﻿HYDROLOGY 
  OF 
  NEW 
  YORK 
  83 
  

  

  Rainfall 
  

  

  Cause 
  of 
  rainfall. 
  The 
  cause 
  of 
  rainfall 
  has 
  been 
  discussed 
  by 
  

   Mr 
  Velschow 
  in 
  the 
  transactions 
  of 
  the 
  American 
  Society 
  of 
  Civil 
  

   Engineers. 
  1 
  This 
  paper 
  may 
  be 
  referred 
  to 
  for 
  a 
  very 
  good 
  dis- 
  

   cussion 
  of 
  the 
  subject. 
  

  

  The 
  subject 
  is 
  also 
  very 
  ably 
  discussed 
  by 
  Alfred 
  J. 
  Henry 
  in 
  

   one 
  of 
  the 
  Weather 
  Bureau 
  Reports. 
  2 
  Mr 
  Henry 
  remarks 
  that 
  the 
  

   theories 
  of 
  rainfall 
  given 
  in 
  books 
  of 
  twenty 
  or 
  thirty 
  years 
  ago 
  

   are 
  not 
  now 
  wholly 
  accepted. 
  Still 
  there 
  is 
  one 
  simple 
  principle 
  

   upon 
  which 
  no 
  disagreement 
  exists 
  — 
  that 
  in 
  order 
  to 
  produce 
  rain 
  

   the 
  temperature 
  of 
  the 
  air 
  must 
  be 
  suddenly 
  cooled 
  below 
  the 
  

   dewpoint. 
  When 
  the 
  air 
  is 
  thus 
  cooled 
  a 
  portion 
  of 
  the 
  vapor 
  is 
  

   changed 
  to 
  the 
  liquid 
  and 
  the 
  particles 
  thus 
  formed 
  may 
  float 
  

   away 
  with 
  the 
  wind 
  or 
  they 
  may 
  increase 
  in 
  size 
  and 
  fall 
  to 
  the 
  

   ground 
  by 
  virtue 
  of 
  gravity. 
  Whether 
  the 
  condensation 
  results 
  

   simply 
  in 
  cloud, 
  or 
  whether 
  rain 
  falls, 
  depends 
  on 
  the 
  magnitude 
  

   of 
  the 
  temperature 
  changes 
  taking 
  place 
  in 
  the 
  air 
  mass. 
  

  

  The 
  precise 
  manner 
  in 
  which 
  air 
  is 
  cooled 
  to 
  produce 
  rain, 
  

   whether 
  by 
  contact 
  or 
  by 
  mixing, 
  is 
  not 
  clearly 
  apprehended. 
  

   Cooling 
  by 
  expansion, 
  as 
  air 
  ascends, 
  is 
  one 
  of 
  the 
  most 
  effective 
  

   causes 
  of 
  rainfall. 
  The 
  ascensional 
  movement 
  is 
  brought 
  about 
  

   in 
  several 
  ways, 
  probably 
  the 
  most 
  important 
  of 
  which 
  is 
  circula- 
  

   tion 
  of 
  air 
  in 
  cyclonic 
  storms, 
  by 
  a 
  radial 
  inflow 
  from 
  all 
  sides 
  and 
  

   an 
  ascensional 
  movement 
  in 
  the 
  center. 
  A 
  very 
  large 
  percentage 
  

   of 
  the 
  rain 
  of 
  the 
  United 
  States 
  is 
  precipitated 
  in 
  connection 
  with 
  

   the 
  passage 
  of 
  storms 
  of 
  this 
  class. 
  3 
  

  

  Mr 
  Henry 
  discusses 
  the 
  precipitation 
  of 
  the 
  United 
  States 
  under 
  

   the 
  following 
  topics: 
  (1) 
  The 
  statistics 
  used 
  and 
  their 
  accuracy; 
  

   (2) 
  geographic 
  distribution 
  and 
  annual 
  allowance; 
  (3) 
  monthly 
  

   distribution 
  by 
  districts 
  and 
  types; 
  (4) 
  the 
  precipitation 
  of 
  the 
  

   crop-gTowing 
  season; 
  (5) 
  secular 
  variations; 
  (6) 
  details 
  of 
  the 
  

  

  1 
  The 
  Cause 
  of 
  Rain 
  and 
  the 
  Structure 
  of 
  the 
  Atmosphere, 
  by 
  Franz 
  A. 
  

   Velschow 
  : 
  Trans. 
  Am. 
  Soc. 
  Civil 
  Eng., 
  Vol. 
  XXXIII, 
  1890, 
  p. 
  303. 
  

  

  2 
  Rainfall 
  of 
  the 
  United 
  States, 
  by 
  Alfred 
  J. 
  Henry, 
  chief 
  of 
  division 
  : 
  

   Ann. 
  Rept. 
  Weather 
  Bureau, 
  1896-97, 
  p. 
  317. 
  

  

  3 
  Abstracted 
  from 
  Mr 
  Henry's 
  paper. 
  

  

  