﻿Rain, 
  Wind, 
  and 
  Cyclones, 
  229 
  

  

  clouds 
  ; 
  but 
  during 
  the 
  night 
  the 
  land 
  cools 
  more 
  than 
  the 
  

   seas, 
  the 
  air 
  descends 
  and 
  the 
  clouds 
  melt 
  away. 
  Thunder- 
  

   storms 
  originate 
  in 
  this 
  way, 
  the 
  air 
  rising 
  locally 
  to 
  such 
  

   great 
  heights 
  that 
  heavy 
  rain 
  and 
  hail 
  result. 
  

  

  The 
  conditions 
  shown 
  on 
  the 
  diagrams 
  that 
  have 
  been 
  consi- 
  

   dered 
  are 
  such 
  as 
  occur 
  near 
  the 
  earth's 
  surface. 
  At 
  low 
  levels 
  

   the 
  winds 
  often 
  approach 
  and 
  recede 
  from 
  the 
  low-pressure 
  

   area 
  much 
  as 
  comets 
  approach 
  and 
  recede 
  from 
  the 
  sun. 
  But 
  

   the 
  wind 
  near 
  the 
  earth's 
  surface 
  encounters 
  considerable 
  

   frictional 
  resistance, 
  and 
  some 
  of 
  it 
  is 
  drawn 
  in 
  and 
  rises 
  in 
  

   the 
  cyclone. 
  Indeed, 
  if 
  it 
  were 
  not 
  for 
  the 
  friction 
  of 
  the 
  air 
  

   and 
  the 
  ground, 
  a 
  cyclone 
  once 
  started 
  should 
  persist 
  by 
  

   reason 
  of 
  its 
  own 
  momentum. 
  

  

  It 
  must 
  be 
  remembered 
  that 
  the 
  water 
  content 
  of 
  the 
  air 
  

   rapidly 
  decreases 
  with 
  elevation, 
  owing 
  to 
  the 
  decrease 
  of 
  

   temperature. 
  Above 
  3000 
  metres 
  the 
  moisture 
  content 
  of 
  the 
  

   atmosphere 
  is 
  very 
  small 
  owing 
  to 
  the 
  low 
  temperatures 
  

   prevailing 
  at 
  that 
  level. 
  Indeed, 
  this 
  lower 
  region 
  of 
  the 
  

   atmosphere 
  has 
  been 
  called 
  the 
  storm 
  layer. 
  In 
  it 
  the 
  tem- 
  

   perature 
  gradients 
  ate 
  very 
  irregular, 
  for 
  all 
  upward 
  currents 
  

   throw 
  down 
  moisture 
  as 
  they 
  rise, 
  and 
  are 
  prevented 
  from 
  

   falling 
  in 
  temperature 
  as 
  much 
  as 
  they 
  otherwise 
  would 
  do 
  

   by 
  the 
  liberation 
  of 
  the 
  latent 
  heat 
  of 
  condensation, 
  whereas 
  

   descending 
  currents 
  undergo 
  simple 
  adiabatic 
  changes 
  of 
  

   temperature. 
  In 
  very 
  cold 
  weather 
  the 
  temperature 
  gradient 
  

   is 
  often 
  reversed 
  by 
  reason 
  of 
  the 
  cooling 
  of 
  the 
  air 
  in 
  contact 
  

   with 
  the 
  ground. 
  

  

  The 
  diagrams 
  of 
  cyclones 
  shown 
  in 
  the 
  figures 
  that 
  have 
  

   been 
  given 
  make 
  it 
  clear 
  that 
  the 
  winds 
  near 
  the 
  earth's 
  surface 
  

   very 
  often 
  cross 
  the 
  gradients 
  at 
  high 
  angles. 
  Gold 
  * 
  has 
  

   carefully 
  considered 
  the 
  relationship 
  of 
  the 
  isobars 
  and 
  winds 
  

   at 
  heights 
  of 
  about 
  3000 
  feet. 
  Assuming 
  that 
  the 
  isobars 
  near 
  

   the 
  earth's 
  surface 
  hold 
  true 
  for 
  greater 
  heights, 
  it 
  is 
  found 
  that 
  

   the 
  strength 
  of 
  the 
  upper 
  winds 
  closely 
  approximates, 
  by 
  calcu- 
  

   lation, 
  to 
  what 
  would 
  be 
  expected 
  from 
  the 
  gradients 
  at 
  the 
  

   earth's 
  surface. 
  Gold 
  remarks 
  "It 
  is 
  to 
  be 
  noted, 
  however, 
  

   that 
  on 
  the 
  average, 
  even 
  for 
  anticyclones, 
  the 
  tendency 
  is 
  for 
  

   the 
  wind 
  at 
  1000 
  metres 
  altitude 
  to 
  blow 
  slightly 
  across 
  the 
  

   isobars 
  from 
  high 
  pressure 
  to 
  low." 
  In 
  a 
  preface 
  to 
  the 
  paper 
  

   referred 
  to, 
  Shaw 
  says 
  : 
  " 
  The 
  general 
  result 
  of 
  the 
  investi- 
  

   gation 
  is, 
  in 
  my 
  opinion, 
  to 
  confirm 
  the 
  suggestion 
  that 
  the 
  

   adjustment 
  of 
  wind 
  velocity 
  to 
  gradient 
  is 
  an 
  automatic 
  

  

  * 
  "Barometric 
  Gradient 
  and 
  Wind 
  Force," 
  Meteorological 
  Office 
  

   Publication 
  190, 
  p. 
  9. 
  

  

  