﻿200 
  ANNUAL, 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  The 
  alto-cumuli 
  appear 
  to 
  be 
  due 
  to 
  local 
  convections, 
  caused 
  either 
  

   by 
  differences 
  of 
  temperature 
  or 
  by 
  wave 
  motion, 
  analogous 
  to 
  those 
  

   that 
  produce 
  the 
  cirro-cumuli, 
  but 
  are 
  at 
  lower 
  levels, 
  roughly 
  2£ 
  

   miles 
  high, 
  usually 
  larger 
  in 
  volume 
  and 
  of 
  more 
  humid 
  air, 
  

   They 
  frequently 
  form 
  in 
  the 
  early 
  forenoon 
  after 
  a 
  clear 
  sunrise 
  

   in 
  consequence 
  of 
  scattered 
  convection 
  in 
  a 
  humid 
  layer 
  of 
  air 
  — 
  humid 
  

   from 
  the 
  evaporation 
  in 
  it 
  of 
  a 
  previous 
  sheet 
  of 
  alto-cumuli, 
  or 
  

   owing 
  to 
  moisture 
  from 
  the 
  spreadout 
  tops 
  of 
  thunderstorm 
  clouds. 
  

   After 
  sundown 
  this 
  type 
  of 
  cloud 
  frequently 
  evaporates, 
  through 
  the 
  

   process 
  of 
  cooling, 
  contracting, 
  and 
  then 
  sinking 
  to 
  a 
  warmer 
  and 
  

   drier 
  level. 
  

  

  5. 
  Alto-stratus 
  (A.-St.). 
  — 
  "A 
  dense 
  sheet 
  of 
  a 
  gray 
  or 
  bluish 
  color, 
  

   sometimes 
  f 
  'owning 
  a 
  compact 
  mass 
  of 
  dull 
  gray 
  color 
  and 
  fibrous 
  

   structure. 
  At 
  other 
  times 
  the 
  sheet 
  is 
  thin 
  like 
  the 
  denser 
  forms 
  of 
  

   cirro-stratus, 
  and 
  through 
  it 
  the 
  sun 
  and 
  the 
  moon 
  may 
  be 
  seen 
  dimly 
  

   gleaming 
  as 
  through 
  ground 
  glass. 
  This 
  form 
  exhibits 
  all 
  stages 
  of 
  

   transition 
  between 
  alto-stratus 
  and 
  cirro-stratus, 
  but 
  according 
  to 
  

   the 
  measurements 
  its 
  normal 
  altitude 
  is 
  about 
  one-half 
  of 
  that 
  of 
  

   cirro-stratus." 
  (PL 
  6, 
  fig. 
  2 
  ; 
  pi. 
  9, 
  fig. 
  2.) 
  

  

  This 
  type 
  of 
  cloud 
  may 
  be 
  formed 
  b} 
  7 
  the 
  flow 
  of 
  warmer 
  moist 
  air 
  

   over 
  colder 
  air 
  beneath, 
  by 
  winds 
  dragging 
  out 
  the 
  tops 
  of 
  thunder- 
  

   storm 
  clouds, 
  by 
  the 
  cooling 
  in 
  place 
  of 
  a 
  layer 
  of 
  humid 
  air, 
  and 
  by 
  

   the 
  relatively 
  small 
  precipitation 
  out 
  of 
  alto-cumulus 
  and 
  cirro- 
  

   cumulus. 
  

  

  The 
  sun 
  and 
  moon 
  when 
  seen 
  through 
  the 
  alto-stratus, 
  alto- 
  

   cumulus, 
  or 
  any 
  other 
  thin 
  cloud 
  of 
  water 
  droplets, 
  are 
  sur- 
  

   rounded 
  by 
  one 
  or 
  more 
  sets 
  of 
  rings 
  of 
  colored 
  light 
  commonly 
  

   only 
  two 
  or 
  three 
  diameters 
  of 
  the 
  sun, 
  or 
  moon, 
  away 
  (pi. 
  26, 
  fig. 
  2). 
  

  

  These 
  small 
  circles, 
  in 
  which 
  the 
  red 
  is 
  farthest 
  from 
  the 
  sun 
  

   or 
  moon 
  are 
  called 
  coronae, 
  and 
  are 
  produced, 
  as 
  just 
  stated, 
  

   by 
  the 
  action 
  (diffraction) 
  of 
  small 
  water 
  droplets 
  on 
  the 
  light. 
  

   The 
  much 
  larger 
  circles, 
  called 
  halos, 
  and 
  whose 
  red 
  portions 
  are 
  

   nearest 
  the 
  sun, 
  or 
  moon, 
  are 
  caused, 
  as 
  previously 
  explained, 
  by 
  

   the 
  passage 
  of 
  light 
  through 
  ice 
  crystals. 
  

  

  6. 
  Strato-cumulus 
  (St.-Cu.). 
  — 
  " 
  Large 
  lumpy 
  masses 
  or 
  rolls 
  of 
  

   dull 
  gray 
  cloud, 
  frequently 
  covering 
  the 
  whole 
  shy, 
  especially 
  in 
  

   winter. 
  Generally 
  strato-cumulus 
  presents 
  the 
  appearance 
  of 
  a 
  

   gray 
  layer 
  broken 
  up 
  into 
  irregular 
  masses 
  and 
  having 
  on 
  the 
  mar- 
  

   gin 
  smaller 
  masses 
  grouped 
  in 
  flocks 
  like 
  alto-cumulus. 
  Some- 
  

   times 
  this 
  cloud-form 
  has 
  the 
  characteristic 
  appearance 
  of 
  great 
  

   rolls 
  of 
  cloud 
  arranged 
  in 
  parallel 
  lines 
  close 
  together. 
  (Roll- 
  

   cumulus 
  in 
  England, 
  Wulst-cumulus 
  in 
  Germany.) 
  The 
  rolls 
  them- 
  

   selves 
  are 
  dense 
  and 
  dark, 
  but 
  in 
  the 
  intervening 
  spaces 
  the 
  cloud 
  

   is 
  much 
  lighter 
  and 
  blue 
  sky 
  may 
  sometimes 
  be 
  seen 
  through 
  them. 
  

  

  