﻿208 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  is 
  objectionable 
  since 
  the 
  cloud 
  in 
  question 
  is 
  indeed 
  a 
  cirrus, 
  differ- 
  

   ing 
  from 
  other 
  cirri, 
  as 
  stated, 
  only 
  in 
  origin. 
  

  

  Mammato- 
  cumulus. 
  — 
  The 
  maminato-cumulus, 
  called 
  also 
  pocky- 
  

   cloud, 
  festoon-cloud, 
  sack-cloud, 
  " 
  rain 
  balls," 
  and 
  several 
  other 
  local 
  

   names, 
  is 
  a 
  sort 
  of 
  miniature 
  up-side-down 
  cumulus. 
  (PI. 
  17, 
  fig. 
  1.) 
  

   It 
  occurs 
  most 
  frequently 
  in 
  connection 
  with 
  severe 
  thunderstorms, 
  

   and 
  appears 
  to 
  be 
  due 
  to 
  irregular 
  descents 
  here 
  and 
  there 
  of 
  cold 
  

   air 
  onto 
  an 
  existing 
  stratus 
  cloud, 
  each 
  place 
  of 
  fall 
  being 
  marked 
  by 
  

   a 
  downward 
  bulge 
  in 
  the 
  cloud 
  base. 
  

  

  Tormado 
  or 
  funnel 
  cloud. 
  — 
  The 
  tornado 
  or 
  funnel 
  cloud 
  (pi. 
  23, 
  

   figs. 
  1 
  and 
  2; 
  pi. 
  24) 
  is 
  only 
  a 
  long 
  funnel-shaped 
  cloud 
  about 
  the 
  

   axis 
  of 
  rotation 
  of 
  a 
  tornado. 
  It 
  hangs 
  down, 
  either 
  straight 
  or 
  

   curved, 
  from 
  the 
  base 
  of 
  a 
  rain 
  cloud 
  — 
  commonly, 
  if 
  not 
  always, 
  a 
  

   cumulo-nimbus 
  — 
  and 
  varies 
  in 
  extent 
  all 
  the 
  way 
  from 
  a 
  mere 
  pro- 
  

   tuberance 
  on 
  the 
  parent 
  cloud, 
  to 
  a 
  crooked 
  trunk 
  reaching 
  all 
  the 
  

   way 
  to 
  the 
  earth. 
  It 
  is 
  caused 
  by 
  the 
  cooling, 
  due 
  to 
  expansion, 
  

   of 
  the 
  rotating 
  air, 
  to 
  a 
  temperature 
  below 
  the 
  dew-point. 
  In 
  short, 
  

   it 
  is 
  just 
  cloud, 
  induced, 
  like 
  other 
  clouds, 
  by 
  cooling, 
  though 
  often 
  

   mixed 
  with 
  dust 
  in 
  its 
  lower 
  portion. 
  

  

  CLOUD 
  MISCELLANY. 
  

  

  CLOUD 
  MATERIAL. 
  

  

  Clouds 
  consist 
  either 
  of 
  water 
  droplets, 
  always 
  when 
  the 
  tem- 
  

   perature 
  is 
  above 
  freezing, 
  or 
  ice 
  crystals, 
  normally 
  when 
  the 
  tem- 
  

   perature 
  is 
  below 
  freezing, 
  but 
  not 
  invariably, 
  because 
  occasion- 
  

   ally 
  the 
  droplets 
  cool 
  without 
  solidifying 
  to, 
  or, 
  perhaps, 
  form 
  at, 
  

   temperatures 
  far 
  below 
  the 
  " 
  freezing 
  point." 
  

  

  The 
  droplets 
  that 
  together 
  constitute 
  a 
  cloud 
  out 
  of 
  which 
  no 
  

   rain 
  is 
  falling 
  vary 
  in 
  size 
  from 
  the 
  vanishingly 
  small, 
  especially 
  

   at 
  the 
  surface 
  and 
  edges 
  where 
  they 
  are 
  likely 
  to 
  be 
  disappearing 
  

   by 
  evaporation, 
  or 
  coming 
  into 
  existence 
  through 
  condensation, 
  up 
  

   to 
  several 
  fold 
  that 
  of 
  the 
  average 
  particle. 
  Most 
  of 
  them, 
  how- 
  

   ever, 
  appear 
  to 
  be 
  about 
  one 
  one-thousandth 
  of 
  an 
  inch 
  in 
  diameter, 
  

   roughly 
  the 
  size 
  of 
  the 
  familiar 
  lycopodium 
  spore. 
  In 
  rain 
  clouds, 
  

   on 
  the 
  other 
  hand, 
  such 
  as 
  the 
  cumulo-nimbus, 
  this 
  range 
  is 
  much 
  

   greater 
  — 
  from 
  the 
  invisibly 
  minute, 
  as 
  before, 
  up 
  to 
  the 
  fully 
  de- 
  

   veloped 
  drop 
  of 
  one-tenth, 
  say, 
  to 
  one-fifth 
  (the 
  maximum 
  possible) 
  

   of 
  an 
  inch 
  across. 
  

  

  The 
  ice 
  crystals, 
  the 
  material 
  of 
  cold 
  clouds, 
  also 
  vary 
  widely 
  in 
  

   size, 
  and 
  include 
  every 
  gradation 
  from 
  the 
  minute, 
  almost 
  micro- 
  

   scopic, 
  needles 
  of 
  the 
  cirrus 
  to 
  the 
  well-known 
  snow 
  flakes 
  of 
  the 
  

   winter 
  storm. 
  Their 
  chief 
  claim, 
  however, 
  on 
  our 
  attention 
  is 
  not 
  

   because 
  of 
  this 
  interesting 
  dimensional 
  range, 
  but 
  owing 
  to 
  their 
  

  

  