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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  upper 
  part 
  is 
  left 
  as 
  a 
  gracious 
  veil 
  that 
  shields 
  one 
  from 
  the 
  sun 
  

   until 
  nearly 
  noon. 
  

  

  The 
  same 
  pamphlet, 
  entitled 
  " 
  Cloud 
  Forms," 
  from 
  which 
  the 
  

   above 
  quoted 
  definitions 
  and 
  descriptions 
  of 
  the 
  ten 
  standard 
  

   cloud 
  types 
  were 
  taken, 
  contains 
  also 
  the 
  following 
  instructions 
  

   to 
  observers 
  : 
  

  

  "(a) 
  In 
  the 
  daytime 
  in 
  summer 
  all 
  the 
  lower 
  clouds 
  assume, 
  

   as 
  a 
  rule, 
  special 
  forms 
  more 
  or 
  less 
  resembling 
  cumulus. 
  In 
  such 
  

   cases 
  the 
  observer 
  maj 
  7 
  enter 
  in 
  his 
  notes 
  ' 
  stratus- 
  or 
  nimbus- 
  

   cumuli 
  form 
  is. 
  1 
  

  

  "(5) 
  Sometimes 
  a 
  cloud 
  will 
  show 
  a 
  mammillated 
  surface 
  and 
  

   the 
  appearance 
  should 
  be 
  noted 
  under 
  the 
  name 
  mammato-cumu- 
  

   lus. 
  (PI. 
  17, 
  fig. 
  1.) 
  

  

  "(c) 
  The 
  form 
  taken 
  by 
  certain 
  clouds 
  particularly 
  on 
  days 
  of 
  

   sirocco, 
  mistral, 
  fohn, 
  etc., 
  which 
  show 
  an 
  ovoid 
  form 
  with 
  clean 
  

   outlines 
  and 
  sometimes 
  irisation, 
  will 
  be 
  indicated 
  by 
  the 
  name 
  

   lenticular, 
  for 
  example: 
  Cumulus 
  lenticularis, 
  stratus 
  lenticularis 
  

   (Cu.-lent., 
  St.-lent.). 
  

  

  "(d) 
  Notice 
  should 
  always 
  be 
  taken 
  when 
  the 
  clouds 
  seem 
  mo- 
  

   tionless 
  or 
  if 
  they 
  move 
  with 
  very 
  great 
  velocity." 
  

  

  SPECIAL 
  CLOUD 
  FORMS. 
  

  

  It 
  may 
  seem, 
  no 
  doubt, 
  that 
  enough 
  cloud 
  forms 
  have 
  already 
  

   been 
  mentioned 
  to 
  include 
  every 
  type 
  known 
  to 
  the 
  heavens, 
  and 
  

   thus 
  to 
  satisfy 
  the 
  most 
  ardent 
  cloud 
  observer, 
  but, 
  in 
  addition 
  to 
  

   the 
  gradual 
  transformation 
  of 
  a 
  cloud 
  from 
  one 
  type 
  to 
  another, 
  

   giving 
  stages 
  that 
  puzzle 
  the 
  expert, 
  there 
  are 
  several 
  occasional 
  

   forms 
  sufficiently 
  distinct, 
  even 
  though 
  some 
  of 
  them 
  belong 
  to 
  the 
  

   general 
  types 
  already 
  mentioned, 
  to 
  justify 
  individual 
  names 
  and 
  

   special 
  descriptions. 
  

  

  Billow 
  cloud. 
  — 
  Billow 
  clouds 
  (pi. 
  9, 
  fig. 
  1), 
  also 
  called 
  windrow 
  

   clouds 
  and 
  wave 
  clouds, 
  occur 
  in 
  nearly 
  equally 
  spaced 
  parallel 
  

   bands, 
  generally 
  with 
  intervening 
  strips 
  of 
  clear 
  sky. 
  The 
  billow 
  

   cloud 
  most 
  frequently, 
  perhaps, 
  is 
  only 
  a 
  special 
  form 
  of 
  the 
  more 
  

   general 
  alto-cumulus. 
  It 
  also 
  is 
  a 
  common 
  form, 
  rather 
  as 
  ripples, 
  

   however, 
  of 
  the 
  cirro-cumulus 
  (pi. 
  6, 
  fig. 
  1). 
  On 
  rare 
  occasions 
  it 
  

   likewise 
  occurs 
  at 
  low 
  levels 
  where 
  it 
  might 
  be 
  called 
  a 
  type 
  of 
  

   strato-cumulus. 
  

  

  But 
  whatever 
  its 
  level, 
  the 
  billow 
  cloud 
  is 
  always 
  produced 
  in 
  the 
  

   same 
  way 
  — 
  by 
  the 
  flowing 
  of 
  one 
  stratum 
  or 
  current 
  of 
  air 
  over 
  

   another 
  of 
  distinctly 
  greater 
  density, 
  thereby 
  creating 
  air 
  billows 
  

   precisely 
  as 
  water 
  billows 
  are 
  formed 
  on 
  the 
  ocean. 
  Now, 
  the 
  

  

  