﻿198 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  in 
  the 
  same 
  region 
  of 
  the 
  sky. 
  These 
  are 
  only 
  thin 
  streaks 
  of 
  snow 
  

   into 
  which 
  a 
  little 
  ball 
  of 
  rising 
  cloud 
  is 
  drawn 
  out, 
  partly 
  by 
  the 
  

   slow 
  descent 
  of 
  the 
  snow 
  itself, 
  and 
  partly 
  by 
  the 
  increase 
  of 
  wind 
  

   velocity 
  with 
  elevation. 
  

  

  The 
  cirrus 
  occurs 
  up 
  to 
  (and 
  usually 
  at) 
  greater 
  heights 
  than 
  any 
  

   other 
  type 
  of 
  cloud 
  — 
  roughly 
  5 
  miles 
  in 
  polar 
  regions, 
  7 
  in 
  middle 
  lati- 
  

   tudes, 
  and 
  9 
  within 
  the 
  Tropics. 
  Since 
  it 
  occurs 
  above 
  all 
  other 
  clouds 
  

   it 
  also 
  is 
  the 
  coldest, 
  ranging, 
  roughly, 
  from 
  50°F. 
  below 
  zero 
  near 
  

   the 
  poles 
  to 
  90° 
  below 
  in 
  the 
  neighborhood 
  of 
  the 
  Equator, 
  the 
  differ- 
  

   ence 
  in 
  temperature 
  being 
  due 
  to 
  the 
  difference 
  in 
  height. 
  Further- 
  

   more, 
  because 
  it 
  is 
  the 
  coldest 
  of 
  the 
  clouds 
  it 
  likewise 
  is 
  the 
  thinnest, 
  

   being 
  formed 
  in 
  air 
  whose 
  water 
  content, 
  owing 
  to 
  its 
  low 
  tempera- 
  

   ture, 
  must 
  be 
  very 
  small. 
  Indeed, 
  the 
  sun 
  and 
  moon 
  are 
  sharply 
  

   outlined 
  through 
  it. 
  Finally, 
  as 
  its 
  temperature 
  is 
  so 
  very 
  low 
  the 
  

   cirrus 
  nearly 
  always 
  (there 
  seem 
  to 
  be 
  occasional 
  exceptions) 
  consists 
  

   of 
  snow 
  crystals. 
  

  

  As 
  just 
  stated, 
  the 
  cirrus 
  occurs 
  at 
  higher 
  levels 
  than 
  does 
  any 
  

   other 
  type 
  of 
  cloud. 
  But 
  it 
  also 
  occurs, 
  under 
  favorable 
  conditions, 
  

   at 
  any 
  other 
  level, 
  even 
  down 
  to 
  the 
  surface. 
  It 
  is 
  only 
  a 
  shallow, 
  

   or 
  thin, 
  cloud 
  of 
  fine 
  snow 
  crystals 
  (except 
  rarely) 
  drawn 
  out 
  by 
  

   the 
  wind 
  into 
  more 
  or 
  less 
  parallel, 
  fine 
  streaks. 
  Its 
  chief 
  cause 
  

   appears 
  to 
  be 
  the 
  further 
  cooling, 
  by 
  expansion 
  due 
  to 
  ascent, 
  of 
  

   air 
  already 
  cold 
  and 
  holding 
  but 
  little 
  moisture, 
  and 
  the 
  dragging 
  

   out 
  of 
  the 
  slight 
  cloud 
  thus 
  formed 
  into 
  streaks 
  and 
  whirls 
  by 
  

   the 
  winds 
  in 
  which 
  it 
  occurs. 
  

  

  When 
  the 
  cirrus 
  slowly 
  disappears, 
  leaving 
  a 
  clear 
  sky, 
  fair 
  

   weather 
  is 
  probable 
  for 
  at 
  least 
  a 
  day 
  or 
  two. 
  On 
  the 
  other 
  hand, 
  

   when 
  it 
  gradually 
  thickens 
  and 
  merges 
  into 
  a 
  nearly 
  continuous 
  and 
  

   formless 
  sheet, 
  rain 
  or 
  snow 
  usually 
  begins 
  within 
  a 
  few 
  hours. 
  

  

  2. 
  Cirro-stratus 
  (Ci.-St.). 
  — 
  "A 
  thin 
  sheet 
  of 
  whitish 
  cloud; 
  some- 
  

   times 
  covering 
  the 
  sky 
  completely 
  and 
  merely 
  giving 
  it 
  a 
  milky 
  ap- 
  

   pearance; 
  it 
  is 
  then 
  called 
  cirro-nebula 
  or 
  cirrus 
  haze; 
  at 
  other 
  times 
  

   presenting 
  more 
  or 
  less 
  distinctly 
  a 
  fibrous 
  structure 
  like 
  a 
  tangled 
  

   web. 
  This 
  sheet 
  often 
  produces 
  halos 
  round 
  the 
  sun 
  or 
  moon." 
  (PL 
  

   5, 
  fig. 
  2.) 
  

  

  The 
  halos 
  here 
  referred 
  to 
  are 
  of 
  two 
  kinds: 
  (a) 
  Those 
  due 
  to 
  

   the 
  refraction 
  of 
  light 
  into 
  rainbow 
  colors 
  on 
  its 
  passage 
  through 
  

   ice 
  crystals; 
  and 
  (b) 
  those 
  produced 
  by 
  the 
  mere 
  reflection 
  of 
  light 
  

   (hence 
  white 
  or 
  colorless) 
  by 
  the 
  faces 
  of 
  these 
  crystals. 
  

  

  The 
  most 
  common 
  of 
  the 
  first 
  class 
  of 
  halos 
  are: 
  (1) 
  The 
  circle 
  

   of 
  22° 
  radius 
  about 
  the 
  sun 
  or 
  moon 
  (pi. 
  5, 
  fig. 
  2) 
  ; 
  (2) 
  the 
  similar 
  

   circle 
  of 
  46° 
  radius; 
  and 
  (3) 
  the 
  circumzenithal 
  arc, 
  a 
  brilliantly 
  

   colored 
  arc 
  having 
  the 
  point 
  directly 
  overhead 
  as 
  its 
  center, 
  but 
  ap- 
  

   pearing 
  on 
  the 
  side 
  next 
  the 
  sun 
  only. 
  The 
  chief 
  halos 
  of 
  the 
  second 
  

  

  