﻿216 
  ANNUAL 
  EEPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  bus 
  cloud, 
  from 
  which 
  rain 
  often 
  falls 
  all 
  day, 
  varies 
  in 
  thickness 
  

   from, 
  say, 
  500 
  feet, 
  up 
  to 
  4 
  or 
  5 
  miles. 
  Its 
  average 
  thickness, 
  how- 
  

   ever, 
  appears 
  to 
  be, 
  roughly, 
  half 
  a 
  mile. 
  All 
  other 
  types 
  of 
  clouds 
  

   generally 
  are 
  intermediate 
  in 
  thickness 
  between 
  the 
  cirrus 
  and 
  the 
  

   nimbus, 
  averaging, 
  perhaps, 
  500 
  to 
  1,500 
  feet. 
  

  

  CLOUD 
  VELOCITY. 
  

  

  The 
  direction 
  and 
  speed 
  of 
  travel 
  of 
  clouds 
  can 
  be 
  determined 
  

   in 
  several 
  ways, 
  most 
  of 
  which 
  involve 
  triangulation 
  of 
  the 
  kind 
  

   used 
  by 
  surveyors 
  and 
  geologists. 
  In 
  the 
  great 
  majority 
  of 
  cases 
  

   the 
  velocity 
  of 
  a 
  cloud 
  is 
  that 
  of 
  the 
  air 
  in 
  which 
  it 
  happens 
  to 
  be. 
  

   In 
  a 
  few 
  cases, 
  however, 
  the 
  movement 
  of 
  the 
  cloud 
  is 
  not 
  that 
  of 
  

   its 
  enveloping 
  atmosphere. 
  Thus 
  the 
  cloud 
  that 
  forms 
  along 
  the 
  

   crest 
  of 
  a 
  mountain 
  is 
  as 
  stationary 
  as 
  a 
  waterfall, 
  and, 
  like 
  the 
  

   waterfall, 
  is 
  continually 
  renewed 
  by 
  fresh 
  material. 
  No 
  matter 
  

   how 
  swift 
  the 
  current, 
  the 
  fall 
  remains 
  fixed, 
  and 
  however 
  strong 
  

   the 
  wind 
  over 
  the 
  mountain, 
  cloud 
  is 
  formed 
  in 
  it 
  as 
  soon 
  as 
  it 
  

   reaches 
  a 
  particular 
  altitude, 
  determined 
  by 
  the 
  humidity 
  and 
  tem- 
  

   perature, 
  and 
  evaporated 
  as 
  it 
  is 
  drifted 
  beyond 
  and 
  to 
  lower 
  levels. 
  

   The 
  crest 
  cloud, 
  therefore, 
  being 
  always 
  stationary, 
  can 
  not 
  be 
  used 
  

   in 
  measuring 
  wind 
  velocity. 
  Neither 
  can 
  the 
  riffle 
  cloud, 
  the 
  lenticu- 
  

   lar 
  cloud, 
  nor 
  the 
  banner 
  cloud, 
  and 
  for 
  the 
  same 
  reason 
  — 
  all 
  except 
  

   certain 
  kinds 
  of 
  the 
  lenticular, 
  are 
  stationary 
  whatever 
  the 
  wind 
  

   velocity. 
  

  

  Neither 
  can 
  the 
  velocity 
  of 
  the 
  wind 
  be 
  determined 
  by 
  the 
  move- 
  

   ment 
  of 
  the 
  parallel 
  rolls 
  of 
  the 
  windrow 
  or 
  billow 
  cloud. 
  These 
  

   clouds 
  rest 
  on 
  the 
  crests 
  of 
  air 
  waves 
  which 
  are 
  caused 
  by 
  the 
  flow 
  

   of 
  one 
  wind 
  sheet 
  over 
  another. 
  The 
  velocity 
  of 
  these 
  waves, 
  and, 
  

   hence, 
  of 
  the 
  clouds 
  that 
  crest 
  them, 
  is 
  intermediate 
  between 
  that 
  

   of 
  the 
  two 
  sheets 
  and 
  therefore 
  does 
  not 
  measure 
  the 
  actual 
  velocity 
  

   of 
  either. 
  

  

  In 
  the 
  great 
  majority 
  of 
  cases, 
  though 
  — 
  that 
  is, 
  with 
  the 
  above 
  

   exceptions 
  — 
  clouds 
  do 
  move 
  strictly 
  with 
  the 
  enveloping 
  air. 
  Hence, 
  

   though 
  everywhere 
  blowing, 
  in 
  the 
  course 
  of 
  time, 
  from 
  all 
  direc- 
  

   tions 
  — 
  " 
  boxing 
  the 
  compass," 
  as 
  the 
  mariner 
  says 
  — 
  in 
  the 
  Tropics 
  

   they 
  are 
  nearly 
  always 
  from 
  easterly 
  points, 
  northeast 
  to 
  southeast, 
  

   and 
  in 
  middle 
  latitudes 
  prevailingly 
  from 
  westerly 
  regions. 
  

  

  Their 
  average 
  velocity 
  when 
  moving 
  in 
  their 
  prevailing 
  direction 
  

   increases 
  with 
  height 
  at 
  approximately 
  the 
  same 
  proportion 
  that 
  

   the 
  density 
  of 
  the 
  atmosphere 
  decreases 
  — 
  that 
  is, 
  at 
  such 
  a 
  rate 
  that 
  

   the 
  product 
  of 
  the 
  velocity 
  (cloud 
  or 
  wind) 
  by 
  the 
  density 
  of 
  the 
  air 
  

   is 
  a 
  constant. 
  

  

  