  

   cloud 
  frequency, 
  even 
  though 
  it 
  may 
  not 
  be 
  as 
  easy 
  to 
  prove 
  some 
  

   of 
  them 
  by 
  direct 
  observation 
  owing 
  to 
  the 
  prevalence 
  of 
  lower 
  

   clouds. 
  The 
  more 
  important 
  levels 
  of 
  maximum 
  cloudiness 
  are 
  : 
  

  

  Fog 
  level. 
  — 
  As 
  every 
  one 
  knows, 
  a 
  fog, 
  whatever 
  its 
  depth, 
  from 
  

   a 
  mere 
  gauzy 
  veil 
  to 
  the 
  deepest 
  and 
  densest 
  layer, 
  is 
  a 
  cloud 
  on 
  the 
  

   earth. 
  Obviously, 
  therefore, 
  the 
  surface 
  of 
  the 
  earth 
  itself 
  is 
  a 
  level 
  

   of 
  maximum 
  cloudiness. 
  

  

  Cumulus 
  level. 
  — 
  Since 
  evaporation 
  tends 
  all 
  the 
  time 
  to 
  saturate 
  

   the 
  atmosphere, 
  and 
  since 
  humidity 
  is 
  held 
  down 
  by 
  convectional 
  

   condensation, 
  and, 
  further, 
  since, 
  over 
  extensive 
  areas, 
  convection 
  

   is 
  roughly 
  the 
  same 
  most 
  of 
  the 
  time 
  through 
  an 
  entire 
  season, 
  it 
  

   follows 
  that 
  the 
  ordinary 
  rain 
  cloud, 
  and 
  the 
  common 
  cumulus 
  cloud, 
  

   must 
  have 
  a 
  roughly 
  standard 
  base 
  level 
  at 
  a 
  moderate 
  elevation. 
  

   55379—24 
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