﻿202 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  cumulus, 
  whatever 
  may 
  be 
  falling. 
  On 
  the 
  other 
  hand, 
  if 
  it 
  looks 
  

   like 
  a 
  rain 
  cloud 
  and 
  is 
  not 
  a 
  " 
  cumulo-nimbus," 
  described 
  below, 
  

   call 
  it 
  nimbus, 
  whether 
  it 
  is 
  raining 
  or 
  not 
  and 
  regardless 
  of 
  

   all 
  edges. 
  

  

  Of 
  course 
  in 
  taking 
  weather 
  notes 
  it 
  commonly 
  is 
  superfluous 
  

   to 
  say 
  both 
  rain 
  and 
  nimbus 
  cloud, 
  because 
  the 
  former 
  nearly 
  

   always 
  implies 
  the 
  latter, 
  except 
  in 
  the 
  case 
  of 
  the 
  thunderstorm, 
  

   which 
  is 
  separately 
  reported. 
  However, 
  if 
  one 
  were 
  noting 
  clouds 
  

   alone 
  he 
  certainly 
  should 
  say 
  nimbus, 
  or 
  occasionally, 
  nimbus 
  

   cumuliformis, 
  whenever 
  there 
  is 
  precipitation 
  of 
  appreciable 
  in- 
  

   tensity 
  without 
  lightning 
  or 
  thunder. 
  

  

  This 
  type 
  of 
  cloud 
  is 
  most 
  frequently 
  formed 
  by 
  some 
  kind 
  of 
  

   mechanical 
  convection, 
  such 
  as 
  ascent 
  due 
  to 
  converging 
  winds, 
  

   the 
  flow 
  of 
  air 
  over 
  mountain 
  barriers, 
  and 
  the 
  over 
  and 
  under 
  

   running 
  of 
  currents 
  of 
  different 
  temperatures. 
  

  

  The 
  fracto-nimbus, 
  or 
  scud, 
  is 
  only 
  a 
  low, 
  ragged 
  detached 
  

   fragment 
  of 
  cloud 
  that 
  often 
  rises, 
  like 
  steam, 
  immediately 
  after 
  

   rainfall 
  on 
  a 
  warm 
  surface, 
  especially 
  the 
  sides 
  of 
  mountains, 
  which 
  

   it 
  ascends 
  like 
  drifting 
  fog. 
  (PI. 
  11, 
  fig. 
  2.) 
  It 
  also 
  is 
  frequently 
  

   dragged 
  out 
  of 
  a 
  crest 
  cloud 
  (which 
  see 
  below), 
  by 
  the 
  swirls 
  of 
  the 
  

   passing 
  wind 
  and 
  hurled 
  down 
  the 
  leeward 
  mountain 
  slope. 
  

  

  8. 
  Cumulus 
  (Cu.). 
  — 
  " 
  Woolpack 
  or 
  Cauliflower 
  Cloud. 
  — 
  Thick 
  

   cloud 
  of 
  which 
  the 
  upper 
  surface 
  is 
  dome-shaped 
  and 
  exhibits 
  

   protuberances 
  while 
  the 
  base 
  is 
  generally 
  horizontal. 
  These 
  clouds 
  

   appear 
  to 
  be 
  formed 
  by 
  ascensional 
  movement 
  of 
  air 
  in 
  the 
  daytime 
  

   which 
  is 
  almost 
  always 
  observable. 
  When 
  the 
  cloud 
  and 
  the 
  sun 
  

   are 
  on 
  opposite 
  sides 
  of 
  the 
  observer, 
  the 
  surfaces 
  facing 
  the 
  

   observer 
  are 
  more 
  brilliant 
  than 
  the 
  margins 
  of 
  the 
  protuberances. 
  

   When, 
  on 
  the 
  contrary, 
  it 
  is 
  on 
  the 
  same 
  side 
  of 
  the 
  observer 
  as 
  the 
  

   sun 
  it 
  appears 
  dark 
  with 
  bright 
  edges. 
  When 
  the 
  light 
  falls 
  side- 
  

   ways, 
  as 
  is 
  usually 
  the 
  case, 
  cumulus 
  clouds 
  show 
  deep 
  shadows." 
  

   (PL 
  7, 
  fig. 
  2; 
  pi. 
  12, 
  figs. 
  1 
  and 
  2; 
  pi. 
  13, 
  figs. 
  1 
  and 
  2; 
  and 
  pi. 
  14, 
  

   fig. 
  1.) 
  

  

  " 
  True 
  cumulus 
  has 
  well-defined 
  upper 
  and 
  lower 
  margins 
  ; 
  but 
  

   one 
  may 
  sometimes 
  see 
  ragged 
  clouds 
  — 
  like 
  cumulus 
  torn 
  by 
  strong 
  

   wind 
  — 
  of 
  which 
  the 
  detached 
  portions 
  are 
  continually 
  changing; 
  

   to 
  this 
  form 
  of 
  cloud 
  the 
  name 
  fracto- 
  cumulus 
  may 
  be 
  given." 
  

   (PL 
  14, 
  fig. 
  2.) 
  

  

  Even 
  the 
  most 
  casual 
  observation 
  shows 
  the 
  cauliflower 
  heads 
  

   and 
  sides 
  of 
  cumuli 
  to 
  be 
  in 
  a 
  state 
  of 
  rapid 
  change 
  and 
  constant 
  

   turmoil. 
  All 
  clouds 
  of 
  this 
  type 
  are 
  caused 
  by 
  the 
  lower 
  air 
  being 
  

   so 
  much 
  warmer 
  than 
  the 
  upper 
  that 
  it 
  is 
  forced 
  to 
  ascend 
  much 
  as 
  

   warm 
  air 
  is 
  pushed 
  up 
  a 
  chimney 
  by 
  the 
  heavier 
  cold 
  air 
  on 
  the 
  

   outside. 
  Hence, 
  even 
  forest 
  fires 
  sometimes 
  cause 
  the 
  formation 
  of 
  

  

  