﻿210 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  may 
  descend, 
  the 
  cloud 
  must 
  settle, 
  if 
  it 
  comes 
  down 
  at 
  all, 
  much 
  more 
  

   slowly. 
  

  

  Cloud 
  droplets 
  of 
  the 
  size 
  just 
  implied 
  — 
  that 
  is, 
  such 
  that 
  1,200 
  

   of 
  them 
  side 
  by 
  side 
  make 
  a 
  row 
  1 
  inch 
  long 
  — 
  do, 
  as 
  a 
  matter 
  

   of 
  fact, 
  fall 
  through 
  still 
  air 
  about 
  8 
  feet 
  per 
  minute. 
  Hence, 
  

   where 
  there 
  are 
  no 
  rising 
  currents, 
  a 
  cloud 
  must, 
  and 
  does, 
  grad- 
  

   ually 
  sink 
  into 
  the 
  lower 
  unsaturated 
  air 
  and 
  evaporate. 
  Clouds, 
  

   however, 
  as 
  explained 
  above, 
  commonly 
  are 
  formed 
  in 
  ascending 
  

   currents, 
  and 
  this 
  ascent 
  generally 
  is 
  distinctly 
  greater 
  than 
  8 
  

   feet 
  per 
  minute, 
  the 
  rate 
  just 
  given 
  of 
  cloudfall 
  in 
  still 
  air. 
  Hence 
  

   cloud 
  is 
  continously 
  formed 
  at 
  that 
  level 
  at 
  which 
  the 
  rising 
  air 
  

   is 
  cooled 
  by 
  its 
  expansion 
  to 
  the 
  dew 
  point. 
  The 
  droplets 
  here 
  

   formed 
  are 
  carried 
  to 
  higher 
  levels, 
  there 
  evaporated 
  or 
  merged 
  into 
  

   raindrops, 
  or 
  carried 
  away, 
  as 
  circumstances 
  determine, 
  but 
  new 
  

   droplets 
  replacing 
  these 
  are 
  as 
  continuously 
  being 
  formed 
  as 
  fresh 
  

   humid 
  air 
  rises 
  to 
  the 
  cloud 
  level. 
  

  

  Clouds, 
  therefore, 
  do 
  not 
  even 
  eventually 
  fall 
  to 
  the 
  earth 
  because 
  

   either 
  they 
  are 
  continuously 
  formed 
  at 
  the 
  condensation 
  level 
  by 
  

   rising 
  currents 
  or, 
  on 
  slowly 
  sinking 
  (8 
  feet 
  per 
  minute) 
  to 
  lower 
  

   levels 
  in 
  still 
  air, 
  or 
  being 
  dragged 
  down 
  by 
  descending 
  air, 
  are 
  

   soon 
  evaporated. 
  In 
  any 
  case 
  the 
  cloud 
  floats 
  at 
  a 
  greater 
  or 
  less 
  

   height 
  and 
  never 
  falls 
  precipitately 
  to 
  the 
  earth 
  as 
  does 
  rain. 
  

  

  HOW 
  RAIN 
  IS 
  PRODUCED. 
  

  

  Few 
  people 
  ever 
  ask 
  how 
  rain 
  is 
  produced. 
  Perhaps 
  this 
  is 
  be- 
  

   cause 
  the 
  phenomenon 
  is 
  too 
  familiar 
  to 
  arouse 
  one's 
  curiosity 
  or 
  

   make 
  him 
  in 
  the 
  least 
  inquisitive. 
  Nevertheless, 
  and 
  however 
  

   childish 
  it 
  may 
  seem, 
  it 
  is 
  both 
  a 
  rational 
  question 
  and 
  a 
  difficult 
  one 
  

   to 
  answer. 
  

  

  The 
  familiar, 
  pretended 
  answer, 
  is, 
  in 
  effect, 
  that 
  somehow 
  the 
  air 
  

   is 
  cooled 
  until 
  condensation 
  occurs 
  on 
  the 
  various 
  nuclei 
  present, 
  and 
  

   that 
  the 
  larger 
  of 
  the 
  droplets 
  thus 
  produced 
  that 
  happen 
  to 
  be 
  well 
  

   up 
  in 
  the 
  cloud 
  fall 
  to 
  lower 
  levels, 
  thereby 
  encountering 
  many 
  other 
  

   particles 
  and 
  through 
  coalescence 
  with 
  them 
  growing 
  into 
  full-sized 
  

   drops. 
  But, 
  as 
  implied, 
  this 
  explanation 
  explains 
  nothing. 
  In 
  the 
  

   first 
  place 
  there 
  are 
  so 
  many 
  nuclei 
  present 
  in 
  the 
  atmosphere 
  — 
  hun- 
  

   dreds 
  at 
  least, 
  and 
  usually 
  thousands, 
  to 
  every 
  cubic 
  inch 
  — 
  that 
  divi- 
  

   sion 
  of 
  the 
  condensed 
  vapor 
  between 
  them 
  leaves 
  every 
  one 
  quite 
  

   too 
  minute 
  to 
  fall 
  with 
  any 
  considerable 
  velocity. 
  Then, 
  too, 
  calcula- 
  

   tion 
  shows 
  that 
  if 
  a 
  particle 
  should 
  fall, 
  in 
  the 
  manner 
  supposed, 
  

   through 
  a 
  cloud 
  even 
  a 
  mile 
  thick 
  and 
  pick 
  up 
  everything 
  in 
  its 
  path 
  

   it 
  still 
  would 
  be 
  a 
  small 
  drop. 
  That 
  is, 
  rain 
  is 
  not 
  formed 
  in 
  this 
  

  

  