﻿0. 
  L. 
  Fassig 
  — 
  March 
  Weather 
  in 
  the 
  United 
  States. 
  325 
  

  

  enabled 
  us 
  to 
  see 
  wliy 
  the 
  winds 
  at 
  any 
  given 
  place 
  change 
  

   rapidly 
  in 
  direction 
  as 
  an 
  area 
  of 
  low 
  or 
  high 
  pressure 
  passes 
  

   over 
  the 
  place. 
  It 
  is 
  also 
  essential 
  to 
  bear 
  in 
  mind 
  certain 
  

   relations 
  existing 
  between 
  temperature 
  and 
  moisture 
  conditions 
  

   of 
  the 
  atmosphere. 
  At 
  a 
  given 
  temperature 
  the 
  maximum 
  

   amount 
  of 
  invisible 
  moisture 
  in 
  a 
  given 
  space 
  is 
  a 
  fixed 
  quan- 
  

   tity. 
  Should 
  the 
  temperature 
  be 
  lowered, 
  some 
  of 
  the 
  moisture 
  

   will 
  assume 
  a 
  visible 
  form 
  such 
  as 
  cloud 
  or 
  rain. 
  Conversely, 
  

   if 
  the 
  temperature 
  be 
  raised 
  the 
  space 
  will 
  become 
  dryer 
  as 
  

   the 
  capacity 
  for 
  moisture 
  is 
  increased 
  by 
  raising 
  the 
  tempera- 
  

   ture. 
  Hence 
  a 
  portion 
  of 
  atmosphere 
  rising 
  from 
  the 
  surface 
  

   of 
  the 
  earth 
  will 
  become 
  relatively 
  more 
  moist 
  as 
  it 
  rises, 
  

   owing 
  to 
  the 
  decrease 
  in 
  temperature 
  upward 
  from 
  the 
  surface. 
  

   As 
  the 
  atmosphere 
  contains 
  at 
  all 
  times 
  more 
  or 
  less 
  moisture, 
  

   a 
  rising 
  current 
  will 
  produce 
  clouds 
  and 
  eventually 
  rain, 
  while 
  

   a 
  descending 
  atmosphere 
  becomes 
  relatively 
  dryer 
  as 
  it 
  de- 
  

   scends. 
  

  

  ISTow 
  let 
  us 
  apply 
  these 
  simple 
  laws 
  to 
  conditions 
  in 
  areas 
  of 
  

   low 
  and 
  high 
  pressure. 
  In 
  a 
  low 
  pressure 
  area 
  we 
  have 
  seen 
  

   that 
  the 
  atmosphere 
  flows 
  in 
  toward 
  the 
  center 
  from 
  all 
  direc- 
  

   tions, 
  consequently 
  at 
  or 
  near 
  the 
  center 
  of 
  the 
  area 
  it 
  must 
  

   rise 
  and 
  be 
  lowered 
  in 
  temperature 
  ; 
  the 
  moisture 
  of 
  the 
  atmos- 
  

   ])here 
  will 
  approach 
  saturation 
  ; 
  clouds 
  will 
  be 
  formed 
  and 
  if 
  

   it 
  continue 
  to 
  rise 
  rain 
  will 
  follow. 
  Hence 
  an 
  area 
  of 
  low 
  

   pressure 
  is 
  apt 
  to 
  be 
  characterized 
  by 
  cloudy 
  and 
  rainy 
  weather. 
  

   The 
  atmosphere 
  in 
  a 
  region 
  of 
  high 
  pressure 
  descends; 
  hence 
  

   it 
  grows 
  warmer 
  as 
  it 
  descends. 
  As 
  it 
  grows 
  warmer, 
  it 
  

   becomes 
  dryer 
  ; 
  any 
  clouds 
  which 
  ma}^ 
  have 
  existed 
  over 
  this 
  

   area 
  tend 
  to 
  become 
  dissolved. 
  Hence 
  an 
  area 
  of 
  high 
  pres- 
  

   sure 
  is 
  an 
  area 
  of 
  clearing 
  or 
  clear 
  weather. 
  We 
  shall 
  see 
  

   later 
  that 
  these 
  principles 
  hold 
  good 
  for 
  average 
  pressure 
  con- 
  

   ditions 
  as 
  well 
  as 
  for 
  the 
  transient 
  conditions 
  shown 
  on 
  our 
  

   daily 
  weather 
  maps. 
  That 
  is 
  to 
  say, 
  if 
  in 
  a 
  certain 
  region 
  we 
  

   iind 
  the 
  mean 
  pressure 
  for 
  the 
  month 
  to 
  be 
  considerably 
  higher 
  

   than 
  in 
  neighboring 
  regions, 
  the 
  weather 
  there 
  will 
  be 
  found 
  

   to 
  be 
  generally 
  fair 
  ; 
  while 
  in 
  a 
  region 
  where 
  the 
  pressure 
  has 
  

   been 
  relatively 
  low 
  the 
  prevailing 
  weather 
  will 
  be 
  found 
  to 
  be 
  

   cloudy 
  and 
  unsettled. 
  We 
  can 
  now 
  see 
  that 
  if 
  we 
  can 
  defi- 
  

   nitely 
  locate 
  areas 
  of 
  mean 
  high 
  pressure 
  and 
  mean 
  low 
  

   pressure, 
  during 
  a 
  specified 
  time, 
  we 
  shall 
  have 
  a 
  means 
  of 
  

   determining 
  readily 
  the 
  prevailing 
  direction 
  of 
  the 
  winds, 
  this, 
  

   in 
  turn, 
  enabling 
  us 
  to 
  determine 
  roughly 
  the 
  temperature 
  and 
  

   the 
  rainfall. 
  

  

  Throughout 
  this 
  discussion 
  I 
  have 
  considered 
  the 
  isobar 
  of 
  

   301 
  inches 
  as 
  defining 
  roughly 
  the 
  outer 
  limit 
  of 
  a 
  high 
  area, 
  and 
  

   the 
  isobar 
  of 
  29*9 
  inches 
  as 
  enclosing 
  the 
  area 
  of 
  low 
  pressure. 
  

  

  