﻿FOGS 
  AND 
  CLOUDS 
  — 
  HUMPHREYS. 
  203 
  

  

  cumuli. 
  (PL 
  15, 
  fig. 
  1.) 
  However, 
  we 
  should 
  remember 
  that 
  the 
  

   fire 
  itself 
  adds 
  a 
  surprisingly 
  large 
  amount 
  of 
  moisture 
  to 
  the 
  

   air 
  — 
  a 
  gallon 
  of 
  water, 
  roughly, 
  for 
  every 
  15 
  pounds 
  of 
  fuel 
  burned, 
  

   so 
  that 
  the 
  fire 
  cumulus 
  is 
  not 
  entirely 
  due 
  to 
  convection. 
  

  

  Since 
  cumuli 
  are 
  caused 
  by 
  rising 
  air 
  currents 
  induced 
  by 
  rela- 
  

   tively 
  high 
  surface 
  temperatures, 
  they 
  are 
  very 
  common 
  in 
  equa- 
  

   torial 
  regions, 
  and 
  also 
  through 
  the 
  middle 
  latitudes 
  during 
  summer. 
  

   Hence, 
  too, 
  they 
  commonly 
  occur 
  over 
  land 
  most 
  frequently 
  during 
  

   the 
  afternoon 
  and 
  over 
  water 
  late 
  at 
  night. 
  For 
  the 
  same 
  reason 
  

   they 
  often 
  follow 
  a 
  coast 
  line 
  over 
  the 
  water 
  at 
  night 
  and 
  over 
  the 
  

   land 
  through 
  the 
  day. 
  Similarly 
  they 
  are 
  common 
  over 
  islands 
  (pi. 
  

   15, 
  fig. 
  2), 
  whose 
  presence 
  frequently 
  is 
  shown 
  in 
  this 
  way 
  while 
  

   they 
  themselves 
  are 
  still 
  below 
  the 
  horizon. 
  

  

  9. 
  Cumulo-nimbus 
  (Cu.-Nb.). 
  — 
  "The 
  thunder 
  cloud; 
  shower 
  

   cloud. 
  — 
  Great 
  masses 
  of 
  cloud 
  rising 
  in 
  the 
  form 
  of 
  mountains 
  or 
  

   towers 
  or 
  anvils, 
  generally 
  having 
  a 
  veil 
  or 
  screen 
  of 
  fibrous 
  texture 
  

   {false 
  cirrus) 
  at 
  the 
  top 
  and 
  at 
  its 
  base 
  a 
  cloud 
  mass 
  similar 
  to 
  

   nimbus. 
  From 
  the 
  base 
  local 
  showers 
  of 
  rain 
  or 
  snow, 
  occasionally 
  

   of 
  hail 
  or 
  soft 
  hail, 
  usually 
  fall. 
  Sometimes 
  the 
  upper 
  margins 
  

   have 
  the 
  compact 
  shape 
  of 
  cumulus 
  or 
  form 
  massive 
  heaps, 
  around 
  

   which 
  floats 
  delicate 
  false 
  cirrus. 
  At 
  other 
  times 
  the 
  margins 
  them- 
  

   selves 
  are 
  fringed 
  with 
  filaments 
  similar 
  to 
  cirrus 
  clouds. 
  This 
  last 
  

   form 
  is 
  particularly 
  common 
  with 
  spring 
  showers. 
  The 
  front 
  of 
  a 
  

   thunderstorm 
  of 
  wide 
  extent 
  is 
  frequently 
  in 
  the 
  form 
  of 
  a 
  large 
  

   low 
  arch 
  above 
  a 
  region 
  of 
  uniformly 
  lighter 
  sky." 
  (PI. 
  16, 
  figs. 
  1 
  

   and 
  2.) 
  

  

  This 
  is 
  the 
  most 
  turbulent 
  and 
  also 
  the 
  thickest 
  of 
  all 
  clouds, 
  

   ranging 
  in 
  depth 
  commonly 
  from 
  1 
  to 
  3 
  miles, 
  and 
  occasionally, 
  in 
  

   tropical 
  regions, 
  even 
  to 
  8 
  or 
  9 
  miles. 
  Its 
  times, 
  places, 
  and 
  modes 
  

   of 
  occurrence 
  are 
  all 
  the 
  same 
  as 
  those 
  of 
  the 
  cumulus, 
  given 
  above. 
  

  

  10. 
  Stratus 
  (St.). 
  — 
  "A 
  uniform 
  layer 
  of 
  cloudlike 
  fog 
  not 
  lying 
  

   on 
  the 
  ground. 
  The 
  cloud 
  layer 
  of 
  stratus 
  is 
  always 
  very 
  low. 
  If 
  

   it 
  is 
  divided 
  into 
  ragged 
  masses 
  in 
  a 
  wind 
  or 
  by 
  mountain 
  tops 
  it 
  

   may 
  be 
  called 
  fracto-stratus. 
  The 
  complete 
  absence 
  of 
  detail 
  of 
  

   structure 
  differentiates 
  stratus 
  from 
  other 
  aggregated 
  forms 
  of 
  

   cloud." 
  (PI. 
  11, 
  fig. 
  2.) 
  

  

  The 
  stratus 
  is 
  the 
  lowest 
  of 
  clouds, 
  averaging 
  around 
  2,000 
  feet 
  

   above 
  the 
  surface. 
  It 
  frequently 
  is 
  formed 
  by 
  the 
  lifting 
  of 
  rela- 
  

   tively 
  warm, 
  humid 
  air 
  by 
  underrunning 
  cold 
  winds 
  ; 
  by 
  the 
  flow 
  of 
  

   a 
  warm, 
  humid 
  wind 
  over 
  a 
  surface 
  stratum 
  of 
  cold 
  air; 
  and 
  by 
  

   the 
  drifting 
  of 
  deep 
  fog 
  from 
  the 
  sea 
  over 
  relatively 
  warm 
  land, 
  as 
  

   in 
  the 
  case 
  of 
  the 
  " 
  velo 
  " 
  cloud 
  of 
  southern 
  California, 
  in 
  which 
  the 
  

   lower 
  portion 
  of 
  the 
  fog 
  is 
  delightfully 
  evaporated 
  away, 
  while 
  the 
  

  

  