﻿FOGS 
  AND 
  CLOUDS 
  HUMPHREYS. 
  201 
  

  

  Strato-cumulus 
  may 
  be 
  distinguished 
  from 
  nimbus 
  by 
  its 
  lumpy 
  or 
  

   rolling 
  appearance, 
  and 
  by 
  the 
  fact 
  that 
  it 
  does 
  not 
  generally 
  tend 
  

   to 
  bring 
  rain." 
  (PI. 
  10, 
  figs. 
  1 
  and 
  2.) 
  

  

  This 
  cloud, 
  of 
  which 
  there 
  are 
  several 
  forms 
  ranging 
  from 
  the 
  

   stratus 
  of 
  uneven 
  density 
  through 
  the 
  great 
  parallel 
  rolls 
  to 
  the 
  

   sheet 
  of 
  well-nigh 
  discrete 
  cumuli, 
  is 
  always 
  due 
  to 
  vertical 
  con- 
  

   vection. 
  The 
  more 
  nearly 
  discrete 
  or 
  separate 
  masses 
  are 
  produced 
  

   by 
  the 
  rising 
  of 
  warm 
  air 
  (thermal 
  convection), 
  while 
  the 
  irregu- 
  

   larities 
  in 
  the 
  stratus 
  form 
  appear 
  to 
  be 
  caused, 
  in 
  large 
  part 
  at 
  

   least, 
  by 
  mechanical 
  turbulence. 
  This 
  last 
  type 
  (pi. 
  10, 
  fig. 
  2), 
  

   might 
  well 
  be 
  called 
  turbulo-cumulus. 
  The 
  shallow 
  depth 
  and 
  

   broad 
  expanse 
  of 
  the 
  strato-cumulus 
  often 
  is 
  due 
  to 
  an 
  overlying 
  

   layer 
  of 
  air 
  of 
  such 
  temperature 
  that 
  the 
  rising, 
  cloud-laden 
  column 
  

   can 
  not 
  pass 
  through 
  it, 
  and 
  hence 
  is 
  forced 
  to 
  spread 
  out, 
  much 
  as 
  

   rising 
  smoke 
  in 
  a 
  room 
  spreads 
  out 
  under 
  the 
  ceiling. 
  

  

  7. 
  Nimbus 
  (Nb.). 
  — 
  "A 
  dense 
  layer 
  of 
  dark, 
  shapeless 
  cloud 
  with 
  

   ragged 
  edges 
  from, 
  which 
  steady 
  rain 
  or 
  snow 
  usually 
  falls. 
  If 
  

   there 
  are 
  openings 
  in 
  the 
  cloud 
  an 
  upper 
  layer 
  of 
  cirro-stratus 
  

   or 
  alto-stratus 
  may 
  almost 
  invariably 
  be 
  seen 
  through 
  them. 
  If 
  a 
  

   layer 
  of 
  nimbus 
  separates 
  in 
  strong 
  wind 
  into 
  ragged 
  cloud, 
  or 
  if 
  

   small 
  detached 
  clouds 
  are 
  seen 
  drifting 
  underneath 
  a 
  large 
  nimbus 
  

   (the 
  • 
  Scud 
  ' 
  of 
  sailors) 
  , 
  either 
  may 
  be 
  specified 
  as 
  fracto-niinbus 
  

   (Fr.-Nb.)" 
  (PL 
  11, 
  fig. 
  1.) 
  

  

  The 
  name 
  of 
  this 
  cloud 
  has 
  evoked 
  much 
  discussion. 
  Nimbus, 
  

   originally 
  meaning 
  cloud 
  and, 
  inferentially, 
  storm, 
  now 
  means 
  snow 
  

   or 
  rain 
  cloud. 
  Hence, 
  many 
  argue, 
  if 
  rain 
  or 
  snow 
  is 
  falling 
  from 
  

   a 
  cloud 
  it 
  certainly 
  is 
  a 
  rain 
  cloud. 
  Likewise, 
  if 
  rain 
  is 
  not 
  falling 
  

   from 
  it, 
  then 
  clearly 
  it 
  must 
  be 
  some 
  other 
  sort 
  of 
  cloud. 
  But, 
  on 
  

   the 
  other 
  hand, 
  if 
  a 
  given 
  cloud 
  while 
  raining 
  happens 
  to 
  be 
  a 
  typical 
  

   rain 
  cloud 
  (nimbus), 
  what 
  was 
  it 
  immediately 
  before 
  we 
  saw 
  the 
  

   rain? 
  Again, 
  if 
  it 
  happens 
  to 
  be 
  raining 
  very 
  hard 
  what 
  shall 
  we 
  

   call 
  the 
  cloud 
  that 
  we 
  can 
  not 
  see 
  for 
  the 
  rain? 
  Suppose 
  that 
  we 
  

   can 
  not 
  see 
  the 
  edge 
  of 
  the 
  cloud 
  that 
  is 
  raining, 
  and 
  generally 
  we 
  

   can 
  not, 
  dare 
  we 
  then 
  call 
  it 
  a 
  nimbus 
  in 
  face 
  of 
  the 
  official 
  defini- 
  

   tion 
  " 
  with 
  ragged 
  edges 
  " 
  ? 
  

  

  These 
  are 
  typical 
  of 
  the 
  questions 
  and 
  quibbles 
  the 
  arbitrary 
  

   official 
  definition 
  of 
  " 
  nimbus 
  " 
  has 
  evoked. 
  

  

  As 
  a 
  matter 
  of 
  fact 
  a 
  trace 
  at 
  least 
  of 
  precipitation 
  may 
  be 
  

   falling 
  from 
  a 
  typical 
  alto-stratus, 
  alto-cumulus, 
  or 
  other 
  form, 
  

   which 
  it 
  would 
  be 
  but 
  confusion 
  to 
  call 
  nimbus. 
  Similarly, 
  a 
  

   typical 
  nimbus 
  from 
  which 
  rain 
  is 
  falling 
  steadily 
  looks 
  but 
  little 
  

   different 
  from 
  what 
  it 
  did 
  immediately 
  before 
  the 
  rain 
  began. 
  

   Hence 
  it 
  is 
  convenient 
  to 
  interpret 
  the 
  definition 
  broadly 
  enough 
  to 
  

   cover 
  both 
  cases. 
  If 
  the 
  cloud 
  is 
  typically 
  alto-cumulus, 
  call 
  it 
  alto- 
  

  

  