﻿SOARING 
  FLIGHT 
  KLEMPERER 
  231 
  

  

  They 
  are 
  characterized 
  by 
  alternating 
  pulling 
  up 
  and 
  diving 
  down 
  

   synchronously 
  with 
  the 
  acceleration 
  phases 
  of 
  the 
  gusts. 
  The 
  im- 
  

   portant 
  thing 
  about 
  them 
  is 
  that 
  they 
  must 
  consist 
  of 
  at 
  least 
  one 
  

   primary 
  oscillation 
  and 
  its 
  first 
  harmonic. 
  Thus 
  in 
  a 
  harmonic 
  gust 
  

   the 
  two 
  branches 
  of 
  the 
  path 
  will 
  look 
  unsymmetrical, 
  the 
  diving 
  part 
  

   being 
  done 
  somewhat 
  steeper 
  than 
  the 
  climbing. 
  The 
  mechanism 
  of 
  

   this 
  performance 
  has 
  been 
  compared 
  to 
  making 
  a 
  car 
  travel 
  uphill 
  on 
  

   a 
  scenic 
  railway 
  structure 
  by 
  oscillating 
  the 
  whole 
  structure 
  hori- 
  

   zontally, 
  in 
  the 
  plane 
  of 
  travel. 
  It 
  is 
  quite 
  possible 
  to 
  derive 
  some 
  

   profit 
  from 
  occasional 
  strong 
  gusts 
  by 
  this 
  rocking 
  method, 
  but 
  it 
  

   would 
  be 
  in 
  vain 
  to 
  hope 
  to 
  rely 
  on 
  this 
  alone 
  for 
  support. 
  

  

  Horizontal 
  and 
  vertical 
  gustiness 
  of 
  the 
  wind 
  may 
  occur 
  combined. 
  

   When 
  they 
  happen 
  to 
  be 
  synchronous, 
  their 
  relative 
  phases 
  are 
  of 
  

   importance. 
  Professor 
  v. 
  Karman 
  has 
  shown 
  that, 
  in 
  proximity 
  to 
  

   the 
  ground, 
  the 
  surface 
  friction 
  causes 
  the 
  combination 
  of 
  freshening 
  

   and 
  descending 
  or 
  of 
  fading 
  and 
  ascending 
  to 
  occur 
  more 
  frequently 
  

   than 
  the 
  reverse. 
  For 
  anj'- 
  particle 
  coming 
  down 
  from 
  above, 
  where 
  

   velocity 
  is 
  greater, 
  brings 
  forward 
  impulse 
  with 
  it; 
  whereas 
  any 
  

   particle 
  that 
  happens 
  to 
  rise 
  from 
  the 
  retarded 
  layer 
  below 
  carries 
  

   braking 
  impulse 
  with 
  it. 
  This 
  reasoning 
  would 
  lead 
  us 
  to 
  believe 
  that 
  

   an 
  airplane 
  flying 
  low 
  over 
  the 
  ground 
  would 
  gain 
  less 
  when 
  headed 
  

   against 
  the 
  wind, 
  but 
  more 
  when 
  fleeing 
  before 
  the 
  wind, 
  as 
  com- 
  

   pared 
  with 
  what 
  it 
  would 
  gain 
  from 
  vertical 
  oscillations 
  without 
  a 
  

   horizontal 
  component. 
  

  

  The 
  same 
  principle 
  can 
  be 
  applied 
  to 
  explain 
  a 
  long-known 
  

   phenomenon 
  which 
  has 
  puzzled 
  early 
  experimenters- 
  A 
  light 
  

   wind 
  vane 
  balanced 
  and 
  mounted 
  on 
  a 
  horizontal 
  axis, 
  intended 
  

   to 
  show 
  the 
  vertical 
  inclination 
  of 
  the 
  wind, 
  seems 
  to 
  indicate 
  an 
  

   upward 
  trend 
  of 
  the 
  average 
  of 
  the 
  irregular 
  fluctuations 
  amount- 
  

   ing 
  to 
  some 
  2 
  to 
  4 
  degrees 
  on 
  a 
  level 
  plain. 
  Of 
  course, 
  the 
  wind 
  

   can 
  not 
  spring 
  from 
  the 
  ground. 
  The 
  explanation 
  is 
  that 
  the 
  

   rising 
  elements 
  are 
  slower, 
  the 
  sinking 
  elements 
  are 
  faster. 
  Thus 
  

   although 
  no 
  more 
  air 
  rises 
  than 
  falls, 
  on 
  the 
  average, 
  the 
  apparent 
  

   angle 
  of 
  the 
  resultant 
  motion 
  is 
  greater 
  for 
  the 
  slower, 
  and 
  smaller 
  

   for 
  the 
  faster 
  particles. 
  The 
  mere 
  time 
  average 
  of 
  the 
  direction 
  

   of 
  an 
  irregularly 
  fluctuating 
  vector 
  has 
  no 
  physical 
  significance. 
  

  

  Really 
  measuring 
  the 
  vectorial 
  fluctuations 
  in 
  the 
  atmosphere 
  

   is 
  a 
  very 
  interesting 
  but 
  delicate 
  technique. 
  The 
  introduction 
  of 
  

   the 
  electrical 
  hot-wire 
  anemometer 
  and 
  directional 
  anemometers 
  

   into 
  micro-meteorology 
  promises 
  many 
  scientific 
  possibilities. 
  

   Electrical 
  anemometry 
  is 
  based 
  on 
  the 
  principle 
  of 
  exposing 
  a 
  fine 
  

   platinum 
  wire 
  electrically 
  heated 
  to 
  the 
  air 
  current 
  to 
  be 
  explored. 
  

   The 
  temperature 
  the 
  wire 
  assumes 
  under 
  the 
  cooling 
  influence 
  of 
  

  

  